Introduction To Planetary Geomorphology Ronald Greeley

Introduction To Planetary Geomorphology Ronald
Greeley download
https://ebookbell.com/product/introduction-to-planetary-
geomorphology-ronald-greeley-4689996
Explore and download more ebooks at ebookbell.com

Here are some recommended products that we believe you will be
interested in. You can click the link to download.
Introduction To Planetary Science The Geological Perspective Gunter
Faure Teresa M Mensing
https://ebookbell.com/product/introduction-to-planetary-science-the-
geological-perspective-gunter-faure-teresa-m-mensing-4158072
Introduction To Planetary Photometry 1st Edition Michael K Shepard
https://ebookbell.com/product/introduction-to-planetary-
photometry-1st-edition-michael-k-shepard-6717572
Introduction To One Health An Interdisciplinary Approach To Planetary
Health Sharon L Deem
https://ebookbell.com/product/introduction-to-one-health-an-
interdisciplinary-approach-to-planetary-health-sharon-l-deem-22053456
An Introduction To Planetary Nebulae Jason J Nishiyama
https://ebookbell.com/product/an-introduction-to-planetary-nebulae-
jason-j-nishiyama-23291572

An Introduction To Planetary Atmospheres Agustin Sanchezlavega
https://ebookbell.com/product/an-introduction-to-planetary-
atmospheres-agustin-sanchezlavega-5146294
An Introduction To Planetary Atmospheres 1st Edition Agustin
Sanchezlavega
https://ebookbell.com/product/an-introduction-to-planetary-
atmospheres-1st-edition-agustin-sanchezlavega-60484634
Introduction To Earth And Planetary System Science New View Of Earth
Planets And Humans 1st Edition Naotatsu Shikazono Auth
https://ebookbell.com/product/introduction-to-earth-and-planetary-
system-science-new-view-of-earth-planets-and-humans-1st-edition-
naotatsu-shikazono-auth-4103268
Mars An Introduction To Its Interior Surface And Atmosphere Cambridge
Planetary Science Nadine Barlow
https://ebookbell.com/product/mars-an-introduction-to-its-interior-
surface-and-atmosphere-cambridge-planetary-science-nadine-
barlow-22442320
Introduction To Modern Analysis 2nd Edition 2nd Kantorovitz
https://ebookbell.com/product/introduction-to-modern-analysis-2nd-
edition-2nd-kantorovitz-44870612

more information – www.cambridge.org/9780521867115

Introduction to Planetary Geomorphology
Nearly all major planets and moons in our Solar System have been
visited by spacecraft, and the data they have returned have
revealed the incredible diversity of planetary surfaces. Featuring
a wealth of images, this textbook explores the geologic evolution
of the planets and moons.
Introductory chapters discuss how information gathered from
spacecraft is used to unravel the geologic complexities of our Solar
System. Subsequent chapters focus on current understandings of
planetary systems. The textbook shows how planetary images and
remote sensing data are analyzed through the application of funda-
mental geologic principles. It draws on results from spacecraft sent
throughout the Solar System by NASA and other space agencies.
Aimed at undergraduate students in planetary geology, geo-
science, astronomy, and Solar System science, it highlights the
differences and similarities of planetary surfaces at a level that can
be readily understood by non-specialists.
Electronic versions of ﬁgures from the book are available at
www.cambridge.org/Greeley.
RO N A L D GR E E L E Y (1939–2011) was a Regents’ Professor in the
School of Earth and Space Exploration, Arizona State University,
Director of the NASA–ASU Regional Planetary Image Facility, and
Principal Investigator of the Planetary Aeolian Laboratory at NASA-
Ames Research Center. He co-authored several well-known books
on planetary surfaces, including The Compact NASA Atlas of the
Solar System and Planetary Mapping (both available from
Cambridge University Press).

Introduction to
Planetary
Geomorphology
Ronald Greeley
Arizona State University

C A M B R I D G E U N I V E R S I T Y P R E S S
Cambridge, New York, Melbourne, Madrid, Cape Town,
Singapore, São Paulo, Delhi, Mexico City
Cambridge University Press
The Edinburgh Building, Cambridge CB2 8RU, UK
Published in the United States of America by
Cambridge University Press, New York
www.cambridge.org
Information on this title: www.cambridge.org/9780521867115
© R. Greeley 2013
This publication is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without
the written permission of Cambridge University Press.
First published 2013
Printed and bound in the United Kingdom by the MPG Books Group
A catalog record for this publication is available from the British Library
Library of Congress Catalog in Publication data
Greeley, Ronald.
Introduction to planetary geomorphology / Ronald Greeley.
p. cm.
ISBN 978-0-521-86711-5 (hardback)
1. Planets – Geology – Popular works. 2. Planets – Crust – Popular
works. 3. Geomorphology. I. Title.
QB603.G46.G74 2013
551.4109990
2–dc23
2011053270
ISBN 978-0-521-86711-5 Hardback
Additional resources for this publication at www.cambridge.org/greeley
Cambridge University Press has no responsibility for the persistence or
accuracy of URLs for external or third-party internet websites referred to
in this publication, and does not guarantee that any content on such
websites is, or will remain, accurate or appropriate.

For
Randall and Lidiette
Thomas, Rebecca, and Jennifer

CONTENTS
Foreword by Robert T. Pappalardo page xi
Preface xiii
Acknowledgments xiv
1 Introduction 1
1.1 Solar System overview 1
1.1.1 The terrestrial planets 1
1.1.2 The giant planets 3
1.1.3 Small bodies, Pluto, and
“dwarf planets” 4
1.2 Objectives of Solar System exploration 6
1.2.1 Planetary geology objectives 6
1.2.2 Astrobiology 7
1.3 Strategy for Solar System exploration 7
1.4 Flight projects 9
1.5 Planetary data 12
1.6 Planetary research results 13
Assignments 14
2 Planetary geomorphology methods 15
2.1 Introduction 15
2.2 Approach 15
2.3 Planetary geologic maps 17
2.4 Geologic time 20
2.5 Remote sensing data 22
2.5.1 Visible imaging data 23
2.5.2 Multispectral data 24
2.5.3 Thermal data 24
2.5.4 Radar imaging data 24
2.5.5 Ultraviolet, X-ray, and gamma-ray data 25
2.6 Geophysical data 25
2.7 Image processing 26
2.8 Resolution 28
2.9 Electronic data records (EDRs) 30
2.10 Cartography 30
Assignments 33
3 Planetary morphologic processes 34
3.1 Introduction 34
3.2 Tectonism 34
3.3 Volcanic processes 37
3.3.1 Volcanic eruptions 38
3.3.2 Volcanic morphology 38
3.3.3 Volcanic craters 40
3.3.4 Intrusive structures 42
3.4 Impact cratering 43
3.4.1 Impact cratering mechanics 43
3.4.2 Impact craters on Earth 44
3.4.3 Impact craters and planetary
environments 50
3.5 Gradation 51
3.5.1 Weathering 51
3.5.2 Mass wasting 53
3.5.3 Processes associated with the
hydrologic cycle 53
3.5.4 Aeolian processes 54
3.5.5 Periglacial processes 56
3.6 Summary 58
Assignments 58
vii

4 Earth’s Moon 59
4.1 Introduction 59
4.2 Lunar exploration 59
4.2.1 Pre-Apollo studies 59
4.2.2 The Apollo era 62
4.2.3 Post-Apollo exploration 65
4.3 Interior characteristics 71
4.4 Surface composition 72
4.5 Geomorphology 75
4.5.1 Impact craters and basins 75
4.5.2 Highland plains 77
4.5.3 Mare terrains 77
4.5.4 Sinuous rilles 78
4.5.5 Volcanic constructs 82
4.5.6 Tectonic features 82
4.5.7 Gradational features 84
4.6 Geologic history of the Moon 85
Assignments 90
5 Mercury 91
5.1 Introduction 91
5.2 Mercury exploration 91
5.5.1 General physiography 94
5.5.2 Impact craters 95
5.5.3 Multi-ring basins 99
5.5.4 Volcanic features 102
5.5.6 Gradation features 104
5.6 Geologic history 104
Assignments 105
6 Venus 106
6.1 Introduction 106
6.2 Venus exploration 106
6.5.1 General physiography 113
Assignments 125
7 Mars 126
7.2 Exploration 126
7.3 Interior 129
7.5.1 Physiography 131
Assignments 146
8 The Jupiter system 147
8.2 Exploration 147
8.3 Jupiter 148
8.4 Io 149
8.4.1 Impact features (none!) 152
8.4.5 Io summary 156
8.5 Europa 156
8.5.1 Impact features 157
8.5.5 Europa summary 162
8.6 Ganymede 162
8.6.6 Ganymede summary 169
8.7 Callisto 170
8.7.4 Callisto summary 174
8.8 Small moons and rings 174
8.9 Summary 175
Assignments 176
9 The Saturn system 177
9.2 Exploration 177
9.3 Saturn 178
9.4 Satellites 178
9.5 Titan 179
Contents viii

9.6 Enceladus 185
9.7 Intermediate-size satellites 188
9.7.1 Mimas 188
9.7.2 Tethys 190
9.7.3 Dione 192
9.7.4 Rhea 193
9.7.5 Iapetus 194
9.7.6 Small satellites 197
9.8 The ring system 198
9.9 Summary 199
Assignments 200
10 The Uranus and Neptune systems 201
10.2 Uranus and Neptune 201
10.3 Uranian moons 202
10.4 Neptunian moons 206
10.5 Summary 208
Assignments 209
11 Planetary geoscience future 210
11.2 Planetary protection 210
11.3 Missions in ﬂight and anticipated
for launch 211
11.4 Extended missions 214
11.5 Summary 214
Assignments 215
Appendices 216
References 221
Further reading 225
Index 227
Contents ix

FOREWORD
Robert T. Pappalardo
Ron Greeley’s Introduction to Planetary Geomorphology is
the single most outstanding and complete compendium of
the science of planetary geology that exists. It is a fully
complete and up-to-date synopsis of the science of planetary
geology, written in Greeley’s characteristically succinct
and clear style. This is the ideal primer for an upper under-
graduate course, and an excellent compendium for the
interested amateur or professional astronomer. The figures
within are all the “right” ones – the very best for illustrating
fundamental concepts and “type examples” of terrestrial
and planetary processes – compiled here in one place.
Ron Greeley passed away suddenly in the fall of 2011,
just a month after submitting this complete book for pub-
lication. It will remain a tribute to his life’s work, encapsu-
lating his passions both for research and for teaching.
Greeley was a scholar and a gentleman, and a pioneer in
the methods of planetary geology. His Ph.D. research at the
University of Missouri at Rolla included field work on the
Mississippi Barrier Islands, where he studied modern liv-
ing forms of organisms that he was researching in the fossil
rock record. This work laid the foundation for his practical
approach to deciphering the processes that have shaped the
surfaces of other planets by studying their modern Earth
analogues. In the laboratory and the field, Greeley would
effectively visit other worlds and other times.
Greeley’s career in planetary geology began in 1967,
when he was called to active military service just a year
after receiving his Ph.D. Fortunately, he was assigned
to NASA’s Ames Research Center to work on Apollo-
related problems. He occasionally mused about whether
this interesting assignment came about by someone’s
misunderstanding of his dissertation topic of “lunulitiform
bryozoans” as somehow related to the geology of the
Moon!
Greeley trailblazed the burgeoning field of planetary
geomorphology at Ames. While the Apollo missions
explored the Moon, Greeley conducted detailed compar-
isons of lunar sinuous rilles with terrestrial volcanic land-
forms in Hawaii and the Snake River Plain of Idaho,
making important contributions to understanding lunar
processes. Then, as early 1970s Mariner 9 photos began
to reveal Mars, Greeley used wind tunnels at Ames to
simulate how aeolian processes might operate on the Red
Planet. His seminal work on terrestrial and planetary
aeolian processes is being applied anew today to explan-
ation of dunes on Saturn’s moon Titan, which were
recently discovered by the Cassini spacecraft.
Greeley was involved in nearly every major spacecraft
mission flown in the Solar System since Apollo. This
includes the Magellan mission to Venus and the Galileo
mission to Jupiter. He contributed to a panoply of
missions to Mars: Mariners 6, 7, and 9, Viking, Mars
Pathfinder, Mars Global Surveyor, the two Mars
Exploration Rovers, and the European Space Agency’s
(ESA’s) Mars Express. He chaired many NASA and
National Research Council (NRC) panels, including
NASA’s Mars Exploration Program Analysis Group, the
Mars Reconnaissance Orbiter Science Definition Team,
the NASA–ESA Joint Jupiter Science Definition Team,
the NRC’s Committee on Planetary and Lunar
Exploration, and most recently the Planetary Science
Subcommittee of the NASA Advisory Council.
xi

For those fortunate enough to have known Ron Greeley
ﬁrst-hand, through his teaching, research, committees, or
friendship, this book will serve as a lasting tribute. For
those learning of him, and from him, for the ﬁrst time,
welcome to this man and his work.
As Greeley would say: a journey of a thousand miles
begins with a single step. In your introduction to the
planets and moons of our Solar System, the journey of
4.5 billion kilometers begins with a turn of the page. An
adventure awaits.
Foreword xii

PREFACE
Planetary geoscience had its inception with the birth of the
Space Age in the early 1960s. In the ensuing decades, it
has evolved into a discipline that is recognized by sections
of professional organizations such as the Geological
Society of America and the American Geophysical
Union, as well as being taught at the university level.
Much of our understanding of the geology of extra-
terrestrial objects is derived from remote sensing data –
primarily images that portray planetary surfaces. In fact,
discoveries such as the dry river beds on Mars, the tectonic
deformation of Venus, and the actively erupting volcanoes
on Jupiter’s moon Io all came from pictures taken from
spacecraft. Thus, the focus of this book is on the geo-
morphology of solid-surface objects in our Solar System
and the interpretations of the processes that led to the
diverse landforms observed. Geomorphology, however,
must be analyzed in the context of broader geoscience;
consequently, in the chapters on the individual planetary
systems, the geophysics and interior characteristics are
reviewed along with our current understanding of surface
compositions and the general geologic histories. Of
course, our knowledge of the Solar System is far from
uniform from one planet to another, dependent upon the
numbers and types of spacecraft that have returned data.
Thus, the chapters on the Moon and Mars are more
detailed than those on the outermost planet systems,
Uranus and Neptune, because dozens of successful space-
craft have visited our nearest planetary neighbors, in con-
trast to the limited data returned from “flybys” of the
Voyager spacecraft to the planets beyond Saturn.
Our journey to explore the geomorphology of the Solar
System begins with introductory chapters that introduce
the planets and other objects of planetary geoscience
interest, discuss the methods used in studying extraterres-
trial objects, and review the fundamental geomorphic
processes on Earth that can be compared with what we
see on other planets and satellites.
Key references are given in the text and listed at the
back of the book. The end of this book includes
additional reading for those who wish to delve into the
chapter topic in more detail. Because images form the
basis of much of planetary geomorphology, figure cap-
tions generally include the basic NASA or other space
agency data for the frames shown, to enable the use of
various electronic search engines for obtaining additional
information.
I hope that you find the exploration of the Solar System
a rewarding experience. While many planets and satellites
show landforms that are quite familiar to geologists,
others hold surprises that have not yet been explained or
understood. Have fun, and maybe you can solve some of
these mysteries!
xiii

ACKNOWLEDGMENTS
I thank the countless graduate and undergraduate students
from planetary geoscience classes who have provided the
stimulus for this book. Students have the marvelous
capacity to ask thought-provoking questions that remind
planetary scientists that there is still a great deal to learn
about the Solar System. I also thank my colleagues who are
at the forefront of planetary exploration for their keen
insight into the complexities of geologic surface evolution.
While they have helped me tremendously in understanding
these complexities, any errors in fact or interpretation con-
tained herein are solely my responsibility.
The preparation of this book was facilitated by the
talents of Sue Selkirk for illustrations, Amy Zink for
preparation of the final versions of the images, and Dan
Ball and the NASA Space Photography Laboratory for
access to the planetary images; I am grateful for their
substantial help. I thank Stephanie Holaday for her tireless
word-processing of many draft iterations as well as track-
ing down permissions for previously published figures.
Her assistance has been invaluable and is much appreci-
ated. Finally, I thank Cindy Greeley for her editing skills
and corrections to my flawed writing!
xiv

CHAPTER 1
Introduction
The early part of the twenty-first century saw the comple-
tion of the reconnaissance of the Solar System by space-
craft. With the launch of the New Horizons spacecraft to
Pluto in early 2006 and its expected arrival in 2015, space-
craft will have been sent to every planet, major moon, and
representative asteroid and comet in our Solar System.
With the return of data taken by spacecraft of these objects,
the study of planetary surfaces passed mostly from the
astronomer to the geologist and led to the establishment
of the field of planetary geology.1
The term geology is
used in the broadest sense to include the study of the solid
parts of planetary objects and includes aspects of geophys-
ics, geochemistry, and cartography. Much of our knowl-
edge of the geologic evolution of planetary surfaces is
derived from remote sensing, in situ surface measurements,
geophysical data, and the analysis of landforms, or their
geomorphology, the primary subject of this book.
In this chapter, an overview of Solar System objects is
given, the objectives of Solar System exploration are out-
lined, and the strategy for exploration by spacecraft is
discussed. In the following chapters, the approach used
in understanding the geomorphology of planets is pre-
sented, including the types and attributes of various data
sets. The principal geologic processes operating on plan-
ets are then introduced, and the geology and geomorphol-
ogy of each planetary system is described in subsequent
chapters. The book ends with a discussion of future mis-
sions and trends in Solar System exploration.
1.1 Solar System overview
Our Solar System consists of a fascinating array of
objects, including the Sun, planets and their satellites,
comets and asteroids, and tiny bits of dust. Most of the
mass of the Solar System is found within the Sun, a rather
ordinary star that generates energy through nuclear fusion
with the conversion of hydrogen to helium. Coupled with
astrophysical models, analyses of meteorites suggest that
the Solar System began to form at about 4.6 Ga (Ga is the
abbreviation for giga or 109
annum, or years).
Planets are relatively large objects that are in orbit
around the Sun. As we learned at a very young age, the
planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, and Neptune. And then there is Pluto! The year
2006 saw an interesting controversy emerge when the
International Astronomical Union (the scientific group
responsible for formal naming of objects in the heavens)
declared that Pluto was no longer a “planet” and demoted
it to a new class of objects called “dwarf planets.” This
issue will be discussed later.
All of the planets originally formed through the accre-
tion of dust and smaller objects, making protoplanets. As
the protoplanets grew in size, still more dust and accreted
materials were swept up, a process that continues even
today. For example, it is estimated that more than 10,000
tons of materials are added to Earth each day. Although
this addition is impressive, it is insignificant in compar-
ison with the orders-of-magnitude larger rates of accretion
in the early stages of planetary formation. In the first 0.5
Ga, so much material was amassed that the heat generated
by impacts probably melted the planets completely, lead-
ing to their differentiation, in which the heavier elements,
such as iron, sank to their interiors to form planetary cores
while the lighter elements floated toward the surface.
1.1.1 The terrestrial planets
Mercury, Venus, Earth, and Mars are called the terrestrial
planets because they share similar attributes to Earth
(which in Latin is terra). As shown in Fig. 1.1, these
planets are small in comparison with the other planets
1
Terms when first used are in bold and defined. These terms are given in
the index, where the page number in bold indicates where the term is
defined.
1

and are found closest to the Sun, leading to their alter-
native description as the inner planets. They are com-
posed primarily of rocky material and have solid surfaces.
In planetary geology, Earth’s Moon is typically included
with the terrestrial planets because of its large size and
similar characteristics.
As the terrestrial planets began to cool and form crusts,
elements combined and crystallized into rocks and min-
erals. For the most part, these elements are silicon, oxy-
gen, iron, magnesium, sodium, calcium, potassium, and
aluminum in various combinations that collectively make
up the silicate minerals. The most important silicate
minerals fall into two groups. Light-colored silicate min-
erals are common in continental rocks on Earth and
include quartz, orthoclase feldspar, plagioclase feldspar,
and muscovite mica. Dark-colored silicate minerals are
common on Earth’s sea floor and are rich in iron and
magnesium; they include olivine, pyroxene, hornblende,
and biotite mica. Silicate minerals are the basic building
blocks of most rocks in the crusts of Earth and the Moon,
and they are thought to make up most of the rocks on
Mercury, Venus, and Mars.
Venus, Earth, and Mars all have significant atmospheres
composed of gasses that are gravitationally bound to the
planets (Table 1.1). Mercury and the Moon are too small
to retain anything but the most tenuous atmospheres,
measurable only by very sensitive instruments. Although
some gasses were accumulated by all of the terrestrial
protoplanets during their initial formation, these primary
atmospheres were lost to space. Secondary atmo-
spheres were later released as gasses escaped from the
interior and interacted with the surface. Earth’s atmo-
sphere may be termed a tertiary atmosphere because it
has been greatly modified by biologic processes.
Figure 1.1. A family portrait of the
planets imaged by spacecraft. The
inner planets (Mercury, Venus, Earth
and Moon, and Mars) are shown to
scale with each other and are enlarged
relative to the giant planets (Jupiter,
Saturn, Uranus, and Neptune), which
are shown to scale with each other.
Earth is somewhat smaller than the
Giant Red Spot (indicated by the
arrow) of Jupiter (NASA PIA01341).
Introduction 2

The presence of large impact craters on the terrestrial
planets (Fig. 1.2) shows that their crusts had cooled and
solidified in the first 0.5 Ga of Solar System history before
all of the miscellaneous debris had been swept up.
1.1.2 The giant planets
Jupiter, Saturn, Uranus, and Neptune are referred to as
giant planets. Relative to the terrestrial planets, these
planets are enormous and contain most of the mass in
the Solar System outside the Sun. Jupiter and Saturn are
composed mostly of hydrogen and helium, while Uranus
and Neptune are composed mostly of water, ices, and
other volatile materials. Collectively, the giant planets
and Pluto are called the outer planets, referring to their
location in the Solar System.
The early history of the giant planets is similar to that of
the terrestrial planets. The giant planets also formed by the
accretion of smaller bodies, with each forming a nucleus
large enough to capture the lighter elements that had
escaped from the inner Solar System to the outer frigid
parts of the Solar System. As this process continued, the
giant planets grew to their large sizes, with heavier ele-
ments sinking to their interior. Most models of the giant
planets suggest that each contains a rock-like core, some
of which are larger than Mars.
Each of the giant planets resembles the Sun in compo-
sition, but not even the largest, Jupiter, was destined to
grow to a size sufficient to initiate nuclear fusion.
However, giant planets do resemble the Sun in one impor-
tant way – each grew and evolved to have a family of
smaller bodies in orbit about them so that each resembles
the Solar System in miniature.
Although the giant planets have no “geology” because
they lack solid surfaces, their satellites are of great interest
for planetary geomorphology (Table 1.2). Collectively,
Table 1.1. Basic data for planets
Orbit semi-
major axis
Name (106
km) (AU)a
Revolution
period (yr)
Diameter
(km)
Rotation
(days)
Mass
(1024
kg)
Density
(g/cm3
)
Escape
velocity
(km/s) Surface Atmosphere
Mercury 57.9 0.39 0.24 4,879 58.65 0.33 5.4 4.3 Silicates Trace Na
Venus 108 0.72 0.62 12,104 243.0 (Rb
) 4.87 5.2 10 Basalt,
granite?
90 bar: 97%
CO2
Earth 150 1.00 1.00 12,756 1.00 5.97 5.5 11 Basalt,
granite,
water
1 bar: 78%
N2, 21% O2
Mars 228 1.52 1.88 6,794 1.03 0.64 3.9 5.0 Basalt, clays,
ice
0.07 bar: 95%
CO2
Jupiter 778 5.20 11.86 142,984 0.41 1,899 1.3 60 None H2, He, CH4,
NH3, etc.
Saturn 1427 9.54 29.46 120,536 0.44 569 0.7 35 None H2, He, CH4,
NH3, etc.
Uranus 2871 19.19 84.02 51,118 0.72 (Rb
) 86.8 1.3 21 None (?) H2, He, CH4,
NH3, etc.
Neptune 4498 30.07 164.79 49,528 0.67 102 1.8 24 None (?) H2, He, CH4,
NH3, etc.
Pluto 5906 39.48 247.9 2,302 6.39 (R) 0.013 2 1.3 CH4, ice Trace CH4
a
1 AU (astronomical unit) = Earth–Sun distance, or ~149.6 × 106
km.
b
R = retrograde.
Figure 1.2. The heavily cratered surface of the Moon, shown in
this view obtained by the Apollo 13 astronauts, represents the
final stages of planetary accretion in the first 0.5 Ga of the Solar
System. The dark, smooth area is Mare Moscoviense on the lunar
far side (NASA 70–H–700).

these moons represent a myriad of objects of different
sizes, compositions, and geologic histories. They are clas-
siﬁed as regular satellites (orbiting in the same direction
as the parent planet’s spin direction) or irregular satel-
lites (orbiting in the opposite direction) that are probably
captured objects. Jupiter’s moons Ganymede and Callisto
are about the size of the planet Mercury. At least three
moons, Jupiter’s satellite Io, Saturn’s Enceladus, and
Neptune’s Triton, are currently volcanically active – in
fact, Io is the most geologically active object in the Solar
System (Fig. 1.3). Other outer planet satellites appear to
have remained relatively unaltered since their initial for-
mation. Many of the geologic processes that operate on
terrestrial planets are also seen on outer planet satellites;
however, because of their different compositions (mostly
ices, plus some silicates) and extremely cold environ-
ments, the outer planet satellites also display features
representing processes unique to the outer Solar System.
1.1.3 Small bodies, Pluto, and “dwarf planets”
Asteroids, comets, and the smaller moons of the outer
planets are often called small bodies, even though the
largest asteroids are hundreds of kilometers in diameter.
Comets consist of primordial material left over from the
early stages of Solar System formation. Most comets are
found in the Oort cloud and the Kuiper belt, both beyond
the orbit of Pluto. The Oort cloud forms a spherical zone
some 3 × 1012
km from the Sun and is the apparent source
of long-period comets (those that take more than 200
years to complete an orbit around the Sun), while the
Kuiper belt is a disk-shaped region extending from
Neptune’s orbit to ~8 × 109
km from the Sun and is the
source for short-period comets (those that orbit the Sun in
less than 200 years). Just to make things a little more
complex, objects that are in orbit in this belt are referred
to as Kuiper belt objects, or KBOs. Some of the outer
planet satellites, such as Neptune’s Triton, could have
been captured from the Kuiper belt.
Often described as “dirty snowballs,” comets are com-
posed of dust grains and carbonaceous (carbon-rich)
materials embedded in a matrix of water-ice (Fig. 1.4).
Study of cometary material collected from Comet Wild 2
by NASA’s Stardust mission (Brownlee et al., 2006) and
returned to Earth suggests that at least some comets are
composed of grains that were heated in the inner Solar
Table 1.2. Basic data for selected satellites
Planet
Satellite
name Discovery
Period
(days)
Diameter
(km)
Mass
(1020
kg)
Density
(g/cm3
) Surface material
Earth Moon – 27.32 3,476 735 3.3 Silicates
Mars Phobos Hall (1877) 0.32 27 1 × 10−4
2.2 Carbonaceous
Deimos Hall (1877) 1.26 13 2 × 10−5
1.7 Carbonaceous
Jupiter Io Galileo (1610) 1.77 3,660 893 3.6 Sulfur, SO2
Europa Galileo (1610) 3.55 3,130 480 3.0 Ice
Ganymede Galileo (1610) 7.15 5,268 1,482 1.9 Dirty ice
Callisto Galileo (1610) 16.69 4,806 1,076 1.8 Dirty ice
Saturn Mimas Herschel (1789) 0.94 396 0.376 1.2 Ice
Enceladus Herschel (1789) 1.37 504 0.74 1.10 Pure ice
Tethys Cassini (1684) 1.89 1,048 6.27 1.0 Ice
Dione Cassini (1684) 2.74 1,120 11 1.4 Ice
Rhea Cassini (1672) 4.52 1,528 23 1.3 Ice
Titan Huygens (1655) 15.95 5,150 1,346 1.9 Methane ice
Hyperion Bond, Lassell (1848) 21.3 360 8 × 10−3
? ? Dirty ice
Iapetus Cassini (1671) 79.3 1,436 16 1.1 Ice/carbonaceous
Phoebe Pickering (1898) 550 (Ra
) 220 0.004 ? Carbonaceous?
Uranus Miranda Kuiper (1948) 1.41 474 0.7 1.3 Dirty ice
Ariel Lassell (1851) 2.52 1,159 14 1.6 Dirty ice
Umbriel Lassell (1851) 4.14 1,170 12 1.4 Dirty ice
Titania Herschel (1787) 8.71 1,578 35 1.6 Dirty ice
Oberon Herschel (1787) 13.5 1,522 30 1.5 Dirty ice
Neptune Triton Lassell (1846) 5.88 (Ra
) 2,704 214 2.0 Methane ice
Pluto Charon Christy (1978) 6.39 1,186 16.2 ? Ice
a
R = retrograde.
Introduction 4

System, were driven outward from the Sun, and then
coalesced to form some comets. This led to the reference
to some comets as “icy dirt balls,” a concept that was
supported in 2005 when the Deep Impact spacecraft
launched a roughly half-ton metal ball into Tempel 1, a
comet measuring 7.6 km by 4.9 km. The impact explosion
released a plume of icy dust, suggesting the properties of
freshly fallen, fluffy snow with dust. Images taken by
Deep Impact and those taken by the NASA NExT space-
craft in 2011 after the impact show that Tempel 1’s surface
has smooth terrains and areas that have been eroded.
Most asteroids are found in the zone between the orbits
of Mars and Jupiter, known as the main asteroid belt.
However, asteroids are also found in orbits of larger
planets and are called Trojan asteroids, while those in
orbits that come close to the Earth are called near-Earth
objects, or NEOs. Asteroids can be further classified in
terms of their spectral properties and comparisons with
meteorites, many of which were derived from asteroids.
Historically, asteroids were thought to be either remnants
of a former planet that broke apart or objects that never
accreted to form a planet early in Solar System history. As
with many ideas in planetary science, this was an over-
simplification. It is now fairly clear that some asteroids
(and the corresponding meteorites) represent fragments of
a larger body that had been differentiated. Thus, “metal-
lic” objects are thought to represent the core of a planet,
while those having signatures of the mineral olivine
would represent a planetary mantle, and “stony” objects
would represent the crust. Other asteroids have the signa-
tures of carbon-rich materials and are considered to rep-
resent “unprocessed,” or primitive, planetary material. In
this regard, many planetologists suggest that some of
these types of asteroids are actually the rocky material
left over from comets that have lost most or all of their
volatile materials.
Numerous missions have flown past, orbited, and even
landed on asteroids, with one mission returning samples to
Earth. The first images of asteroids up-close were taken in
Figure 1.4. This view of the 5 km in diameter Comet Wild 2 was
taken by the Stardust spacecraft in January 2004 (NASA Stardust
Project).
Figure 1.3. One of the moons of Jupiter, Io, is the most
volcanically active object in the Solar System. This image was
taken by the New Horizons spacecraft in 2007 during its flyby of
the Jupiter system on the way to Pluto. The huge umbrella-
shaped plume at the top of the image is pyroclastic material
rising 290 km from the active volcano Tvashtar. Also visible (left
side) is a smaller (60 km high) plume erupted from the volcano,
Prometheus (NASA PIA09248).

1991 and 1993 by NASA’s Galileo spacecraft in the main
asteroid belt (Fig. 1.5) and included the discovery that
asteroids could even have their own small moons. In 2003,
Japan launched the Hayabusa spacecraft, which rendez-
voused with the NEO Itokawa (Fig. 1.6) in 2005; the
spacecraft touched down on the asteroid and collected
samples that were returned to Earth in 2010 for analyses.
Pluto was discovered telescopically in 1930 and for 76
years was classified as a planet. But it does not fit neatly
into either the terrestrial planet or the giant planet classi-
fication; it is relatively small and has an orbit that is
substantially inclined to the general ecliptic plane and at
times is inside the orbit of Neptune. In the past few
decades, many more large objects have been discovered
in orbit around the Sun, including Eris, which is slightly
larger than Pluto. It is estimated that, as a minimum, some
several dozen large objects reside within the zone of
Pluto’s orbit, and many hundreds could well be found in
the Kuiper belt. These factors led the International
Astronomical Union to “demote” Pluto as a main planet
in 2006 and to define a new category, the so-called “dwarf
planets,” currently consisting of Pluto, Ceres (formerly
classified as an asteroid), Haumea, Makemake, and Eris,
some of which have one or more moons. None of
these objects has been visited by spacecraft, but the
Dawn spacecraft will visit Ceres in 2014, and the New
Horizons spacecraft is slated to fly past Pluto in 2015.
1.2 Objectives of Solar System exploration
October 1957 saw the launch of the Soviet orbiter Sputnik
around Earth and the beginning of the “Space Age.”
About the size of a basketball, Sputnik did little more
than send a “beep–beep” radio signal, but it was the
starting gun for the space race. The United States
responded with President Kennedy’s decision to send
men to the Moon before the end of the 1960s and the
formation of the National Aeronautics and Space
Administration, or NASA, in October 1958. Although
the decision was motivated by politics and military con-
siderations (an orbiting spacecraft has the ability to deliver
warheads to any place on Earth), the National Academy of
Sciences was asked to define the scientific goals for Solar
System exploration. After careful consideration by a
group of distinguished scientists, the principal goals
were defined as determining: (1) the origin and evolution
of the Solar System, (2) the origin and evolution of life,
and (3) the processes that shape humankind’s terrestrial
environment. Although these goals have evolved over the
years, the basic concepts remain the foundation for Solar
System exploration.
1.2.1 Planetary geology objectives
Geologic sciences figure prominently in the goals for
Solar System exploration. Basic geologic questions
Figure 1.5. The first close-up view of an asteroid was obtained
by the Galileo spacecraft in October 1991, shown in this view of
asteroid Gaspra, which is of dimensions 19 km by 12 km by 11 km.
Gaspra’s irregular shape suggests that it might be a piece of a
larger object that fragmented from one or more collisions. More
than 600 impact craters ranging in size from 100 to 500 m are
visible on Gaspra’s surface (NASA JPL P-40449).
Figure 1.6. An image of the asteroid Itokawa, taken by the JAXA
Hayabusa spacecraft, which touched down on the surface,
collected samples, and returned them to Earth in the fall of 2010.
This asteroid measures 535 m by 294 m by 209 m and appears to
be a “rubble-pile” of boulders, the biggest of which is about
50 m across. More than 1,500 small grains were collected, and
initial analyses show the presence of silicate minerals, such as
olivine.
Introduction 6

include the following. (a) What is the present state of the
planet? (b) What was the past state of the planet? (c) How
do the present and past states compare with those of other
objects in the Solar System?
The question dealing with the present state seeks to
determine the composition, distribution, and ages of
rocks on the surface, identify active geologic processes,
and characterize the interior.
Determining the past state of a planet, including Earth, is
a fundamental aspect of geology and involves determining
its geologic history. For example, is the present state repre-
sentative of previous conditions on the planet, or has there
been a change or evolution in the surface or interior?
Answering these questions is typically accomplished
through geologic mapping, coupled with the derivation of
a stratigraphic framework and geologic time scale.
Comparative planetology addresses the third aspect in
the geologic study of planets. Once the present and past
states have been assessed, the results are then compared
among all of the planets to determine their differences and
similarities. This comparision enables a more complete
understanding of geologic processes in general and of the
evolution of all solid-surface objects in the Solar System.
1.2.2 Astrobiology
Are we alone? That fundamental question has been posed
in various forms throughout humankind’s history and
constitutes one of the key motivations in the exploration
of space. The term astrobiology was coined to encompass
all aspects of the search for present and past life, including
research on the conditions for the origin of life and study
of the environments conducive for biological processes.
The NASA Astrobiology Institute (NAI), which was
formed in 1998 and is headquartered at the NASA-Ames
Research Center in California, consists of an international
consortium of universities and institutions conducting a
wide variety of research projects in astrobiology. The NAI
organizes annual spring meetings to review the latest
results in astrobiology (http://nai.nasa.gov); these meet-
ings are well attended and open to the public.
The Viking mission to Mars in the mid 1970s was the
first project to search for life beyond Earth. Experiments
for the two Viking landers (Fig. 1.7) were developed to
search specifically for life-forms and to assess possible
biological processes. The results from these experiments
were negative, and the general search for life was out of
vogue for some 20 years. However, during this period,
careful considerations were given as to how astrobiology
questions should be addressed. For example, when tar-
geting specific planetary objects for astrobiology explo-
ration, at least three factors should be considered: the
presence of water (preferably in the liquid state), a source
of sufficient energy to support biological processes, and
the availability of organic chemistry and other elements
essential for life processes (primarily carbon, nitrogen,
hydrogen, oxygen, phosphorus, and sulfur). With current
data, the search narrows to Mars, Jupiter’s moon Europa,
and possibly Saturn’s moons Enceladus and Titan. If the
search is expanded to include potential past environ-
ments, objects such as Jupiter’s moon Ganymede might
be included.
In 1996 a meteorite (designated ALH84001) found in
Antarctica was thought to have been ejected from Mars
and was suggested to show evidence for biology.
Although much of this evidence has been rejected, interest
in astrobiology increased substantially, especially as
related to the exploration of Mars. The current search
strategy focuses on identifying the present and past envi-
ronments conducive for biology and is a “win–win”
approach. Obviously, if life or the signs of life (e.g.,
fossils) are found, the result would be truly profound
(Fig. 1.8). However, a negative result is equally intrigu-
ing; if present or past environments are found that are
amenable for life, but life is not found, then one must
ask why not, and what is it about Earth that would make
our planet unique for life if indeed we are truly “alone?”
As the field of astrobiology has moved forward, life has
been found to be much more pervasive on Earth than had
previously been suspected. In recent years life-forms have
been found in extreme conditions of temperature, pres-
sure, pH, and other environmental parameters, showing
that biology can occur in a much greater range of settings
than previously suspected, thus widening the search for
life throughout the Solar System.
1.3 Strategy for Solar System exploration
Determining the present and past states of planets and
comparative planetology requires observations and meas-
urements from orbit, placement of instruments on plane-
tary surfaces, and the return of samples to Earth. Thus, the
general exploration of the Solar System by spacecraft
follows a strategy involving a series of missions of
increasing capabilities. However, even before spacecraft
are launched, Earth-based telescopic observations are
made to determine the fundamental characteristics of
1.3 Strategy for Solar System exploration 7

planetary objects, such as their size and density, and the
presence or absence of atmospheres.
The first exploratory missions are usually “flybys,” in
which spacecraft zoom past planetary objects and, over a
period of only hours or a few days, collect data. Although
limited, these data provide the first glimpses of the object
up-close and are far better than those obtained from Earth-
based telescopes. For example, in 1979 and the 1980s the
spectacular Voyager 1 and 2 spacecraft (Fig. 1.9) revealed
the complexities of the moons of Jupiter, Saturn, Uranus,
and Neptune during brief flybys of those planetary
systems.
Next in exploration comes the use of orbiting space-
craft. Remaining in orbit for days, months, or even years,
orbiters provide the opportunity for more complete map-
ping and observations of potential seasonal changes.
Spacecraft in polar or near-polar orbits can obtain remote
sensing data for the entire planet, enabling assessments of
the surface complexity, collection of geophysical data,
and measurements of topography. Thus, one of the pri-
mary advantages of orbiters is the collection of global
data.
Once a planet has been surveyed from orbit, the mis-
sions that follow can include landed spacecraft. Landers
enable “ground-truthing” of the remote sensing data
obtained from orbit. Such data include in situ measure-
ments of surface chemistry and mineralogy, determina-
tions of the physical properties of the surface enviroment,
and geophysical measurements, including seismometry.
Landed missions are significantly enhanced by surface
mobility as afforded by robotic systems, such as the
Mars Exploration Rovers (Fig. 1.10). The advantage of
Figure 1.7. The first
successful landing on
Mars was the Viking 1
lander, shown in this
diagram with its
principal components.
Introduction 8

landers and rovers is the ability to obtain data directly
from planetary surfaces and near-surface materials, as
from drill cores, which was first done robotically by the
Soviets on the Moon. The disadvantage is the relatively
limited number of sites that can be visited; can you imag-
ine characterizing the complex geology of the Earth from
only a handful of stations on the surface?
Samples returned from planetary objects represent the
next stage in exploration. These enable sophisticated lab-
oratory analyses of compositions, measurements of phys-
ical properties, and searches for signs of past or present
life. Although significant advances in instruments that can
be applied on robotic missions have been made in recent
years, none can approach the accuracy and precision
afforded by full laboratory facilities on Earth.
Particularly critical for geology are the ages of rocks
determined on returned samples using techniques based
on the decay of radioactive materials (see Section 2.4).
While some measurements can be made from robotic
spacecraft, the complexities of obtaining and properly
handling samples in order to make the measurements
have not been solved satisfactorily for determining ages.
The ultimate in planetary science is human exploration.
Humans have the ability to analyze and synthesize data
quickly, make decisions on the spot, and respond to the
results. No machine can match these attributes. But, of
course, sending humans into space is both risky and
costly. Currently, it is far more cost-effective to send
robotic spacecraft throughout the Solar System.
However, the time will come when humans will be
required for the ultimate step in exploration.
Figure 1.11 shows the “score-board” for the different
stages of Solar System exploration. Nearby objects, such
as our Moon, have been explored extensively, while
most of the outer Solar System has been viewed only
by flyby missions. Despite this uneven coverage, we are
now well poised to address many of the fundamental
aspects of the origin and evolution of the major planetary
objects.
1.4 Flight projects
Getting a NASA spacecraft “off the ground” is a long
process that involves many constituencies, including
NASA, Congress (which appropriates the money), the
aerospace industry (which builds much of the hardware),
Figure 1.8. What are the signs of life that might be sought in the
search for present or past life beyond Earth? From our “Earth
bias,” we might think that we know what fossils look like. But
even on Earth, some cases are not so clear: (a) living
cyanobacteria (courtesy of Jennifer Glass, Arizona State
University), (b) synthetic non-biological filaments containing
silica and the mineral witherite (from Garcia-Ruiz, J. M., Hyde,
S. T., Carnerup, A. M. et al. (2003), Self-assembled silica–
carbonate structures and detection of ancient microfossils,
Science, 302, 1,194–1,197. Reprinted with permission from the
AAAS), and (c) an image of martian meteorite ALH84001
(courtesy of NASA Astrobiology Institute).

Figure 1.9. The Voyager project involved two spacecraft that explored the outer Solar System in 1979 and into the 1980s
with flybys of Jupiter and Saturn (Voyagers 1 and 2) and Uranus and Neptune (Voyager 2), providing the first clear images of their
major moons.
Figure 1.10. The Mars Exploration
Rovers, Spirit and Opportunity,
landed in early 2004. Shown here is
Spirit before launch, compared with
the flight-spare of the Mars
Pathfinder rover on the right (NASA
PIA04421).
Introduction 10

the science community, and the public. NASA is an inde-
pendent federal agency, meaning that there is no
“Department of Space” and its Administrator is appointed
by the President. Direct science input is through NASA
committees, with members appointed from universities,
NASA centers, and other research organizations. The
National Academy of Sciences (through its working
organization, the National Research Council, or NRC)
provides science guidance. This is accomplished by
means of formal reports prepared by scientists from
the planetary community that recommend missions and
research activities covering a ten-year period, known
informally as decadal surveys (NRC, 2011). Individuals
influence Solar System exploration by making their opin-
ions known through communication with NASA,
Congress, and the Administration. Coordinated input
is often conducted through organizations such as The
Planetary Society (http://www.planetary.org/home/),
the National Space Society (http://www.nss.org/), and
the Mars Society (http://www.marssociety.org/).
In principle, the various constituencies work together to
derive the specific goals for a mission, the means to
achieve those goals (e.g., the kind of spacecraft), and the
budget to make it all happen. In practice, the process is
often more haphazard, yet most of the constituencies are
still involved to varying degrees. The time from initial
mission concept to the return of data is usually years, or
even decades.
Once a project has been approved, the mission is
assigned to a NASA research center or run through the
Jet Propulsion Laboratory (JPL) in California or the
Applied Physics Laboratory (APL) in Maryland, both
of which are NASA contract research centers. Flight pro-
jects go through various phases from design and develop-
ment through mission operations. Early in the process,
a Science Definition Team (SDT) is appointed from
the scientific community, which has the responsibility
for determining the specific objectives for the mission.
After this has been completed, an Announcement of
Opportunity is released by NASA, enabling proposals to
be submitted for building the spacecraft and providing the
science payload or instruments. Individuals and organiza-
tions can then compete for selection, which is made
through peer-reviews of the proposals.
Figure 1.11. Status of Solar System exploration by spacecraft important for planetary geomorphology.

NASA missions can be categorized as (1) strategic
missions, (2) Principal Investigator (PI)-led missions,
and (3) supporting missions. Strategic missions include
the Mars Science Laboratory and the Cassini spacecraft in
orbit around Saturn. Such missions cost multiple billions
of dollars and might be flown once or twice per decade.
PI-led missions are proposed, designed, and executed by a
planetary scientist, who assembles the science team,
industry partners, and a planetary research center, such
as the JPL or APL. PI-led missions of interest for geo-
science are found in the Discovery Program (such as the
MESSENGER mission to Mercury) and the New Frontiers
Program (such as the New Horizons Pluto mission).
Specific missions within these programs are “cost-
capped,” with New Frontiers being at the upper level of
$1 billion.
Supporting missions are designed to collect data to
enable follow-on missions. While science is not usually
the primary motivation, such data are often used for sci-
entific research, and the missions typically have a cadre of
scientists involved. For example, the Lunar Reconnais-
sance Orbiter has the primary goal of obtaining data
necessary for the eventual return of humans to the
Moon, but these data are of high value for science as well.
For strategic and supporting missions, scientists can
propose to be the PI for an instrument or suite of instru-
ments as part of the payload. The selected PI forms the
science team, designs the instrument, has it built, and
implements the experiments through operation of the
instrument and collection of the data. In some cases,
facility instruments are provided directly by NASA and
scientists can propose to be a member or team leader of
that instrument science team; the team is then responsible
for carrying out the investigation.
The European Space Agency (ESA) also flys planetary
missions, but operates differently from NASA. The ESA
is composed of 17 member nations and is headquartered in
Paris, with its primary operations center in the
Netherlands. Once a mission has been selected for flight,
the ESA develops and builds the spacecraft and is respon-
sible for its operation. The scientific payload, however, is
competed for among the member nations through their
science communities; if selected, that nation is responsible
for funding and delivering the instrument or suite of
instruments to the ESA.
Operation of an active flight project is exciting and
complicated! After launch, the mission goes through
cruise (the journey from Earth to its destination), nominal
operations (at the target for the duration approved in the
budget), and, if all goes well, an extended mission (a
specific period of time following the nominal mission
and budgeted separately). During cruise, the instruments
are typically turned on briefly for calibration and check-
out before arrival at the target; otherwise they are either in
a dormant state or turned off. During planetary operations,
the data needed to meet the objectives of the mission are
obtained and returned to Earth through the Deep Space
Network (DSN), which consists of large antennas located
at Goldstone (southern California), Madrid (Spain), and
Canberra (Australia). This distribution enables complete
coverage of spacecraft, regardless of the time of day or the
position of the spacecraft in the Solar System.
Science operation of a spacecraft involves fundamen-
tally two aspects: sending commands to the spacecraft
(called uplink) and receiving the data from the spacecraft
(called downlink), both through the DSN. Putting the
plan together for the uplink involves integrating the
desires of all the instrument teams to fit the power, on-
board computer processing, and other resources of the
spacecraft. As one might imagine, there are often compet-
ing wishes for these resources among the scientists, and
compromises almost always are required for the final plan.
After each instrument has sent its commands, data are
downlinked and, again, there is often competition for
downlink resources. Modern instruments generally can
take far more data than can be returned, and decisions
must be made to satisfy the overall mission objectives.
1.5 Planetary data
As soon as a successful mission goes into operation, the
science flight team plans the acquisition of data (such as
targeting areas to be imaged), collects the data, and ini-
tiates their analysis. Some of these data are posted on the
website for that particular mission (go to the general
NASA website http://www.nasa.gov/, or the ESA website
http://www.esa.int/, and look for the specific mission by
name). These data are for general public interest and often
have not been calibrated or verified for accuracy. It is
considered “bad form” for the science community to pub-
lish results from such data before they are officially
released for scientific analysis. Such release is done on a
project schedule after validation by the science team,
posted in the Planetary Data System (PDS, http://pds.jpl.
nasa.gov/), and publicly announced. Because of the pace
of mission operations, the volume of data from modern
missions, the complexity of the data, and the possibility of
Introduction 12

errors in the data stream, releases typically occur no earlier
than about six months from their acquisition. Once
released, the NASA data are available to everyone.
Following (or during) a mission or set of missions,
NASA will organize a Scientific Data Analysis Program.
These programs provide funds to support the analysis of
data by the community through open competition and peer
review. Such programs are usually of a limited duration,
such as three years. In addition, each scientific discipline,
such as the Planetary Geology and Geophysics Program at
NASA, has funds for basic research, including geologic
mapping, laboratory studies, and integrated data analysis.
These, too, are open through competition and peer review.
The NASA Research Opportunities (ROSES; http://
nspires.nasaprs.com/external/) posts the procedures and
schedules for proposing for these and other opportunities
from NASA.
1.6 Planetary research results
Knowledge of the Solar System is expanding rapidly and
is enabled primarily by data returned from spacecraft
missions. Even for planetary scientists, it is often difficult
to keep up with the advances in exploration. Typically, the
first results from flight projects are announced through
press releases from NASA or the space agency responsi-
ble for the mission. While the releases are generally pre-
pared by the project science team, many of the ideas
presented are not very mature. The next stage is the oral
presentation of results at scientific meetings. By this time,
the results and the ideas have been more widely discussed
within the science teams and have been somewhat refined.
Although abstracts (short summaries of the content) of the
presentations are published for the meeting, the abstracts
are usually submitted months before the actual meeting;
with active flight projects, the abstracts that are submitted
are often simply placeholders and might not have much
real content, unlike the oral presentation itself.
Key scientific meetings for planetary science are the
Lunar and Planetary Science Conference (LPSC), held
every March in Houston, the American Geophysical Union
(AGU) meeting held each fall in San Francisco, the Division
of Planetary Science (DPS) meeting of the American
Astronomical Society held each fall, the European
Geosciences Union (EGU) meeting and the Europlanet
meeting held in Europe, the Geological Society of America
(GSA) fall meeting, and the Meteoritical Society meeting
held each fall. These meetings all publish abstracts of the
presentations, which are usually available on-line from the
sponsoring scientific organizations. The AGU, GSA, and
EGU meetings are very large and include a wide variety of
subjects in addition to planetary science.
Most large scientific meetings are attended by profes-
sional science writers who are very skilled in extracting
new and exciting results. Their articles are then published
in venues such as Science News, Space News, and The
Planetary Report.
Traditionally, the first papers from flight projects are
published in Science or Nature, often as special sections or
editions of the journal. These papers are “peer reviewed,”
meaning that scientists not involved with the project but
who are knowledgeable of the field have reviewed and
evaluated the results.
The first full papers from planetary missions are typi-
cally published a year or two after data acquisition. By this
time, the ideas have matured and the manuscripts have
been rigorously peer-reviewed. Key journals include
Icarus, the Journal of Geophysical Research – Planets
(an AGU publication), and Planetary and Space Science.
Additional sources of planetary information are speci-
alized topical meetings. These range in size from small
workshops involving a dozen or so people to international
conferences attended by hundreds of participants. Topics
can range from the latest results from a large flight project
to highly specialized research topics. In most cases,
abstracts of papers are available at the meeting and full
peer-reviewed papers are published in journals or as a
special conference book.
Planetary science series of books published by organ-
izations such as the University of Arizona Press and
Cambridge University Press contain collections of review
papers, with most individual volumes focusing on specific
planetary objects. These books typically follow interna-
tional meetings that are organized to synthesize new, as
well as mature, results from spacecraft missions and gen-
eral investigations.
While this outline has focused on results from new
planetary flight projects, the venues listed are also where
results from active planetary research projects can be
found. As noted throughout the text, various key websites
are identified for sources of information. These and related
websites relevant for planetary exploration and data are
listed in Appendix 1.1 at the end of this book, and can
also serve as “spring boards” for additional websites. An
example is the Java Mission-planning and Analysis for
Remote Sensing (JMARS) website (http://jmars.asu.edu)
for a geospatial information system (GIS) that enables
1.6 Planetary research results 13

rapid searches and provides analytical tools for planetary
data. For example, a user can construct maps that combine
images, topographic information, and multispectral data
for areas and scales of the user’s choice. Currently,
JMARS principally covers Mars and is being adapted for
the Moon and Earth. Public downloads are readily acces-
sible through the website. In addition, NASA maintains a
network of Regional Planetary Image Facilities that have
sets of images available for viewing and staff who can
answer specific questions; Appendix 1.2 lists these facili-
ties and their locations.
In summary, the exploration of the Solar System affords
a great opportunity to study geology and geomorphology in
a wide variety of settings and over time scales from the
earliest formation of planetary crusts to geologically active
planets. As shown in Fig. 1.12, there is an enormous
potential for such studies when considering the total surface
areas of planets and satellites amenable for geology.
Figure 1.12. The surface areas of the
rocky planets, the Moon, and the
larger outer planet satellites as a
function of Earth’s land surface area
(Earth = 1); note that the surface area
of Mars is just about equal to the
surface of Earth not covered by
water, while the surface area of
Venus is nearly 3.5 times that of
Earth’s land surface.
Assignments
1. Briefly explain how the analysis of geologic features
on planetary surfaces is relevant to the search for life
beyond Earth.
2. Go to the website for Science News and summarize a
scientific result from a currently operating spacecraft
that is relevant to planetary geology.
3. Compare and contrast the types of planetary data
returned from an orbiting spacecraft and a landed
spacecraft.
4. Go to the websites for planetary missions currently
being conducted by spacefaring nations and agencies,
such as the ESA, and identify one spacecraft each for
an inner planet, a gas giant planet, and a small body
(such as an asteroid). List the launch dates, dates of
operations at the “target” planetary body, and one or
two key results relevant to planetary geology for each
of the three spacecraft identified.
5. Examine the tables for the primary characteristics of
the planets and satellites. Give one example of how the
environment for a terrestrial planet of your choice
would influence the geology in comparison with an
icy satellite of your choice.
6. Go to the website for Space News and summarize one
budget issue for the current year that has an impact on
planetary exploration.
Introduction 14

CHAPTER 2
Planetary geomorphology methods
2.1 Introduction
For many years, the study of the geomorphology of the
Earth was primarily descriptive. In the middle of the
twentieth century, the emphasis shifted to a more process-
oriented approach, with the goal of understanding the
reasons behind a landform’s appearance. The analysis of
planetary surfaces has gone through a similar history.
When the first close-up images of the Moon and planets
were obtained, their surfaces were described, and some
attempts were made to interpret their origin and evolution.
Unfortunately, some of these attempts were rather imma-
ture. Planetary scientists with a geology background drew
on their experiences with Earth, taking a simplified “ana-
log” approach; i.e., if it looks like a volcanic crater, it must
be of volcanic origin. Scaling the sizes of features and
considerations of planetary environments took a back seat
to the simple “look alike” answer.
As the Apollo program drew to a close in the early 1970s
and the exploration of the full Solar System emerged, plan-
etary geomorphology became more process-oriented, with
attempts to take differences in planetary environments into
account, while maintaining fundamental geologic principles.
In this chapter, the following question will be addressed:
how can one study the geology of a planet or satellite without
actually going there? This will include the approaches used
in planetary geomorphology and the types of data that are
commonly available for the study of planetary surfaces.
2.2 Approach
The general approach in planetary geomorphology involves
three elements: (a) analysis of spacecraft data, (b) laboratory
and computer simulations of key geologic processes in
different planetary environments, and (c) the study of ter-
restrial analogs. Each element has its advantages and
disadvantages but collectively provides a powerful means
to decipher present and past planetary surface histories.
The starting point is the analysis of planetary data,
typically in the form of images. From these studies, the
overall terrains and varieties of landforms are identified
and characterized. Various hypotheses are proposed to
explain the possible formation and evolution of the land-
forms observed. With further study and new data, the
number of hypotheses can be reduced, or new ideas
emerge. The history of the study of craters on the Moon
is a good case to review. Beginning with telescopic views,
the origin of lunar craters was debated for centuries, lead-
ing to the time of the Space Age. Even with the return of
data from spacecraft sent to the Moon in preparation for
the Apollo program, there were two primary competing
ideas for craters, impact versus volcanic origins. Images
of lunar craters showed features that were used to support
both ideas. While the characteristics of volcanic craters on
Earth were fairly well understood, little thought had been
given to extrapolation of volcanic processes to the low-
gravity, airless environment of the Moon. In the early days
of the Space Age, impact cratering as a process was little
appreciated in the geologic context, and there was no
understanding of the physics of the process. At about the
same time as robotic missions were returning new, close-
up data for the Moon, experiments to study the physics of
impact events were initiated. Although similar work had
been conducted for decades by the military to understand
how projectiles could penetrate armor, much of this work
was classified; moreover, the work was more applicable to
man-made targets than to natural, rocky material. It is
interesting to note that in the late 1880s the American
geologist G. K. Gilbert dropped small cannon balls into
mud targets (Fig. 2.1) to see what might happen. Gilbert
was very interested in the origin of lunar craters, and
proposed that the Imbrium feature on the Moon was the
result of an impact, as discussed in Chapter 4.
15

Building on the work of Gilbert, Don Gault at NASA-
Ames Research Center constructed in the early 1960s a
facility for conducting sophisticated impact experiments
using a hydrogen gas-gun (Fig. 2.2). This gun can fire
projectiles at velocities as high as 7.5 km/s into a target
contained in a vacuum chamber to simulate the Moon. In
some experiments, the target is placed on a platform that
can be dropped at the time of the impact to simulate
reduced gravity conditions (much like the “weightless”
feeling when an elevator descends rapidly). Results from
this facility enabled the fundamental physics of impacts to
be derived and provided critical insight into the geologic
aspects of impact cratering (Fig. 2.3).
At about the same time as impact experiments were
being conducted, Gene Shoemaker of the US Geological
Survey (USGS) was synthesizing results of his field stud-
ies of Meteor Crater in Arizona (Fig. 2.4), which included
assessments of rock structure and deformation, including
the presence of overturned stratigraphy in the crater rim,
which was so well seen in the Gault experiments
(Fig. 2.3). Subsequently, field sampling at Meteor Crater
led to the discovery of coesite and stishovite, the high-
pressure forms of quartz that are formed by impact pro-
cesses. Concurrently, other geologists were scouring
remote sensing data to identify possible impact structures
on Earth (Fig. 2.5), followed by field investigations.
The mid 1960s also saw investigations of nuclear
explosion craters at the Nevada test site and their study
by Hank Moore of the USGS for comparisons with
Figure 2.1. Geologist G. K. Gilbert
performed experiments in the late
1880s to simulate impact processes,
shown in this photograph of small
cannon balls that were dropped into a
target of stiff mud (courtesy of the US
Geological Survey).
Figure 2.2. The Vertical Gun at NASA-Ames Research Center
consists of an “A” frame mounted with a gun barrel that uses
compressed gasses to launch small projectiles at velocities as high as
7.5 km/s into a vacuum chamber tank to simulate impact cratering
processes. The “A” frame can be rotated so that the gun can fire
projectiles at different impact angles from near-horizontal (seen
here) to vertical.
Planetary geomorphology methods 16

features seen on the Moon. Computer codes were also
being developed for large explosions, which would be
applied later to planetary impact processes.
Through this combination of laboratory experiments,
analysis of remote sensing data, and field studies, a gen-
eral model of impact processes emerged that could be
applied successfully to the interpretation of planetary
data. The study of impact craters set the stage for the
approach used in investigations of other geomorphic pro-
cesses, such as aeolian activity and volcanism.
2.3 Planetary geologic maps
Geologic maps represent a fundamental tool for charac-
terizing the geology and geomorphology of an area and
deciphering its history. The British planetary geologist
John Guest once said “a geological map is (to a geolo-
gist) like a graph to a physicist; it allows an understand-
ing of many observations in a comprehensive form that
would be otherwise difficult.” The basic elements of a
geologic map show the distribution of three-dimensional
rock units (Fig. 2.6), the configuration of the rock units
exposed on the surface of the area mapped, structural
features, such as faults, and the ages of the rocks and
structural features. As is true for all maps, geologic maps
include a scale, orientation (e.g., a north arrow), a legend
explaining the symbols on the map, and the location
of the map area (typically indicated by geographic
coordinates).
The formation is the basic rock unit in mapping.
Formations consist of material of similar rocks, all formed
at the same time, in the same place, and by the same
process. For example, a lava flow resulting from a single
eruption in Hawaii could be treated as a formation that
would be different from a lava flow erupted from Mount
Etna in Sicily at the same time, even though both might be
of the same type of rock.
Some formations can be subdivided into members. For
example, during a given eruption sequence, a lava flow
might be covered by ash from an explosion; the lava
flow and the ash could be called members of the same
formation. Two or more formations that share common
Figure 2.3. Cross-section of a target produced in the Ames Vertical
Gun Range (Fig. 2.2). The target consisted of layers of loose sand grains
dyed different colors mixed with epoxy resin. After the “shot,” the
target was baked to fuse the grains and epoxy resin, and then sawn
into a cross-section. Shown here is the inverted stratigraphy in the
crater rim (the “overturned flap” of ejecta), which is characteristic of
impact craters. The crater is about 0.4 m across.
Figure 2.4. An aerial view of Meteor Crater in northern Arizona, the
best-preserved impact structure on Earth. This crater 1.2 km in
diameter was formed by a 30 m iron meteoroid some 50,000 years
ago (courtesy of Mike Malin).
Figure 2.5. A radar image (C-band) taken on NASA’s Shuttle Radar
Topography Mission of the Manicouagan crater, Quebec, Canada;
this impact structure of diameter 100 km formed about 214 million
years ago. Erosion (mostly by glaciation) has removed about 1 km
of rock from its original surface, exposing the deep structure,
including the “root” of the central uplift, which is now surrounded
by Manicouagan reservoir, seen here in dark gray (NASA
PIA03385).

attributes in a given area are sometimes combined into a
group. For example, a series of lava eruptions and ash
flows mapped as formations and members on a volcano
could be defined as a stratigraphic group.
Formations can be more complicated than the simple
case of the lava flow. Imagine a lake basin that receives
run-off and sediments from the surrounding mountains.
As the streams empty into the lake, there is a decrease in
the speed of the water flow and, hence, a decrease in their
ability to carry sediments. Coarser materials, such as
boulders, would be deposited close to the shore, with
progressively smaller rocks and sediments being depos-
ited outward from the shore. Over time, the sediments in
this sequence continue to accumulate and can eventually
be lithified (turned into rock). If one were to see only the
part formed close to shore, the rock would be a conglom-
erate (a rock composed of large, rounded rock fragments);
farther from shore, the rock would be sandstone (rock
composed of sand-size particles); still farther from the
shore, the rock would be shale (rock composed of very
fine grains, such as clay). Thus, three different rocks are
found, and each could be treated as a different formation;
however, the boundaries between these different rocks
would be gradational (smaller-size materials away from
shore) and would represent the local environment of for-
mation (i.e., progressively less energy to carry the sedi-
ments away from the shoreline). In cases such as this, the
conglomerate, sandstone, and shale would be called facies
(parts) of the same basic formation.
Structural attributes of rock units are indicated by var-
ious map symbols to show faults, folds, and the “attitude”
of the rock units. Attitude refers to orientation, such as
horizontal, vertical, or tilted.
Geologic maps also indicate the ages of the rock units
and the timing of deformation. These relations are por-
trayed as a stratigraphic column, in which the oldest
materials are at the bottom of the sequence and the young-
est are at the top. In geology, time is usually indicated
from the bottom to the top, to reflect the principle of
superposition. This principle states that in any sequence
Figure 2.6. Geologic maps
(top) show three-dimensional
rock units (bottom cross-
section), such as limestone,
sandstone, and intrusive
rocks, and structural features
such as faults. The relative
ages of the units and
structural features are
commonly indicated in a
stratigraphic column
organized from youngest
(top of column) to oldest
(bottom of column).

of undisturbed rocks those on the bottom must be the
oldest because they had to be present before the subse-
quent rocks could be put on them.
Geologic mapping on Earth began in the eighteenth
century. In the ensuing years, stratigraphic columns
reflecting local sequences of rocks have been defined for
most regions of our planet and have been correlated
(connected) into regional and global associations. From
this synthesis, a generalized geologic time scale that
divides the history of the Earth into formal eras, periods,
and epochs was defined. Early versions of the geologic
time scale indicated only the relative ages of rocks, and it
was not until dating methods based primarily on radio-
active decay were developed that absolute ages could be
assigned, as will be discussed below.
As for the Earth, planetary geologic maps are critical for
understanding surface histories and for providing a frame-
work for other observations. This understanding was rec-
ognized very early in Solar System exploration, and the
first planetary geologic maps were compiled by the US
Geological Survey (USGS) for the Moon by Robert
Hackman and Gene Shoemaker, from telescopic observa-
tions (Fig. 2.7). Their techniques were later codified and
standardized for planetary mapping by Don Wilhelms and
Ken Tanaka, also of the USGS.
Geologic maps of Earth are commonly assembled from a
combination of remote sensing data and field work, all
combined on standard maps, which usually include topog-
raphy. The identification of formations and other rock units
on planets is based primarily on remote sensing data using
photogeological techniques that are commonly employed
for Earth. Images enable obvious features, such as lava
flows, to be identified, while the general appearance of
terrains is used to infer different rock units. Compositional
mapping on the basis of infrared and other data is also used
to distinguish units, when such data are available. Further
insight is provided by quantitative studies of albedo
(a measure of the reflectivity of the surface) and surface
textures at the sub-meter scale derived from radar signa-
tures. Unfortunately, a full suite of remote sensing data is
seldom available for planetary surfaces.
After the rock units and structures have been identified
and their distributions mapped, the next step is to place
them in a chronological sequence. In addition to super-
position, embayment and cross-cutting relations are
used to determine the relative ages among units. For
example, embayment refers to the “flooding” aspect of
some units, as seen on the Moon (Fig. 2.8), in which the
“flooding” unit is the younger. For the application of
cross-cutting relations, a rock unit, fault, or other structure
that cuts across another must be the younger, as shown in
Fig. 2.9.
Planetary geologic mapping is hampered by a lack of
field observations except for those at the local Apollo sites
and a handful of “ground-truth” sites gained from robotic
landers. Consequently, planetary geologic maps are for-
matted a little differently in comparison with those for
Earth. On planetary maps the rock unit descriptions are
divided into two parts, observations and interpretations.
The observation part describes the characteristics of the
unit in objective terms on the basis of the available data,
while the interpretation part explains the possible origin
and evolution of the unit according to the opinion of the
author. In principle, the observation part should remain
Figure 2.7. Part of the geologic map of the Kepler region on the
Moon, published in Geotimes in 1962. This was one of the early
prototype maps to show that geologic maps could be produced for
planetary surfaces (reprinted with permission from Geotimes
magazine, a publication of the American Geological Institute).

valid (at least until new data are available), but the inter-
pretation part could change or be different, depending on
the analyst.
The first planetary geologic maps by Hackman and
Shoemaker set the stage for a series of mapping programs
that extended to Mars, Mercury, Venus, and outer planet
satellites and that continues today through the USGS. In
these programs, each planet is divided into map quadran-
gles of systematic scales, which serve as cartographic
bases for geologic mapping.
2.4 Geologic time
Geologic time can be considered from two perspectives,
absolute time and relative time. In absolute time, rocks
and geologic events, such as faulting, are determined as
being of a specific age expressed in years. In relative geo-
logic time, rocks and events are simply stated as being older
or younger than other rocks or events, without expressing
their age differences in years. Determining the absolute
ages of rocks is accomplished primarily by using radiogenic
“clocks” that are based on the principle that certain unstable
radioactive elements (i.e., isotopes) contained in some
rocks decay or convert to more stable isotopes at a known
rate. If we know this rate and can measure the amounts of
unstable and stable isotopes in a sample, it is possible to
determine the age of the rock. Of course, radioactive iso-
topes of the right type must be available for measurement in
the rock sample and, unfortunately, not all samples contain
these isotopes. Consequently, only some rocks can be
dated. Typically, radiogenic dating provides the age of a
rock in years from its formation.
How is geologic time assessed on other planets and satel-
lites? Practical limitations require that rock samples be ana-
lyzed in laboratories on Earth to determine radiometric ages
because automated dating systems for robotic spacecraft
have not yet been developed. Consequently, planetary abso-
lute dates have been obtained only for rock samples from the
Moon and for meteorites, some of which are from Mars.
Because no direct samples are available from other
planets, only relative ages can be assigned with confi-
dence. The principles of superposition, embayment, and
cross-cutting relations are routinely applied to planet and
satellite surfaces for this purpose.
An additional method for establishing the relative ages
of planetary surfaces is based on the size–frequency dis-
tribution of impact craters. Old surfaces have been
exposed to the impact environment for longer than have
younger surfaces and statistically should contain more
impact craters (Fig. 2.8). By counting the number of
craters superposed on planetary surfaces, their age relative
Figure 2.8. View of the Taurus–Littrow region of the Moon, showing
flooding by dark mare lavas into the more heavily cratered highlands
on the right. The arrow points to a lava-“embayed” crater. The paucity
of cratersin themare (left side)indicates the relative youth ofthelavain
comparison with the highlands. The star indicates the location of the
Apollo 17 landing site. The darker part of the mare surface marks the
presence of dark mantle deposits of volcanic origin (NASA AS17–0939).
Figure 2.9. A view of Rima Ariadaeus, taken by the Apollo 10
astronauts. This linear rille, formed by faulting, is about 2 km wide
and cuts across older terrain (NASA AS10–4646).

to other surfaces can be determined. This concept was
developed for the Moon and was verified when rock
samples were returned to Earth for analysis and radiogenic
dating (Fig. 2.10). Research by NASA planetologist Don
Gault showed that, with time, cratered surfaces reach a
stage, called equilibrium, in which craters of a given size
are obliterated by impact erosion at the same rate as they
are formed, as shown in Figs. 2.11 and 2.12. Thus, only
surfaces that have not yet reached equilibrium for the
crater sizes being considered can be dated (Fig. 2.13).
In practice, difficulties can arise in using crater statistics
for age determinations. For example, non-impact craters,
such as those formed by volcanic processes, might be
indistinguishable from impact craters, and, if non-impact
craters were present, the surface would appear anoma-
lously old. In addition, secondary craters are formed by
the impact of rocks ejected from primary impact craters.
Their presence adds to the total crater population and must
be taken into account by various models that predict how
many secondary craters would form as a function of the
primary crater size. Unfortunately, such models are imper-
fect, and it is difficult to determine the presence and
Figure 2.10. Number of craters larger than 4 km per unit surface area
versus the age in gigayears (Ga), calibrated against absolute dates
obtained from lunar samples. This curve enables extrapolation of
ages to surfaces lacking samples on the basis of crater counts (from
Spudis, 1996, after Heiken et al., 1991; reprinted with permission
from Smithsonian Institution Press).
Figure 2.11. Photographs showing a NASA experiment (called Mare Exemplum) to simulate the evolution of a cratered surface. In this experiment, a
box 3.2m by 3.2m was filled with loose sand, smoothed (upper left), and then impacted with bullets of different sizes (ranging from birdshot to high-
powered rifle). Placement of each shot followed a grid system and a random-number generator; the ratio of differently sized impacts was based on
the size–frequency distribution of craters seen on lunar mare surfaces. The end of the series is in the lower right. The series illustrates how crater
counts can be used to date surfaces; surfaces in the top photographs represent younger surfaces in comparison with those in subsequent
photographs. Note, however, that in the last row of photographs the crater size frequencies are essentially the same, representing cratering
equilibrium, in which craters are being destroyed at the same rate as that of crater formation; thus, it is not possible to date surfaces within the last
row of photographs (courtesy of Don Gault).
2.4 Geologic time 21

number of secondary craters. Other considerations include
target properties that could cause variations in crater sizes.
For example, experiments show that craters formed in
targets containing fluids are larger than craters formed
in dry targets. This difference could cause the size–
frequency distribution for the volatile-containing target
to be interpreted as representing an older surface.
Despite these difficulties and uncertainties, impact crater
statistics are commonly used as a means for obtaining rela-
tive ages for different planetary surfaces. Ages derived from
crater counts have been compared with (and calibrated
against) radiometric dates obtained from lunar samples and
demonstrate the validity of the technique, at least on the
Moon, where surface-modifying processes are minimal.
This result suggests that crater counts can be used to obtain
dates for surfaces on other “airless” bodies, such as Mercury.
Great caution must be exercised, however, in using crater
counts on planets where differences in erosion might occur
as a function of location, or where significant differences in
target properties may alter the crater morphology.
In principle, crater counts can also be used to derive
absolute ages for planetary surfaces, as discussed by
Michael and Neukum (2010). This has been done with
some confidence on the Moon where cratered surfaces
have been sampled and radiometric ages determined
(Neukum et al., 2001); extrapolation of the calibrated crater
curve (Fig. 2.10) to surfaces that have not been sampled
enables estimates of their ages. The same can be done for
surfaces on other planets by extrapolation of the calibrated
lunar crater counts. This requires, however, that correct
adjustments can be made for gravity (which influences the
sizes of craters), impact flux as a function of location in the
Solar System (proximity to the asteroid belt, as with Mars,
which experiences a higher impact rate than that on the
Moon), and the potential for degradation in the presence of
an atmosphere, as on Venus. These and other factors result
in complex algorithms for extrapolation, and potentially
large error bars on the results, depending on the assump-
tions and uncertainties in the age calculations.
2.5 Remote sensing data
Most of our knowledge of the geomorphology of Solar
System objects is derived from remote sensing, defined as
the collection of information without coming into physical
contact with the object of study. This is accomplished by
designing instruments that can be carried on some “plat-
form” to collect useful information. Typically, platforms
on Earth include airplanes and spacecraft, but can also
include balloons, helicopters, or robots operating on the
surface. To the extent possible, similar platforms are used
Figure 2.12. An experiment similar to that shown in Fig. 2.11 to
illustrate stages in crater modification by impact degradation. The
arrows in the first and last images point to a crater that is subjected
to small impacts, which gradually wear down the crater rim and fill in
the crater floor until it is nearly “erased” from the surface (NASA-
Ames photograph AAA481–8, courtesy Don Gault).
Figure 2.13. A diagram of idealized crater size–frequency
distributions (number of craters per unit area versus crater diameter)
for three surfaces; surface 1 is the oldest, reflected by its having the
greatest number of craters and craters of largest sizes. The steep
parts of all three curves represent crater production, while the less
steep parts represent crater equilibrium. Relative age-dates can be
determined either by the “break point” between production and
equilibrium (in which progressively older surfaces have their break
point at larger crater size), or by the position of the production curve,
which shifts to the right with increasing age.

in Solar System exploration, with most information com-
ing from spacecraft located well above the surface.
Remote sensing instruments make use of electromag-
netic (EM) radiation (Fig. 2.14), which is generated
whenever there is a change in the size or direction of an
electrical or magnetic field. For example, electrons shift-
ing from one orbit to another orbit around an atomic
nucleus results in X-rays and visible (light) radiation,
while fluctuations in the electrical/magnetic field generate
microwaves and radio waves of the sort used in radar
systems. Remote sensing instruments are designed to
detect specific parts of the EM spectrum. The energy
detected depends on the interaction of the energy with
the surface being analyzed, generating spectra that are
distinctive for various properties of the surface. For exam-
ple, sunlight shining on a planetary surface can be
reflected, absorbed, and/or transmitted, depending on the
EM wavelength, the temperature, and characteristics of
the surface materials, such as composition and grain size.
Remote sensing systems are classified as either passive
or active. Passive system use natural radiation, such as
sunlight, whereas active systems illuminate the surface
with an artificial energy source. For example, radar imag-
ing systems beam energy toward a surface, some of which
is reflected and then recorded by a radar sensor. Active
systems also include non-imaging systems, such as laser
and radar altimeters, which measure the distance from the
instrument to the surface.
2.5.1 Visible imaging data
Nearly every spacecraft sent in the exploration of the Solar
System has carried a camera or imaging system as part of
its scientific payload. Currently, most imaging systems
use charge-coupled devices (CCDs) as the detector,
rather than film. However, in planetary geomorphology,
images from previous missions are still useful, and it is
important to be familiar with the imaging systems used in
these missions, as described in Appendix 2.1.
CCDs were invented in 1969 by Bell Laboratories and
are used in a variety of solid-state imaging devices. Today,
modern digital cameras all use CCD technology, including
simple cell-phone cameras and sophisticated video systems.
A CCD “chip” consists of a layer of metallic electrodes and
a layer of silicon crystals, separated by an insulating layer of
silicon dioxide. When used as an imaging system, the CCD
chip is structured as an array of picture elements, or pixels.
Light focused onto the chip by a lens causes a pattern of
electrical charges to be created. The charge on each pixel is
proportional to the amount of light received and provides an
accurate representation of the scene. Each charge can be
transmitted separately and then reconstructed using conven-
tional image-processing techniques.
CCD imaging systems can be either line arrays or two-
dimensional arrays. In line arrays, a single line of CCDs
sweeps across the scene as the spacecraft (or aircraft)
moves over the terrain, building up the image. Two-
dimensional arrays consist of a chip with CCDs on an
X–Y coordinate system and are used as framing cameras in
which the CCDs record the scene as a “snapshot.”
Figure 2.14. The electromagnetic (EM) spectrum, showing
important wavelengths used in remote sensing. Radar systems are
subdivided and given letter designations that were arbitrarily
assigned (see Table 2.1).
2.5 Remote sensing data 23

In simple systems, images are produced in a single
wavelength range as a “black and white” (and shades of
gray) picture. The wavelength range can be either narrow
(as in the near-infrared) or “broadband” to produce a
panchromatic picture. Color images are produced by
obtaining data for more than one wavelength over the
same scene. For example, three frames could be exposed,
one each through red, blue, and green filters. Each filter is
sensitive to a given wavelength and the data correspond-
ing to that wavelength are recorded. The three frames are
then combined to produce a color image.
2.5.2 Multispectral data
Mapping surface compositions using remote sensing tech-
niques is critical in planetary science. The crystal structure
of minerals behaves in characteristic ways when exposed
to EM energy. In the visible–near-infrared (Fig. 2.14), or
VNIR, crystals absorb energy, resulting in absorption
bands that are diagnostic for specific minerals or groups
of minerals. To take advantage of this phenomenon, mul-
tispectral spectrometers are designed to measure the
reflected energy as a function of wavelength using filters
and detectors that are responsive to the absorption bands
of interest. Generally, the narrower the band widths cover-
ing the VNIR spectrum, the more precise the analysis. For
example, the Near-Infrared Mapping Spectrometer
(NIMS) which flew on the Galileo mission to Jupiter
could develop a 408-wavelength spectrum for each pixel
over the range from 0.7 to 5.2 microns.
Multispectral spectrometers can be either “point” sys-
tems or mapping systems. In point systems, a single line is
traced across a surface, measuring the spectra as a com-
positional profile over terrains. The advantage of this
approach is that the instrument is relatively simple and
the total data volume is small. The disadvantage is that
sampling of the surface is very limited and important areas
might be missed. Mapping spectrometers have two-
dimensional arrays of detectors that collect data over an
area rather than as a line-trace. Mapping spectrometers are
much more useful for geologic studies.
2.5.3 Thermal data
Surface materials radiate, or emit, energy (“heat”) in the
0.5–300 micron range of the EM spectrum, which can be
recorded as digital files or transformed into images.
Detectors that can measure this energy (sensitive thermom-
eters known as bolometers) were developed by the military.
For example, such a detector flown on an aircraft over terrain
at night can pick up the heat generated by vehicle engines or
even body heat from individuals. In the mid 1960s, some
thermal detectors were declassified for civilian use, leading
to remote sensing systems for use on Earth and in planetary
exploration. The Thermal Emission Spectrometer (TES),
developed by Phil Christensen of Arizona State University
and flown on the Mars Global Surveyor spacecraft, revolu-
tionized our understanding of the surface of Mars. The
energy recorded by a thermal detector, such as the TES, is
a function of many complex variables, including surface
composition and texture, the atmosphere between the surface
and the detector, and the detector sensitivity. Understanding
the physics of the transfer of energy from the surface to the
recorder enables the determinations of factors such as the
mineralogy and grain sizes of surface materials.
2.5.4 Radar imaging data
Dense clouds obscure two objects of planetary geologic
interest, Venus and Titan, a moon of Saturn. Because of its
long wavelength (Fig. 2.14), radar can “see through”
clouds and has been used to map both Venus and Titan
using synthetic aperture radar (SAR) imaging systems.
Radar is an active remote sensing technique in that the
energy is generated artificially, in effect illuminating the
scene to be imaged. SAR systems are mounted on a mov-
ing platform (an airplane or spacecraft) and send short
pulses of radio energy obliquely toward the side, striking
the surface at an angle. Some of the energy is reflected
back to the spacecraft, where it is received as an echo, by
which time the motion of the spacecraft has carried it to a
new position. Several thousand pulses are sent per second,
at the speed of light, resulting in an enormous data set that
must be highly processed. To construct an image, three
factors must be integrated: (1) the round-trip time from the
instrument to the surface, (2) the Doppler shift due to the
motion of the spacecraft, and (3) the radar reflectivity of
the surface, which is a function of composition, surface
roughness, and other factors.
Radar images (Fig. 2.5) can be confused with images in
the visible part of the EM spectrum, and there are signifi-
cant differences that must be understood for proper inter-
pretation. First, the brightness and apparent shadows in a
radar image do not result from sunlight, but from “illumi-
nation” by the radar beam. Thus, the geometry of the
bright and dark terrains depends on the position of the
spacecraft and characteristics of the surface such as topo-
graphy and composition.

Different parts of the radio wavelength EM spectrum
are used in SAR imaging, designated with letters of the
alphabet (Table 2.1). These were defined mostly during
the Second World War as classified information, and the
letters were arbitrarily assigned. In addition, radar data are
typically polarized, by which means the waveform ener-
gies both in the “send” and in the “receive” phases are
filtered into either a horizontal plane or a vertical plane.
Thus, the data can occur in one of four combinations: (a)
HH for horizontal send, horizontal receive; (b) VV for
vertical send, vertical receive; (c) HV for horizontal send,
vertical receive, or (d) VH for vertical send, horizontal
receive. Each mode has advantages and disadvantages
depending on the application and the nature of the surface.
The Soviet Venera 15 and 16 and the US Magellan
missions carried radar imaging systems to Venus to obtain
the first detailed views of the surface from orbit. NASA’s
Cassini orbiter, sent to Saturn, carried a radar system to
obtain images of the moon Titan.
Very-long-wavelength–low-frequency radar systems
are capable of penetrating into the subsurface, depending
on the nature of the surface materials. In general, some of
the radar energy is reflected from subsurface boundaries,
such as contacts between rock units, and is recorded; from
the geometry of the received signal, the depth to the
boundary can be calculated. Instruments using this prin-
ciple were used on the Moon during Apollo and were
flown on the European Space Agency Mars Express mis-
sion and on NASA’s Mars Reconnaissance Orbiter.
Penetrating radar is likely to fly on missions to the outer
Solar System as a means of investigating the ice structure
of some satellites, such as Jupiter’s moon Europa.
2.5.5 Ultraviolet, X-ray, and gamma-ray data
At very short wavelengths (Fig. 2.14), EM energy is
strongly influenced by gasses, and the ultraviolet (UV)
part of the EM spectrum is used to study planetary
atmospheres and surfaces that lack atmospheres. UV
spectra provide useful information on some of the phys-
ical properties of the surface, such as grain size and the
presence of frost. X-ray spectrometers detect energy
generated by the Sun, in which the X-ray spectra are
diagnostic for elements such as aluminum, silicon, and
magnesium. Gamma rays are produced from radioactive
decay and from the bombardment of surfaces by cosmic
rays from deep space. Gamma-ray spectrometers meas-
ure this energy and can be used to map the distributions
of some elements, such as titanium. For example, X-ray
and gamma-ray spectrometers were flown on Apollos 15
and 16 to map parts of the Moon.
2.6 Geophysical data
Various techniques are employed to obtain geophysical
data relevant for planetary geomorphology, including the
use of instruments to measure altimetry, gravity, and mag-
netic fields. Topographic data are fundamental for most
studies and are derived from images (see Section 2.10) or
from altimeters that measure the distance from the space-
craft to the ground. For example, topographic data from
the Mars Orbiter Laser Altimeter (MOLA) are commonly
used to make images that have the appearance of photo-
graphs in which the illumination direction and angle, as
well as the vertical exaggeration, can be controlled.
Both radar systems and lasers have been used in plan-
etary exploration for altimetry, in which the time between
energy emission from the spacecraft and energy receipt
from the surface yields the altitude. The spacecraft posi-
tion referenced to the overall shape of the planet then
enables the surface topography to be derived for compar-
ison with the geomorphology.
Mapping variations in local gravity is a common tech-
nique in geophysical studies on Earth and for mineral
prospecting. Areas containing high-density materials
Table 2.1. Radar bands
Designation Wavelength Frequency Examples
Ku, K, Ka 0.8–2.4 cm 37.5–12.5 GHz Aircraft–Earth
X 2.4–3.8 cm 12.5–8 GHz SIR–Earth
C 3.8–7.5 cm 8–4 GHz SIR–Earth
S 7.5–15 cm 4–2 GHz Magellan–Venus
L 15–30 cm 2–1 GHz Seasat–Earth
P 30–100 cm 1 GHz–300 MHz MARSIS–Mars
2.6 Geophysical data 25

will have slightly higher gravitational accelerations than
areas of low-density materials, when differences in local
topography are taken into account. In planetary explora-
tion, mapping gravity variations, as was done for the
Moon, provides insight into subsurface structure and the
possible relation to surface features.
Magnetic fields vary in space and time on planets. On
Earth, measuring the orientations of the fields “locked” in
rocks helped build the case for the theory of plate tectonics
and showed that the magnetic poles have shifted with
respect to the spin axis. Not all planets have magnetic
fields today, but magnetometers flown on spacecraft have
revealed the presence of remnant magnetic fields. For
example, the Mars Global Surveyor spacecraft recorded a
remnant field in the oldest rocks exposed on the surface.
Its absence in the younger terrains suggests that there was
a fundamental change in the interior of Mars early in its
history, thus demonstrating that geophysical measure-
ments combined with surface geology provide powerful
tools for deciphering planetary histories.
2.7 Image processing
Digital “snapshot” cameras and computers typically have
built-in routines to manipulate images, such as “red-eye”
removal. Such digital image processing has become so
commonplace that many people do not understand what is
involved. In this section, some of the principles and prac-
tices of planetary image processing are outlined.
The basic units of digital images are pixels (an abbrevia-
tion for “picture elements”), which can be arrayed in hori-
zontal lines and vertical rows to make a picture. The amount
of light received by the detector for each pixel is encoded by
its brightness level. In 8-bit encodement (2 raised to the 8th
power), 256 shades of gray can be assigned, with each level
referring to a specific “DN” (digital number), in which a
DN of 0 is black (no light received) and a DN of 255 is a
perfectly white level. These are typically shown on an
image by a DN histogram that gives the distribution of the
various levels of gray in an image (Fig. 2.15). Because the
human eye can discriminate only about 30 shades of gray,
this means that digital images potentially contain much
more information than can be visually detected. Various
algorithms can be applied to extract this information after
calibration of the image.
Calibrations. Digital image detectors are not uniform in
their response to light. Within any given array, some detec-
tor pixels will be more sensitive than others. Consequently,
after the camera has been assembled, but before it goes into
use, it must be calibrated to take these differences into
account. In its simplest form, an image of a “flat field,”
which consists of a perfectly uniform, perfectly illuminated
surface is taken by the camera. Each pixel is then adjusted
by adding or subtracting DNs until all the pixels have the
same value. This mapping of DNs is then standardized for
Figure 2.15. Images of Mars to illustrate some common image
processing techniques: (a) calibrated image showing a histogram
(bottom of image) of DN levels within a relatively narrow range
centered at about 134; (b) a “stretched” image in which the DN levels
are spread over a wider range than in (a) and shifted toward a darker
(lower DN) level; (c) a low-pass filter image resulting in a smoother
appearance; (d) a high-pass filter image in which differences in
brightness are enhanced; and (e) a sharpened image in which
boundaries among DN changes are emphasized (part of Mars Orbiter
Camera image mc27–256).

that particular camera so that, when images are subsequently
taken, they can be calibrated or adjusted pixel-by-pixel.
Despite the best efforts to maintain cleanliness, dust
grains find their way into imaging systems and can pro-
duce artifacts, such as “donuts” (Fig. 2.16). As part of the
calibration routine, these and other artifacts are mapped so
that they can be taken into account and cosmetically
corrected, as noted below.
Calibrations are also typically performed during flight
because detectors can change with time. Such calibrations
are accomplished by taking images of known surfaces, such
as the Moon or star fields, with individual pixels adjusted,
just as is done in pre-flight calibrations. New artifacts can
also occur, as when cosmic rays “zap” the detector and
degrade or knock out one or more pixels. These artifacts
are also mapped and can be corrected cosmetically.
Stretches. Stretching digital images involves shifting
the distribution of DN levels (Figs. 2.15(a) and (b)), or
bringing about a simple increase in brightness by moving
all the DNs to a higher level without changing the shape of
the histograms, or redistributing the DNs following some
specific function, such as a Gaussian distribution. More
complicated stretches involve giving one part of the dis-
tribution more weight than other parts in order to empha-
size detail.
Filters. Filtering involves manipulating multiple pixels
as sets within the image. For example, boxcar filters give a
weighted value to each pixel as a function of the value of its
neighbors (the “box”), which is then slid across the image,
adjusting each pixel one-by-one. In a low-pass filter the
value of the central pixel is the average value of the neigh-
boring pixels, and the image tends to be smoothed, enhanc-
ing broad changes in the scene (Fig. 2.15(c)); the larger the
boxcar, the smoother the result.
In high-pass filters the DN values from the low-pass
filter are subtracted from the image, leaving only the
smaller variations in the scene and producing a somewhat
sharper image (Fig. 2.15(d)). Edge-enhancement filters
decrease the contrast where pixels have similar values and
enhance the contrast where pixels change, in order to
emphasize the boundaries in the scene (Fig. 2.15(e)).
Common “snapshot” digital cameras automatically
apply some form of stretching and filtering to produce
pleasing images. Once in the computer, stretching, filter-
ing, and various color-enhancement techniques are
applied with user-friendly “black box” programs that are
based on the processes outlined above.
Geometric projections. In most spacecraft images, the
position of each pixel is referenced to some system, such
as geographic coordinates by latitude and longitude. This
allows the image to be re-cast into standard projections.
For example, an image might be taken that is oblique, or
viewed looking at the terrain at an angle similar to the
view from an airplane window. Because the geometric
position of each pixel is known, they can be shifted so
that the image is portrayed orthographically as though it
were taken as viewed looking straight down on the terrain
(Fig. 2.17). Alternatively, the image can be re-projected
into a standard cartographic product, such as a Mercator
projection, depending on the intended use.
Mosaics. Multiple frames can be put together as
mosaics (Fig. 2.18), in which the boundaries between
individual frames are seamless. This begins with the iden-
tification of individual tie points that consist of specific
features, such as small craters, that are visible on more
than one frame. The pixels in the frames are then re-
projected geometrically so that all of the features match.
Various filters are then applied so that the pixel DN values
along the frame boundaries are averaged to reduce the
Figure 2.16. “Donuts” are image blemishes (arrow) caused by dust
grains in camera systems. These and other artifacts can be removed
in image processing by mapping the pixels that are affected and
then assigning DN values to them on the basis of the values of the
surrounding unaffected pixels. This represents cosmetic processing
and users need to be aware that the assigned pixel values are
artificial (NASA Viking Orbiter 826A68).
2.7 Image processing 27

contrast. This overall process can be done automatically
or by hand.
Cosmetic processing. A wide variety of processes can
be applied to generate images that are more pleasing to the
eye. For example, individual pixels or blocks of pixels
might have DN “0” levels because parts of the detector
have been damaged, or because data were lost during
transmission from the spacecraft to Earth. The simplest
method of ﬁlling in the missing data is to use some
average value of the neighboring pixels for the missing
pixel. Similarly, blemishes, such as those caused by dust
grains, can be removed by reference to the calibration ﬁles
and the missing data can be applied. One must remember
that cosmetically improved images include DN values that
are not real (Figs. 2.16, 2.19, and 2.20).
2.8 Resolution
The scale of individual pixels in a digital image and the
resolution of that image are somewhat related, but are
distinctive parameters that are often confused, even in
the planetary science community. Put simply, the scale
of a pixel is related to the dimension of the terrain pro-
jected onto the detector through the imaging system lens.
Thus, it is dependent primarily on the optics, the distance
of the camera from the terrain, and the size of each pixel of
the detector. The pixel scale then can be stated as some
length per pixel, such as 10 m per pixel, meaning that 10 m
Figure 2.17. Geometric projections involve shifting individual
pixels into new positions; for example, this image of Mercury was
taken at an oblique angle from the Mariner 10 spacecraft, viewed
off to the side (a), which was then orthographically reprojected
(b) so that the scale is near-uniform over the entire scene (as
though viewed “straight-down” from the spacecraft; NASA
Mariner 10 FDS 27321).
Figure 2.18. Mosaics combine more than one image to matching
pixels along frame boundaries and assigning average values to the
pixels, giving the appearance of a single image. (a) This set of
Mariner 10 images of the Caloris basin on Mercury was assembled by
hand; (b) the same scene as a computer-generated mosaic (courtesy
of the Jet Propulsion Laboratory).

on the ground is registered as a single DN value by the
pixel detector.
Resolution is a much more complicated parameter than
pixel scale, since it refers to the smallest object that can be
identified in the image. Thus, the contrast of the object in
relation to its background, the shape of the object, the
responsitivity of the particular detector to the composition
and surface texture, and other factors all play a role in
resolution, as well as the size of the object in relation to the
pixel scale. For example, a stark white object ten times
larger than the pixel scale might not be seen on an image if
the object were placed on a pristine snow field, while
some linear features, such as fault traces, might be
detected even though the width of the fault might be
smaller than the pixel scale, simply because of its shape
and the contrast with the surrounding terrain.
Because of the complications in defining resolution,
digital images are often (erroneously) stated to have a
“resolution” of x meters per pixel, when this phrase
actually refers to the pixel scale or angular resolution.
Obviously, it takes more than one pixel to discern an
object, depending on the object’s shape. To further com-
plicate the issue, some images will be stated to have a
resolution of x meters per line pair of pixels, which is an
attempt to take into account the need for more than one
pixel for object detection.
Figure 2.18. (cont.)
Figure 2.19. Various cosmetic image processing techniques remove
artifacts such as “donuts” (Fig. 2.16), filling-in pixels or lines of pixels
“dropped” during electronic data transmission (a) by assigning
average values of surrounding pixels to the dropped pixels, and by
removing reseau (“r,” registration marks built into the imaging
system used for precise location of surface features) to produce a
final image (b).
2.8 Resolution 29

Pixel scale can also be erroneously set with the image
processing tools commonly available on computers. Some
of these tools resize the image, in effect adding pixels to fit
some format, and leading to an imaginary pixel scale that
is smaller than the actual pixel scale of the original data.
When using such tools, it is best to use a “sanity check,”
keeping in mind that the pixel scale cannot be better than
that of the original image.
2.9 Electronic data records (EDRs)
EDRs are the files from the mission (this refers to all data,
not just images). For images, there are different levels of
processing, as outlined below.
Level 0 refers to the raw data (no processing) as
received from the spacecraft. This version is generally
preferred by scientists who need to conduct quantitative
studies, such as photometry (precise measurements of
surface brightness), for which they have their own algo-
rithms for customized processing. Level 0 is often the
preferred archival method to preserve the original files in
order that algorithms developed later can be applied.
Level 1 data are decompressed. To make data acquis-
ition and transmission more efficient, various data com-
pression techniques are applied; these can be either
hardware techniques built into the imaging system or
software techniques that can be updated on board the
spacecraft. For example, all pixels that have a DN of
zero (black) might be automatically eliminated from the
data transmission. Compression is either lossless (preserv-
ing all the data) or lossy, in which some data are lost. In
level 1, the compression is “reversed” to restore the orig-
inal image to the extent that this is possible.
Level 2 data have the calibration files applied to pro-
duce radiometrically corrected images in which the
brightness levels are correctly given, taking into account
illumination, etc. On some missions, radiometric calibra-
tion is done on level 1 files.
Level 3 data have been custom processed for specific
uses; for example, images might now be in some uniform
map projection.
Level 4 data are further processed and might be merged
with other data; for example, level 4 images might have
topographic information incorporated.
2.10 Cartography
Maps are essential for exploration. No doubt, early
humans scratched simple maps in the dirt to describe
hunting grounds, clan boundaries, and other geographic
locations critical for their survival. Planetary maps are
generated from image mosaics using conventional carto-
graphic projections. The USGS is supported by NASA to
produce maps with a variety of scales, projections, and
portrayal methods. Most of the terrestrial planets and the
satellites that have been adequately imaged have been
Figure 2.20. Lunar Orbiter photographs were transmitted and
reconstructed by assembling strips of film, resulting in the distinctive
pattern seen in (a); mosaicing and image processing results in a
smooth, continuous image as shown in (b).

mapped in standard cartographic series, or quadrangles.
Indexes and availability of cartographic materials are
maintained on the USGS website (http://pdsmaps.wr.
usgs.gov) and include geologic maps and a gazetteer of
officially named planetary surface features.
Deriving a coordinate reference system for planets is
challenging. Once the spin axis for an object has been
determined, the equator can be set with latitude running
north and south from the equator. The prime meridian is
arbitrarily defined, using some recognizable feature, such as
a small crater. Now come some potential problems!
Longitude can run either east or west from the prime meri-
dian, and both have been used in planetary science, even on
the same planet. Thus, some instrument data sets give
coordinates in east longitude, while others use west longi-
tude. In principle, so long as east or west is designated, there
should be no problems, but in practice, especially with some
digital files where E or W is not designated directly with the
values, errors can arise. For example, both systems have
been used on Venus, and in the early planning stages for a
mission that would have sent small probes through the
atmosphere toward the surface, the scientists used one sys-
tem and the engineers used the other system; had the mis-
sion flown (it did not) and the error not been caught, the
probes would have descended through a part of the venusian
atmosphere totally different from that targeted.
Today, the generally accepted use on maps follows the
planetographic system, in which west longitudes are
used for objects that spin in the same direction as that in
which the object orbits. For example, looking down on a
planet’s north pole, the planet would spin in a counter-
clockwise direction and the planet would orbit the Sun in a
counter-clockwise direction. When viewing the planet
toward the equator from a fixed position in space, the
longitudes would increase in value toward the west as
the planet rotated. For an object that is in retrograde
motion (spinning in the opposite direction to its orbit,
such as Venus), longitudes increase toward the east.
The “zero” reference elevation also poses problems;
what does one use on a planet not having a sea level?
Different systems have been used on different planets. For
example, on the Moon and Venus, the mean radius of the
planet is taken as the “zero” contour, while on Mars, the
original reference was based on the triple point of carbon
dioxide, the main component of the atmosphere. The
triple point of a substance is the pressure–temperature
condition at which all three phases (solid, gas, liquid)
exist. As discussed in Chapter 7, a different system is
used on Mars currently. Thus, depending on data
availability and the specific planet, different systems are
used throughout the Solar System, and, as with longitude,
one must be familiar with the conventions used for the
specific planetary object of interest.
Topographic maps are generated using a variety of
techniques, including photogrammetry, photoclinometry,
and altimetry (see Section 2.6). Photogrammetry has long
been used in making topographic maps of Earth from
aerial photographs. This technique is based on stereo-
scopic models in which two or more images are taken of
the same terrain but from different viewing angles. From
knowledge of the geometry of the camera system optics
and the altitude from which the images are taken, the relief
of the terrain can be derived. Photoclinometry, also
known as “shape from shading,” uses the amount of
light reflected from the surface to determine the slope of
that surface; this technique requires that the illumination
and viewing geometry be known and that the surfaces are
homogeneous with regard to texture, composition, and
other variables that influence their reflectivity. In these
cases, the reflectivity of each pixel is measured, from
Figure 2.21. Digital elevation models (DEMs) enable topography to
be portrayed by a variety of techniques, including as shaded-relief
maps, shown here for the Olympus Mons shield volcano on Mars.
Manipulation of DEMs can provide oblique views of terrain, as might
be seen from an airplane window, and the illumination direction and
angle can be changed to include early-morning to mid-day views
(MOLA topographic DEM courtesy of the US Geological Survey).
2.10 Cartography 31

Table 2.2. Terms used for features in planetary nomenclature (US Geological Survey)
Featurea
Description
Albedo feature Geographic area distinguished by amount of reflected light
Arcus, arcūs Arc-shaped feature
Astrum, astra Radial-patterned features on Venus
Catena, catenae Chain of craters
Cavus, cavi Hollows, irregular steep-sided depressions, usually in arrays or clusters
Chaos, chaoses Distinctive area of broken terrain
Chasma, chasmata A deep, elongated, steep-sided depression
Collis, colles Small hills or knobs
Corona, coronae Ovoid-shaped feature
Crater, craters A circular depression
Dorsum, dorsa Ridge
Eruptive center Active volcanic centers on Io
Facula, faculae Bright spot
Farrum, farra Pancake-like structure, or a row of such structures
Flexus, flexūs A very low curvilinear ridge with a scalloped pattern
Fluctus, fluctūs Flow terrain
Flumen, flumina Channel on Titan that might carry liquid
Fossa, fossae Long, narrow depression
Insula, insulae Island (islands), an isolated land area (or group of such areas) surrounded by, or nearly
surrounded by, a liquid area (sea or lake)
Labes, labēs Landslide
Labyrinthus,
labyrinthi
Complex of intersecting valleys or ridges
Lacus, lacūs “Lake” or small plain; on Titan, a “lake” or small, dark plain with discrete, sharp
boundaries
Landing site name Lunar features at or near Apollo landing sites
Large ringed feature Cryptic ringed features
Lenticula, lenticulae Small dark spots on Europa
Linea, lineae A dark or bright elongate marking, may be curved or straight
Lingula, lingulae Extension of plateau having rounded lobate or tongue-like boundaries
Macula, maculae Dark spot, may be irregular
Mare, maria “Sea;” large circular plain; on Titan, large expanses of dark materials thought to be liquid
hydrocarbons
Mensa, mensae A flat-topped prominence with cliff-like edges
Mons, montes Mountain
Oceanus, oceani A very large dark area on the Moon
Palus, paludes “Swamp;” small plain
Patera, paterae An irregular crater, or a complex one with scalloped edges
Planitia, planitiae Low plain
Planum, plana Plateau or high plain
Plume, plumes Cryo-volcanic features on Triton
Promontorium,
promontoria
“Cape;” headland promontoria
Regio, regiones A large area marked by reflectivity or color distinctions from adjacent areas, or a broad
geographic region
Reticulum, reticula Reticular (netlike) pattern on Venus
Rima, rimae Fissure
Rupes, rupēs Scarp
Satellite Feature A feature that shares the name of an associated feature. For example, on the Moon the
craters referred to as “Lettered Craters” are classified in the gazetteer as “Satellite
Features.”
Scopulus, scopuli Lobate or irregular scarp
Sinus, sinūs “Bay;” small plain
Sulcus, sulci Subparallel furrows and ridges
Terra, terrae Extensive land mass

which the slope of the terrain covered by that pixel is
determined. For example, a pixel covering ground that is
flat and horizontal, with the Sun directly overhead, would
appear brighter than a pixel of the same type of surface,
but tilted with respect to the horizontal. An overall topo-
graphic map is then generated by integrating the slopes for
the areas covered by each pixel.
Once the topography has been derived, whether from
images or from other techniques, digital elevation mod-
els (DEMs) can be constructed. Topography then can be
shown by contour lines, by colors, or as shaded-relief
maps (Fig. 2.21) to portray the terrain as it might appear
to a viewer from above.
Names for planetary objects and surface features are
determined by the International Astronomical Union
(IAU) through a committee and various subcommittees
(typically, one for each planet). Names on planetary sur-
faces derive from a variety of sources, including historic
telescopic usage on maps of the Moon made centuries
ago. In the Space Age, there has been an attempt to set
specific themes for naming surface features. For example,
small craters on Mars are named for Earth villages or
towns of less than 100,000 population, while volcanoes
on Io are named for ancient gods dealing with fire, such as
Prometheus.
Classes of surface features typically are Latinized, as
given in Table 2.2. With features named for people
(mostly craters) it is required that the individual be
deceased for at least five years before the name is applied.
The USGS maintains a gazetteer of named features, which
can be accessed at their website http://planetarynames.wr.
usgs.gov/.
Table 2.2. (cont.)
Featurea
Description
Tessera, tesserae Tile-like, polygonal terrain
Tholus, tholi Small domical mountain or hill
Unda, undae Dune
Vallis, valles Valley
Vastitas, vastitates Extensive plain
Virga, virgae A streak or stripe of color
a
Singular, followed by plural form.
Assignments
1. Discuss the fundamental differences between images
produced from the visible part of the electromagnetic
spectrum and images produced from radar systems.
2. Discuss the fundamental concept of using impact
crater counts for age-dating planetary surfaces and
explain the difference between equilibrium and pro-
duction distributions.
3. Outline the advantages and disadvantages in the use of
laboratory simulations, computer modeling, and ter-
restrial field analog studies to understand geologic
processes on other planets.
4. Explain the difference between pixel scale and reso-
lution for images.
5. Visit the USGS website for planetary maps and review
how names are assigned to surface features and sum-
marize the process.
6. Discuss how an astrobiologist might use a geologic
map of Mars to plan a future landed mission to
search for evidence of past or present life beyond
Earth.
Assignments 33

CHAPTER 3
Planetary morphologic processes
3.1 Introduction
Earth is a dynamic planet. That simple statement can be
supported by our own direct observations. Earthquakes,
river banks collapsing during flooding, erupting volca-
noes – all are experienced or documented on the news
every year and show that our planet is everchanging.
These examples represent three of the four fundamental
processes that shape Earth’s surface: tectonism, grada-
tion, and volcanism.
The fourth fundamental process, impact, which is gen-
erally less often observed, is also documented, sometimes in
quite newsworthy events as when a meteoroid plunges
through the roof of a house. As the geologic record shows,
the history of Earth can be profoundly altered by impacts,
such as the well-known Chicxulub structure in the Yucatan
peninsula of Mexico. This structure, now buried beneath a
kilometer of sediments, has been mapped by geophysical
methods and drill-holes to be more than 80km in diameter
and is estimated to have formed from an impact that released
the energy equivalent of some 10 billion tons of TNT. The
resulting fireball ignited world-wide fires, generated enor-
mous amounts of CO2 from the vaporization of limestone
present at the impact site, and triggered tsunamis throughout
the Gulf of Mexico and adjacent waters. As is now widely
accepted, these catastrophic events led to mass extinctions,
including that of the dinosaurs, and marked the boundary
between the Cretaceous Period and the Tertiary Period 65
million years ago. It was not so much the direct impact that
led to extinctions, but the effects on the surface environ-
ment, including firestorms, enhanced greenhouse processes,
and disruption of the food chain.
Geologic exploration of the Solar System shows that
the surfaces of the terrestrial planets, satellites, and small
bodies, such as asteroids, have been subjected to one or
more of the four fundamental processes. In some cases,
the processes are currently active; in other cases, the
geomorphology of the surface reflects events that hap-
pened in the past but are no longer taking place.
Learning to recognize the landform “signatures” left by
tectonism, gradation, volcanism, and impacts is one of the
main goals of planetary geomorphology.
The relative importance of the surface-modifying pro-
cesses among the planets is a function of many factors,
including the history of the object and the local environ-
ment. For example, gradation on Earth is dominated by
water, but in the current cold, dry martian environment,
wind dominates. Thus, we must understand how gravity,
surface temperature, the presence or absence of an atmo-
sphere, and other variables influence the manner in which
the processes operate and lead to specific landforms.
In the following sections, tectonism, volcanism, grada-
tion, and impact processes are described and illustrated,
using examples taken primarily from Earth to serve as a
basis for planetary comparisions.
3.2 Tectonism
Road-cuts along many highways show ample evidence for
rock deformation, or tectonic processes. As can be seen in
Figs. 3.1 and 3.2, Earth’s crust can be broken along faults
or bent into distinctive folds. These and other features can
be related, in part, to the style of tectonic deformation of
the crust, such as tension or compression. However,
knowledge of global-scale crustal deformation on Earth
was not gained until the unifying concept of global plate
tectonics was formulated in the 1960s. This insight was
important for understanding the evolution of Earth’s crust
and is critical in the interpretation of other planets.
Determining the styles of tectonic deformation on the
planets provides clues to their general evolution and the
configuration of their interiors.
Seismic and other geophysical data show that the inte-
rior of Earth consists of distinctive zones. The outer zone,
34

Another Random Document on
Scribd Without Any Related Topics

CHAPTER XIV.
WHAT BEFEL BARBAROSSA.
The Emperor Charles the Fifth had been very indignant when he
heard of the sack of Fondi, and the attempt to seize the Duchess.
Some months afterwards, when Muley Hassan, whom Barbarossa
had driven from Tunis, appealed to him for assistance, Charles, who
was ambitious of military renown, resolved at once to rid the coast
of a dangerous invader, and avenge an injured prince, by heading an
expedition against Hayraddin.
The united strength of his dominions was therefore called out upon
this enterprise, which he intended to increase his already brilliant
reputation. As the redresser of wrongs, his cause was popular, and
drew on him the applause of Christendom. A Flemish fleet conveyed
his troops from the Low Countries; the galleys of Naples were loaded
with the Italian auxiliaries, and the Emperor himself embarked at
Barcelona with the flower of his Spanish nobility, and considerable
reinforcements from Portugal. Andrea Doria commanded the
Genoese galleys, and the Knights of Malta equipped a small but
powerful squadron, and hastened to the rendezvous at Cagliari.
All this mighty armament to hunt down a Lesbian pirate, the son of
an obscure potter!
Hayraddin was, however, no contemptible foe. Ambitious and
relentless, a skilful and a generous chief, his lavish bounties among
his partizans made them his blind adherents: while his wondrous
versatility had enabled him to ingratiate himself with the Sultan and
his Vizier. It was therefore to be war to the knife between the
Crescent and the Cross.

As soon as Barbarossa heard of the Emperor's formidable
preparations, he called in all his corsairs from their different stations,
drew from Algiers what forces could be spared, summoned Moors
and Arabs from all quarters to his standard, and inflamed their
fanaticism by assuring them he was embarking in a holy war.
Twenty thousand horse and a considerable body of foot answered
his summons, and drew together before Tunis. Hayraddin knew,
however, that his greatest dependence must be on his Turkish
troops, who were armed and disciplined in the European manner. He
therefore threw six thousand of them, under Sinan, the renegade
Jew, into the fortress of Goletta commanding the bay of Tunis; which
the Emperor immediately invested.
Three separate storming parties attacked the fort; Sinan raged like a
lion at bay: frequent sallies were made by his garrison, while the
Moors and Arabs made diversions. But nothing could withstand the
fury of the assailants; and a breach soon appeared in the walls of
the fortress, which the Emperor pointed out to Muley Hassan.
"Behold," said he, "the gate through which you may re-enter your
kingdom!"
With the Goletta, Barbarossa's fleet fell into the Emperor's hands;
and he was driven to extremities. Having strongly entrenched
himself within the city, he called his chiefs to a council of war, and
proposed to them, that before sallying out to decide their fate in
battle, they should massacre ten thousand Christians whom he had
shut up in the citadel.
Even his pirate chiefs were staggered at this proposal; and
Barbarossa, seeing they would not support him in it, yielded the
point with a gesture of disgust at their want of hardihood. Charles
and his chivalry were meanwhile painfully toiling, under a blazing
African sun, across the burning sands which encompass Tunis,
without so much as a drop of water to cool their tongues:

"Non e gente Pagana insieme accolta,
Non muro cinto di profonda fossa,
Non gran torrente o monte alpestre e folta
Selva, che 'l loro vïaggio arrestar possa."
La Ger. Lib., Canto I.
Hayraddin, sallying out upon them with his best troops, made a
desperate onset, but was so vigorously repulsed that his forces
surged back to the city, and he himself was irresistibly borne along
with them like a straw on the tide.
Meanwhile, a pale girl, a Christian slave, who had been within
earshot of the council, carried the report of Barbarossa's ferocious
proposal to the keepers of the citadel. They were revolted at his
cruelty, and her entreaties, backed by the clamours of the despairing
wretches in their charge, prevailed on them to release the Christian
prisoners and strike off their fetters. Forth came Tebaldo Adimari,
the pride of Fondi; forth came many a grey-haired senator, illustrious
cavalier, and venerable hidalgo, some in their full strength, others
wasted with long captivity, but nerved at this moment to strike a
blow for freedom. Unarmed as they were, they flung themselves on
the surprised guard, and turned the artillery of the fort against
Barbarossa himself as he and his discomfited troops poured back in
disorderly retreat. O, fell rage and despair of the defeated pirate,
late the sovereign of two kingdoms, as he now heard Christian war-
cries defying him from his own battlements! gnashing his teeth, and
cursing the comrades whose humanity compelled him to spare those
who were now manning the walls, he sought safety in ignominious
and precipitate flight.
Then what a cheer arose, as the Christians saw the turbans in
retreat, and themselves masters of the city! The Emperor was first
made aware of the turn affairs had taken, by the arrival of deputies
from Tunis, who brought him the keys, and piteously besought him

to check the violence of his troops. In vain! They were already
sacking the city, killing and plundering without mercy; and thirty
thousand defenceless people were the victims of that day, while ten
thousand more were carried away as slaves.
It is said that Charles lamented this dreadful slaughter, and that he
declared the only result of his victory which gave him any
satisfaction was his reception by the ten thousand Christian captives,
who fell at his feet, blessing him as their deliverer. In all, he freed
twenty thousand slaves, whom he sent, clothed at his own expense,
to their own homes; and they, as may well be supposed, made
Europe ring with their praises of his goodness and munificence. It
was a bright day for Fondi when Tebaldo Adimari returned! Though
the Duchess was at Naples, and though Isaura was in her train, he
had seen them both on his way home, and ratified his vows of love
and constancy. The Duchess had promised to smile on their
espousals, which were shortly to take place; and meanwhile his
friends and relations got up a festa to welcome him, and there was
church-going and bell-ringing, and eating and drinking, and dancing
and singing, without any drunkenness, stabbing, or even quarrelling.
If such was the public joy in a little town of four thousand people at
the return of a young fellow of no mark or likelihood whatever,
except that he was comely, merry, brave, ingenuous, with a good
word for everybody and with everybody's good word,—it may be
supposed what a stir the Emperor's arrival at Naples made, and how
that pleasure-loving capital nearly exhausted itself in demonstrations
of welcome. The mole, when he landed, was so crowded, that you
may be sure a grain of millet thrown upon it would not have found
room to reach the ground. Nothing was to be heard but bell-ringing,
acclamations, and the thundering of cannon; nothing to be seen but
gold, velvet, silk, and brocade, festoons of flowers, triumphal arches,
processions, deputations, triumphal cars, prancing steeds, waving
plumes, and bronzed cavaliers looking up at the balconies of fair
women waving their handkerchiefs, among whom, rely on it, were
Vittoria Colonna and Giulia Gonzaga.

Charles, with his Spanish gravity ever uppermost, took it all very
soberly; heard what people had to say, enjoyed it in his way, said
very little himself, and in the proverb style; went to the cathedral,
heard Fra Bernardino Ochino preach, and afterwards observed,
composedly, "That man would make the stones weep!"—his own
eyes being quite dry all the while. Also if anything inexpressibly
funny were said, he remarked, "How very diverting!" but did not
smile. He was best at business, and he entered upon Giulia's affairs.

CHAPTER XV.
MORE ABOUT THE CARDINAL.
Itri, the birthplace of the notorious Fra Diavolo, is a regular robber's-
nest, picturesquely placed on the side of a lofty hill, and crested by a
ruined castle.
In Ippolito de' Medici's time the castle was not ruined; and there was
also a monastery, where he and his attendants were suitably
entertained.
On the afternoon of the 2nd of August, after a meal which we
should call luncheon, but which the early habits of those days
distinguished as dinner,—succeeded by a moderate siesta,—the
court-yard was all alive with preparations for a gallant riding-party, in
the full heat and glare of the day. Groups of cowled and bare-
headed monks stood curiously about, admiring the Cardinal's
beautiful mare; and groups, too, of robber-like, shaggy-looking men,
and bright-eyed women and girls with golden bodkins in their hair,
hung about the gates and passed their comments on the cortége.
The Cardinal came forth, talking to the Prior, whose pale, attenuated
face and hollow eyes formed a notable contrast to the vivid colouring
of his own healthy, well-fed countenance. He was within an ace of
losing his good looks from too much eating and drinking. In dress,
the Cardinal was superb, with a touch of the church militant. A smile
was on his lip as he patted his mare and examined her trappings,
saying,
"She will not serve me that sorry trick again, I hope."
"Fear not, my Lord Cardinal," said his groom; and he threw himself
into the saddle. The Florentines also mounted their horses.

At this moment, Piero Strozzi stepped forward, saying, "This, from
my father," with a meaning smile; and gave him a billet.
This Piero was son of Felippo, and had something of the same cold,
sly look.
The billet only contained these words: "All goes well." The Cardinal
read it with a gay smile, and tossed it back to Strozzi.
"Good news to start with," said he to his companions, as they rode
out of the yard.
"The sun can scarce be hotter in Africa than it is here to-day, I
think," said Donati, one of the fuorusciti.
"Not a whit too hot for me; I enjoy it," said the Cardinal. "And the
road is in our favour, for it is all down-hill."
"Facile descensus," said Capponi. "What a vibrating haze!"
"We shall enjoy the shade and the coolness at Fondi," said Ippolito.
"You know I have undertaken to show you the fairest lady in Italy."
"And I maintain, beforehand, that she cannot be so fair as the
Marchesana del Vasto," said Donati.
"Allowing for difference of years, you mean," said Capponi. "The
Duchess is a little past her prime."
"No such thing," said Ippolito quickly; and he used the spur, though
there was no need. The mare sprang forward; the others were
obliged to quicken their pace, and they had ridden a mile or two
before another word was spoken.
Then the Cardinal slackened his speed, and began to talk of matters
quite different; of the brilliant African campaign; of the likelihood of
Muley Hassan holding his own, now he was reinstated; of the
probable movements of Barbarossa; of the glut of Moorish slaves in
the market, and so forth.
Arrived at Fondi, the Cardinal was preparing to alight, when the
Duchess's grey-haired seneschal came forward and announced the

mortifying intelligence that his lady was from home.
It may be matter of surprise that the Cardinal should not have been
apprised of her absence at Itri; but, in fact, he had learnt from what
he had considered good authority, that she was to return to Fondi a
little before this time, so that he had made sure of finding her at her
castle.
His chagrin was extreme; not only because he had counted much on
this visit, and had now no hope of seeing her before he sailed, but
because he had given out to his companions that he possessed such
perfect knowledge of her movements and such security of a cordial
reception, that he was now open to their raillery, whether or no they
spared it.
The seneschal, who knew him well, respectfully besought him to
partake of such poor refreshment as the castle afforded; but the
Cardinal was vexed, and rode off again, without compassion for man
or beast.
The Florentines looked at one another and shrugged their shoulders,
but were too wise to remonstrate. They followed him, panting,
across the steaming plain, where groups of cream-coloured oxen,
cropping the rank herbage, looked up at them with dreamy,
wondering eyes. When they reached the covert of cypress, poplar,
and gnarled old olives, they loitered dangerously in the shade; and
then, when well chilled, spurred on again, making themselves and
their horses hotter than ever. And of course, as there was a descent
all the way going, there was an ascent all the way back.
Arrived at Itri, the Cardinal, throwing himself from his horse, called
loudly for iced water.
"My lord, you are very hot," said Giovanni Andrea, with seeming
kindness. "Let me prevail on your Eminence to take this broth
instead. It will be safer, and will repair your strength."
The Cardinal took the broth, which was temptingly seasoned, and
turned away with a sigh of relief. It was the early supper-hour, and
the tables were already spread in the vaulted refectory, with

abundance of better cheer than the Prior's larder usually afforded,
some of which had been brought by his illustrious guest. And soon
the hungry visitors took their places, and a long Latin grace was
said, and the first course of confetti was served; and then the
trencher of each man was filled with a large piece of meat that had
been stewed with almonds and sugar.
And while this was being disposed of, the Cardinal's servants and
rubicund lay-brothers covered the table with dishes of boiled meat,
fowls, small birds, kids, wild boar, and other viands. And after this
course, another was to succeed, of tarts and cakes covered with
spun sugar.
But before the banquet reached this stage, the Cardinal, who had
scarcely spoken since he sat down to table, and who had frequently
changed colour, suddenly exclaimed—
"Take me hence—I am strangely ill!"——
Every eye was upon him in a moment—many started from their
seats—one or two noted gourmands feigned deafness, and helped
themselves to the best. Bernardino Salviati, the Cardinal's personal
attendant, caught him in his arms.
"Lean on me, my Lord Cardinal," said he. "We will bear you to your
chamber."
"Treachery, treachery, Salviati!" murmured the Cardinal, almost
inarticulately. "I am poisoned."
Giovanni Andrea, his other supporter, making believe to wipe the
clammy dew from his face, held the handkerchief over his mouth, so
as to muffle his voice. Above it glared the Cardinal at him fiercely.
"Stand back!" said Salviati to him, roughly.
"My Lord Cardinal is delirious, he raves," said Giovanni Andrea,
shrinking away.
"Prior! don't let that man come near me," said Ippolito, faintly.

The Prior, with solicitude, bent his ear to his lips, but only saw them
move. The next instant they were contorted with a spasm.
By this time, they had carried him to his bed-room, which, though
the best guest-chamber of the monastery, was furnished with ascetic
plainness; a crucifix, a bénitier, and a wooden pallet, comprising
most of its moveables, the meagreness of which contrasted
strangely enough with the crimson satin cushions and mattresses
the Cardinal had brought with him, and which belonged to his horse-
litter.
"Air! air!" he said, feebly, as his friends pressed round him.
"It will be well, I think, for all of you to leave the chamber," said the
Prior, "except Salviati, Brother Marco, and myself. The Cardinal is in a
high fever—I will open a vein for him."
"Not on your life," gasped Ippolito.
Meanwhile, all retired from the room except those whom the Prior
had named.
"Marsh miasma, no doubt," said Donati, as he returned to the
refectory. "There was a pestiferous vapour on the marshes to-day."
"And he would ride so fast," said Capponi, resuming his seat at
table. "For my part, I wonder we are not ill too. I feel quite spent,
and want something solid. I dare say a good night's rest will set him
up again. He is of a full habit, like many of the Medici: it does not do
for them to over-heat themselves. He takes everything too violently.
What excellent beccaficoes! I prefer, however, thrushes stuffed with
bergamots."
While these two were composedly resuming their repast, there were
others who did not even sit down to table, but stood apart in a little
knot, anxiously debating whether the Cardinal had or had not
exclaimed,
"Ahí! tradimento!"—

Anxious looks were cast towards the door; and once or twice an
envoy was despatched to the sick room. The first of these came
back with disturbed aspect, saying,
"His Eminence positively refuses to be bled, and the Prior is at his
wit's end."
"What a pity!" said Strozzi. "There is no finer remedy."
"If it were any one else," pursued the first, "the Prior might take the
matter into his own hands; but 'tis ticklish meddling with a Cardinal."
"Especially when that Cardinal's a Medici," said young Strozzi, with
his father's unpleasant smile. "I'll go and see to it myself."
Presently Strozzi returned, saying mysteriously,
"A courier is instantly to be despatched to the Pope, to beg of him a
certain oil he possesses, known to be a sure antidote to all poison."
"Poison!" repeated they all.
"Can it be so?" said Capponi, wiping his lips, and rising from table.
"This ought to be looked to."
"Nay, I say not that it is so, I only say that he thinks so," replied
Strozzi. "At all events, I'm going instantly to despatch a messenger."
"Sad, sirs, sad!" said Capponi, looking his companions in the face, as
Strozzi passed out.
"Nay, I expect not that it will turn out anything serious," said Donati.
"The Strozzi are tender on the subject of poison," observed Messer
Giunigi, the fourth Florentine, under his breath, "since the death of
Madonna Luisa."
"Hush, sir, that touches me nearly too," gravely said Capponi, who
was of kin to Madonna Luisa's husband.
Here the Prior came forth, very irate.
"The Cardinal will none of my assistance," said he, "and yet I have
been held to know something. He is out of his head, and yet exacts

obedience as if he were himself. Not content with obstinately
refusing to lose blood, which would reduce the fever at once, and
leave him as cool as a cucumber, he insists that a courier on a fleet
horse shall instantly be despatched to Fondi for a certain Jew
physician, named Bar Hhasdai, in whom he has more faith than in all
the Christian leeches in Italy. The Jew hath never been baptised,
therefore I cannot consent to send for him."
"Nay, but," said Donati, solicitously, "if the Cardinal himself desires
him, I see not how you are exonerated from having him, baptised,
or otherwise."
"Send for him yourself, then," said the Prior; "you have plenty of
your own people."
"That will I readily," said Donati, and he left the refectory for that
purpose.
Those who remained behind, discussed the chances of the Pope's
sovereign remedy arriving in time to be of use, and talked over the
present political aspect of affairs in Rome, Florence, and Bologna;
and of the various deaths of the Medici—which was almost as dreary
a subject as their lives.
Meanwhile, there lay the poor Cardinal on his crimson satin
mattresses, with his once ruddy, handsome face, now pale as ashes,
pressed against a crimson satin pillow fringed with gold—nothing
white, nothing cool and comfortable about him—there he lay,
alternately flushing and chilling, torn with pain and languishing with
sickness and faintness—and all the while ideas were rushing through
his distracted head like clouds across a racking sky; and the one
predominant thought was, "Treachery! treachery!" Now, he who had
conspired, knew what it was to be conspired against. Oh! what a
long, long night! He scarcely knew or cared that people from time to
time looked in on him, stooped over him to hear if he breathed,
touched his heart, his wrist, drew the coverlet closer over him, and
went away. He scarcely knew or cared whether many were around
him or only the faithful Salviati. His thoughts were following a fleet

horse tearing along the road to Fondi, and striking sparks as it
clattered down the lava paved street. Then he seemed to see the
yellow-faced Jew, in a red night-cap, peering forth from one of the
high, unglazed windows, as the courier shouted out his name—and
behind him that Hebrew youth, whether son or acolyte, whom the
Cardinal had seen at his door in passing, only a few hours before,
with his pale, delicate face, and long, spiral curls, and look of
sadness and submission. How singular that that face, only once
seen, and seen for a moment, should have stereotyped itself on his
mind as the type of Isaac about to be sacrificed!—and now he
seemed to see him collecting medicines, while the old Jew hastily
threw on his furred gaberdine and came down to the door.
A din of wild church music seemed to come through the air, and to
wax insufferably loud, and then die wailing away like a requiem over
the Pontine marshes. And then, wild shouts of "Palle! palle!" and
citizens, half-dressed and half-armed, rushing through streets, and
some of them crying "Liberty! liberty at last!" And then there was an
awful, crushing struggle at a cathedral door; and partisans were
rallying round some one who was being borne into the sacristy; and
blood was flowing and swords were clashing, and all the while an old
pontiff at the altar, who seemed charmed into stone, was holding
aloft the consecrated wafer, and the little tinkling bell was
perpetually ringing till its shrillness seemed as if it would crack the
tympanum of his ears; and sweet childish voices were singing:—
"Et in terra pax! hominibus bonæ voluntatis!"
Then all melted away, and he was aware of a long, long suite of
marble halls, their silk and gilding covered with dust; and of an old,
old man with hoary hair borne through them in the arms of his
servants, and saying with a sigh, as he wistfully looked around
them:
"This is too large a house for so small a family!"
After this stalked the dread pageant of his sins—sins of omission and
sins of commission—sins that seemed so little once, and that

seemed so crushing now—and as he moved his weary head, gibing
faces seemed grinning and skinny fingers pointing at him round the
bed; and when he closed his burning eyelids, he seemed to see
them still, and to hear a voice say, "Son, thou in thy lifetime
receivedst thy good things."
Oh! where were the sacraments of the Church? Where were they?
Why did not some one think of them and bring them? Why had he
not voice enough to ask for them? or strength enough to sign for
them? And if he had, could they do him any good?
He knew not how time went. It seemed one long, long night, but in
fact it covered a few days. Bar Hhasdai arrived at last—he had been
absent when sent for. The Christian hangers-on scowled and spat on
him as he passed. He looked loftily down on them, and he passed
on; following the pale-faced Giovan Andrea. Pausing at the door, the
Jew looked full at him.
"I want a dog," said he.
"A dog?" repeated the steward, aghast.
"Yes: a four-footed one; not a Christian. And a roll of bread."
He passed into the sick room, where the faithful Salviati rose from
the Cardinal's bedside. The Prior, who was telling his beads, drew his
robe closer round him and retired as far from the Jew as possible.
Bar Hhasdai took up a lamp, and held it full in the Cardinal's
unwinking eyes.
"He does not see it," said he.
He laid the palm of his hand against his heart: then taking some
crumb of the roll the steward had brought him, he rubbed it against
his own face and offered it to the lapdog Giovan Andrea held under
his arm. The little dog immediately ate it.
"What next?" thought the steward, in wonder. The Prior stood
transfixed, curiously on the watch. Salviati's eyes had something

imploring in them: the faithful fellow had not once left his master,
and was now haggard with his long vigil.
The Jew silently took another piece of bread and rubbed the
Cardinal's clammy face with it: then offered it to the little dog. The
little dog smelt it, and resolutely refused to taste it.
"You see," said Bar Hhasdai, fixing the steward with his eye, "the
Cardinal is poisoned." Then, to the Prior, "Let him have the
sacraments of your Church."
Giovan Andrea reeled back, but recovered himself in time to escape
falling.
"Wretch!" exclaimed Salviati, springing towards him in rage and
despair; but Giovan Andrea glided like a serpent from beneath his
grasp, and clapped the door after him.
"He will not escape justice," said the Prior. "I have given orders that
he shall be watched."
Salviati cast himself on his expiring master in a paroxysm of grief. At
the sound of his wild cry, others rushed in: and the Jew quietly
passed out. Extreme unction was administered.
Thus perished the brilliant Ippolito de' Medici, who would deserve
more pity if he had not designed some very similar end for his
cousin Alessandro. He was abundantly regretted; for his
companionable qualities and lavish bounties had endeared him to a
very large circle of friends, who did not scan his faults too closely;
while his death was hailed with intense satisfaction by his enemies.
Paul the Third made a frivolous excuse for not sending him the
specific he so urgently requested. Probably it would not have saved
him; but the animus of his Holiness was not shown to his advantage
on the occasion.
As for the wretched Giovan Andrea, he made straight for the outer
gates when he quitted the Cardinal's chamber; but was there
collared by a stalwart lay-brother, who, with the assistance of two of
Ippolito's retainers, conveyed him to the lock-up room. Here he

remained a short time, in full anticipation of being put to the torture;
which too surely came to pass. At first he denied any guilt; but that
most odious process being persisted in, his agony at length wrung
from him the admission that he had administered poison to the
Cardinal, having ground it between two stones, which he had
afterwards thrown away.
Where had he thrown those stones?
Upon a rubbish-heap outside the buttery-window.
Search was made for the stones. They were found, with marks of
some foreign substance upon them. They were shown him: he said
they were the same.
The Cardinal's retainers were so enraged with the wretch, that they
were with difficulty restrained from falling upon him and putting him
to death. Felippo Strozzi had strongly charged his son to deliver him
out of their hands, that a regular judicial examination might take
place at Rome, and Alessandro's guilt, as the prompter of the crime,
be established.
The younger Strozzi, therefore, sent Giovan Andrea, under a
sufficient guard, to Rome, where his examination took place; and in
the first instance he confirmed his former confession, and stated
that he had received the poison from one Otto di Montacuto, a
servant of Duke Alessandro's, to be employed as he had used it.
Yet, after this, he denied both his former confessions, and, in spite
of all that Strozzi could say or do, was actually let off! He thereupon
went straight to Florence, and remained some days in the Duke's
palace, openly under his protection. He then retired to his native
place, Borgo di San Sepolcro, a little town under the Apennines,
some forty miles from Florence. And here, after remaining in safety a
few months, whether or no on account of any fresh proof of his
crime, he was stoned to death in a sudden outburst of popular
indignation.
As for the wicked Duke, his employer, I shall only say that his
murder was most horrible: so that Ippolito's death was amply

avenged. We may all be very glad to have done with the subject.

CHAPTER XVI.
THE DUCHESS AND THE MARCHIONESS.
It was given out to the world that Ippolito had been carried off by
fever, caught on the marshes during his hot ride to and from Fondi;
and this filled the tender-hearted Duchess with grief, as she knew
not but that, had she been at home, he might yet be alive. She
dwelt with mournfulness on his long-cherished attachment, wept
over his poems, recalled his brightest points, and even questioned
herself whether she ought to have accepted him; but the answer
always was no. And surely she was right; for whatever Ippolito's
society-attractions might have been, and however his character
might have been purified by household association with a better
nature, his worse qualities would undoubtedly have cropped out as
long as he remained an unconverted man. Might not she have
converted him? Why, Vittoria, who knew her best, would have told
you that, at this time, Giulia was not even converted herself. She
was very sweet, very amiable and charming; but she had not the
faith which saves. Vittoria, with her higher views and deeper nature,
was almost out of patience with her sometimes.
"What is it you want? What is it you need?" she would say to her;
trying to rouse her to a nobler life. "I can tell you: you want the Holy
Spirit; and He will come to you if you seek Him: but unsought, He is
unfound."
"O Vittoria! why will you torment me so?" said Giulia, fretfully. "I
want rest; I want peace."
"Rest and peace? Why, you have a great deal too much of both to be
good for you; and as for your lawsuit, that is a mere mosquito-sting,
that draws neither blood nor tears. Fie on you, Giulia! with all your
advantages, you ought not to sit and wail about nothing. I think you

loved Ippolito more than you say you did, or you would not give way
so."
"I did not love Ippolito at all," said Giulia, nettled. "I suppose one
may be sorry for a friend, without having been in love with him. You
do injustice to the memory of my dear Duke, to suppose I could ever
forget him."
"As to that," said Vittoria, "considering your good Duke's years and
infirmities, it is difficult for any one to see why you should be
inconsolable. I am sure I am quite ready to do justice to all his
qualities of head and heart; but, if I am to speak sincerely, I must
own that your deploring him in the way you have done has always
seemed to me a little exaggerated."
"I never asked you to speak sincerely," returned Giulia; "and people
generally make that a pretext for saying things that are
disagreeable. As for exaggeration, nobody possessed of any feeling
could consistently accuse me of having too much of it."
"I am the last person to make an inconsistent accusation," observed
Vittoria, "and my own irreparable and immense loss is too world-
known for any one to say I want feeling. I think, cousin, there is no
one in Italy, unless yourself, who has not compassionated me in
having been bereaved of my beloved, adored Pescara, a man of
infinite virtues, graces, and attractions; in war a hero, in wisdom a
sage; in love and constancy a perfect phœnix,—reft from me, me
wretched! in the very prime and flower of his life."
"Well, and I was very sorry for it," said Giulia, "as sorry as it was
possible to be for a man I had never seen, because I could feel for
you, cousin; and I went into the deepest mourning—"
"The outward garb has little to do with inward woe, Duchess," said
Vittoria, severely, "else I had worn weeds for ever"—and she
plunged into her pocket for her handkerchief.
"Well, and so should I have done, Marchioness," said Giulia. And
then they both burst into tears.

"Oh, Giulia," said Vittoria, in a stifled voice, after crying some time,
"why will you try me so?"
"Why, you began," said Giulia. And then they embraced, like Brutus
and Cassius; and Vittoria's good and kindly nature recovering its
ascendancy, she said with her charming smile:
"I really thank you, Giulia, for upsetting me, for I have wanted the
relief of a good cry for some time."
"You dear thing," said Giulia, kissing her—"that was just my feeling
too."
So, after this little squall, there was bright sunshine. And as this was
only a day or two before the 17th of August, when the Emperor was
expected to land on his return from Africa, Vittoria proposed to
Giulia that they should witness the procession together from the
balcony of a friend's palace in the best situation.
Giulia said half reluctantly, "I don't affect such worldly scenes much
—"
"Nor do I, certainly," said Vittoria. "But yet I should like to show my
loyalty to the Emperor; and the scene will not be a mere show, but
will have a kind of historic interest; and will doubtless figure
hereafter on the historic page. So that, if I go, surely you may."
"Ah, well, we will go together," said Giulia, who really liked the idea.
So these two illustrious ladies were among the fairest of the fair
whose eyes "rained influence" on the gay pageant; and, the same
evening, the staid, sober Emperor left the banquet early, and sought
out the widow of his brave though not blameless general, Pescara;
and he liked her so well, that the following year, when he and she
were in Rome, she was almost the only lady whom he condescended
to visit.
On the present occasion, Giulia was with her; and something
happening to be said by the Viceroy, Don Pedro di Toledo, who
accompanied the Emperor, about her roses having paled in
consequence of her vexatious lawsuit, Charles inquired into it, and in

his dry, succinct way, desired Don Pedro to see to it, and let the
affair be adjusted. So, when the Emperor was gone, the Viceroy
undertook the investigation of the rival ladies' claims; and the result
was, that he advised the Duchess to be satisfied with her ample
dowry, and the addition made to it by her husband.
This did not content Isabella, who laid claim to thirteen thousand
ducats for pin-money, and required that a judicial disposition she
herself had made should be declared void! She offered, as a set-off,
to give up five hundred ducats per annum to Giulia; but again
changed her mind. So that Giulia, nearly worried out of her life by
this unreasonable woman, again appealed to the Emperor, who
deputed a commission of three members of his council to give
judgment as the case required. This unpleasant affair extended
through great part of another year.
Nothing brings out the unromantic features of human nature so
unpleasantly as a lawsuit. Giulia was in a constant turmoil; and she
lacked those leadings to a better life, which Ochino might have
afforded her; for he had been summoned to Venice by Cardinal
Bembo, who was anxious to hear him.
This cardinal was not a good man, though I suppose there are good
cardinals now and then; however, he was at least a distinguished
man and a great scholar. And being an epicure in pulpit eloquence,
he wrote to Vittoria Colonna, begging her to use her known
influence with Fra Bernardino, to induce him to preach at Venice
during the ensuing Lent. Vittoria complied with his behest; and
Ochino consequently went to Venice, where the impression that he
made may be judged-of from the following passage in a letter from
the Cardinal to the Marchioness:
"I send Vossignoria notes of Fra Bernardino's sermons, to which I
have listened with a pleasure I cannot express. Certainly, I never
heard so capital a preacher, and I cannot wonder at your estimation
of him. He discourses in quite another manner from any one I have
ever heard; and in a more Christian spirit; bringing forward truths of

the utmost weight, and enforcing them with loving earnestness.
Every one is charmed with him: he will carry away all our hearts."
And again:
"I write to you, Marchioness, as freely as I talk to Fra Bernardino, to
whom I this morning opened my whole heart. Never have I had the
pleasure of speaking to a holier man. I ought to be now at Padua,
on account of a business which has engaged me all the year, and
also to get out of the way of the constant applications with which I
am assailed on account of this blessed cardinalate; but I could not
bear to lose the opportunity of hearing some more of his excellent
sermons."
And again:
"Our Fra Bernardino, whom I must call mine as well as yours, is at
present adored in this city. There is not a man or woman who does
not cry him up to the skies. Oh, what pleasure! oh, what delight, oh,
what joy has he not given! But I will reserve his praises till I see
Vossignoria, and meantime pray God to prolong his life for the glory
of the Lord and the good of man."
What a pity that this enthusiasm was so short-lived! Ochino was
soon afterwards chosen Director of the Capuchins. His influence over
his brother friars was then great; and many of them, before they
were well aware of it, became imbued with the reformed opinions.
Purgatory, penance, and papal pardons crumbled and fell before his
powerfully wielded hammer, the doctrine of justification by faith.
Side by side with him laboured Pietro Martire Vermigli, who
possessed more scholarship, and who, while Ochino filled the pulpit,
furthered the same cause by delivering lectures on the Epistles of St.
Paul. Many monks, many students, many nobles attended these
lectures. At length their tone became so different from that of the
Church, that the Viceroy interdicted him from preaching and
lecturing. But Pietro Martire appealed to Rome, and obtained the
removal of the interdict.

CHAPTER XVII.
ISCHIA.
Giulia was recruiting her health, meantime, at Vittoria's charming
island-home of Ischia,
"Where nothing met the eye but sights of bliss."
—where a graceful simplicity, indeed, reigned, but under the
regulation of the purest taste,—where duties, softened into
pleasures, filled up every hour; and where leisure, never
degenerating into laziness, was alternately dedicated to poetry,
music, and painting, to the enjoyment of the most exquisite beauties
of nature, to the cultivation of the mind, and to offices of charity and
devotion. Among the poets and eminent men who here "invoked the
muses and improved their vein," and who helped to make this
remote rock famous, were Musefilo, Filocalo, Giovio, Bernardo Tasso,
and many others. Bernardo Tasso thus sang the praises of this
charmed islet—

"Superbo scoglio, altero e bel ricetto
Di tanti chiari eroi, d'imperadori,
Onde raggi di gloria escono fuori,
Ch' ogni altro lume fan scuro e negletto,
Se per vera virtute al ben perfetto
Salir si puote ed agli eterni onori
Queste più d'altre degne alme e migliori
V'andran che chiudi nel petroso petto.
Il lume è in te dell' armi; in te s'asconde
Casta beltà, valore e cortesia,
Quanta mai vide il tempo, o diede il cielo.
Ti sian secondi i fati, e il vento e l'onde
Rendanti onore, e l'aria tua natia
Abbia sempre temprato il caldo e il gelo!"
Nor did younger and gayer poets want younger and gayer beauties
to inspire them than the two noble widows; for Vittoria's household
comprised six or eight nobly-born girls who were being trained under
her eye, and whom her conscientiousness prevented from turning
over to the sole superintendence of the Mother of the maids.
"You might take more interest than you do, Giulia," said she, "in the
education of your damsels. It would do them good, and you, too."
"Ah, nothing could be more tiresome to me," said Giulia. "I am most
happy to leave them to Donna Caterina!"
"I doubt, however," said Vittoria, "whether we have even the right to
keep fellow-creatures about us, of like affections and passions with
ourselves, without providing some legitimate outlet for them, or
supplying them with sufficient motives for their restraint."
"My girls seldom go into passions," said Giulia; "and I should think it
impertinent to inquire into their affections."
"Why now, you incorrigible Giulia, did not you tell me of your fits of
suppressed laughter while you were overhearing (actually eaves-

dropping) that love dialogue between Tebaldo and Isaura? and of
your laughing at her to her face, afterwards, in the presence of the
other girls?"
"I gave her a pearl necklace," said the Duchess.
"Not till she married, months afterwards."
"Well, I own I let myself down on that occasion."
"As to letting yourself down, it is your keeping yourself up that I
complain of—"
"O, what a beautiful butterfly!—"
"My dear Giulia, don't run after it and put yourself in a fever. You are
not quite a child now!"
"No, but I was a child once; and when I was a child-Duchess of
thirteen, I thought that if I did not keep my maids at a distance,
they would not respect me. And my mother's word had always been,
'Never associate, child, with servants.'"
"Servants and slaves, that may apply to very well," said Vittoria, who
had not surmounted class-prejudices, "but your maids-of-honour are
well-born, and though for a time they occupy subordinate positions,
eventually they will marry respectably, it is to be hoped."
"And that hope is enough to enliven them, I suppose," said Giulia.
"My dear Duke said to me, very soon after our marriage:
'Pargoletta!'—you know he loved to call me 'pargoletta,' or
'animetta,' or 'dolce alma mia,'—he said, 'Pargoletta, don't have
much to say to your maids; they are light and frivolous, and will do
you no good.' And I loved to obey him; and I love to obey him still,
for he was a wise man."
"They might do you no good, but you might do them great good
now," said Vittoria.
"O, my dear, that set have long married off, and had their portions—
so many ducats, a bed, bedding, and ewer and basin."

"The new set, then—"
"Here's a strawberry, I declare," said Giulia, diving into the leaves on
the bank upon which they were sitting. "Do have it!"
"No, thank you. The—"
"I could no more preach and pray with my maids as you do, Vittoria,
than I could fly!"
"Why not?"
"I should die of shame."
"Nonsense," said the Marchioness, laughing.
"I really should. It would be so ridiculous."
"Quite otherwise, I think, if you undertook it in the right spirit."
"But I never could. It is not in me. They would all begin to laugh—"
"They must be under very poor control, then," said Vittoria.
"Besides, it would be so uncalled for—it would take their thoughts
off their proper work."
"What is their proper work?"
"To do vast quantities of embroidery and fine needlework."
"Well, I think your proper work is to care for their souls."
"That's Fra Silvano's office."
"Does he fulfil it?"
"Not very well, I'm afraid. He chatters and laughs with them too
much."
"I should like to see him chatter and laugh with my maids," said
Vittoria, kindling. "He should not do so twice."
"Ah," said Giulia, after a pause—"I wish I were as good as you,
Vittoria—"
"My dear soul, I am not good."

"You are a great deal better than I am. Such as I am, I am and ever
shall be."
"Hush, we can none of us say that!"
"At any rate, there is no good thing in me, to impart to others. And
the girls do very well as they are—they stick to their needles."
"What do they think of the while?"
"Of their needles, I suppose."
"If they do, they are better than I am," said Vittoria, almost with a
groan. "Oh, Giulia, don't believe it!"
"Well, I suppose nonsense of some sort may pass through their
heads," said Giulia, rather uneasily. "How am I to keep it out?"
"By putting something better in. Not merely by preaching and
praying, but by supplying proper, innocent food for their
imaginations and fancies. You know I read my girls pleasant tales
and dialogues sometimes, and lend them books of poetry and
history."
"Well, your girls are certainly better conducted than mine," said
Giulia. "They giggle less."
"A canister with very little in it always rattles," said Vittoria. "I hate
giggling."
"So do I; and, do you know, my dear Vittoria, that is one reason why
I have so little to say to my maids."
"It is the very reason why you should say the more. You should fill
the canisters."
"I will try then," said the ingenuous Giulia, "when I return to Fondi."
She returned there very soon: and Vittoria Colonna went to Lucca;
"in an unostentatious manner," says the old chronicler, "attended by
only six gentlewomen."
Why she went to Lucca, except that it was just then rife with the
Reformed opinions, and ready to throw off the yoke of Rome, the

chronicler sayeth not. From Lucca she proceeded by easy stages to
Ferrara, mounted on her black and white jennet, with housings of
crimson velvet fringed with gold, and attended by six grooms on
foot, in cloaks and jerkins of blue and yellow satin. She herself wore
a robe of brocaded crimson velvet, with a girdle of beaten gold; and
on her head a travelling-cap of crimson satin, well becoming her
"trecce d'oro," and large, mild blue eyes.
Arrived at Ferrara, she was delightedly welcomed by Duke Ercole
and Duchess Renée. Here was a house divided against itself. The
poor Duchess—highly intelligent and a little crooked—now in her
twenty-ninth year, had been harshly dealt with by her husband, only
a twelvemonth back, for harbouring and comforting those arch-
heretics Calvin and Clement Marot; and was now kept very much in
check by the terrors of the Church, though in heart as much a
Reformer as ever.
To grace "the divine Vittoria," whose poetical fame was known all
over Italy, and whose eulogist, Bernardo Tasso, was secretary to the
Duchess of Ferrara, Duke Ercole invited the most distinguished
literati of Venice and Lombardy to meet her. Oh, what a feast of
reason and flow of soul! What reciprocations of compliments and
couplets! What ransacking of heathen mythologies for metaphors
and allusions! And then, in the retirement of the Duchess's closet,
poor Renée could, with a full heart, ask Vittoria how things were
going at Naples, whether Fra Bernardino were really as moving a
preacher as was reported, and whether Juan di Valdés were sound
on the doctrine of justification.
And perhaps they had a snatch of serious reading together, and
Vittoria might recite to her a few of her sacred sonnets, copies of
which were coveted even by cardinals; and if the Duke came in and
constrained them to change the subject, there was the clever little
Princess Anne to exhibit, who was being educated, for the sake of
emulation, with Olympia Morata. Certes, Vittoria was made much of!
But the air of Ferrara did not agree with her health, and she was
soon obliged to move southwards. Among the dreams and schemes

Welcome to our website – the perfect destination for book lovers and
knowledge seekers. We believe that every book holds a new world,
offering opportunities for learning, discovery, and personal growth.
That’s why we are dedicated to bringing you a diverse collection of
books, ranging from classic literature and specialized publications to
self-development guides and children's books.
More than just a book-buying platform, we strive to be a bridge
connecting you with timeless cultural and intellectual values. With an
elegant, user-friendly interface and a smart search system, you can
quickly find the books that best suit your interests. Additionally,
our special promotions and home delivery services help you save time
and fully enjoy the joy of reading.
Join us on a journey of knowledge exploration, passion nurturing, and
personal growth every day!
ebookbell.com

Introduction To Planetary Geomorphology Ronald Greeley

More Related Content

Similar to Introduction To Planetary Geomorphology Ronald Greeley

Recently uploaded

Introduction To Planetary Geomorphology Ronald Greeley