Life in the universe

547 Pages·2012·35.004 MB·English

by  Jeffrey O Bennett, G Seth Shostak

Life
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THIRD EDITION
Jeffrey Bennett
UNIVERSITY OF COLORADO AT BOULDER
Seth Shostak
SETI INSTITUTE
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Library of Congress Cataloging–in–Publication Data
Bennett, Jeffrey O.
Life in the universe / Jeffrey Bennett, Seth Shostak. — 3rd ed.
p. cm.
Includes index.
ISBN 978–0–321–68767–8
1. Exobiology. 2. Life—Origin. I. Shostak, G. Seth. II. Title.
QH327.B45 2012
576.8’39—dc22
2010038371
ISBN-10: 0-321-68767-1
ISBN-13: 978-0-321-68767-8
Copyright ©2012, 2007, 2003 Pearson Education, Inc., publishing as Addison-Wesley
1301 Sansome St., San Francisco, CA 94111. All rights reserved. Manufactured in the
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1 2 3 4 5 6 7 8 9 10—CRK—13 12 11 10
DEDICATION
The quest to understand life on Earth
and the prospects
for life elsewhere in the universe touches on the most profound
questions of human existence.It sheds light on our origins,teaches
us to appreciate how and why our existence on Earth became
possible,and inspires us to wonder about the incredible possibilities
that may await us in space.We dedicate this book to all who wish
to join in this quest,with the sincere hope that knowledge will help
our species act wisely and responsibly.
All this world is heavy with the promise of
greater things,and a day will come,one day in
the unending succession of days,when beings,
beings who are now latent in our thoughts and
hidden in our loins,shall stand upon this earth
as one stands upon a footstool,and shall laugh
and reach their hands amidst the stars.
H.G.Wells (1866–1946)
Brief Contents
PART I PART IV
INTRODUCING LIFE IN THE UNIVERSE LIFE AMONG THE STARS
1 A Universe of Life? 1 11 Habitability Outside the Solar System 359
2 The Science of Life in the Universe 16 12 The Search for Extraterrestrial Intelligence 396
3 The Universal Context of Life 50 13 Interstellar Travel and the Fermi Paradox 435
Epilogue: Contact—Implications for the Search and
PART II
Discovery 476
LIFE ON EARTH
APPENDIXES
4 The Habitability of Earth 101
A Useful Numbers A-1
5 The Nature of Life on Earth 150
B Useful Formulas A-2
6 The Origin and Evolution of Life on Earth 192
C A Few Mathematical Skills A-3
D The Periodic Table of Elements A-12
PART III
E Planetary Data A-13
LIFE IN THE SOLAR SYSTEM
Glossary G-1
7 Searching for Life in Our Solar System 237
Photo Credits P-1
8 Mars 260
Index I-1
9 Life on Jovian Moons 293
10 The Nature and Evolution of Habitability 326
iv
Detailed Contents
Preface viii KEY ASTRONOMICAL DEFINITIONS 53
About the Author xiii COSMIC CALCULATIONS 3.1How Far Is a Light-Year? 57
How to Succeed in Your SPECIAL TOPIC 3.1How Do We Know That
the Universe Is Expanding? 62
Astrobiology Course xiv
MOVIE MADNESSThe Day the Earth Stood Still 69
COSMIC CONTEXTFigure 3.33. Interpreting a Spectrum 88
PART I
INTRODUCING LIFE IN THE UNIVERSE
PART II
LIFE ON EARTH
1 A Universe of Life? 1
1.1 The Possibility of Life Beyond Earth 2
4 The Habitability of Earth 101
1.2 The Scientific Context of the Search 4
4.1 Geology and Life 102
1.3 Places to Search 8
4.2 Reconstructing the History of Earth and Life 104
1.4 The New Science of Astrobiology 11
4.3 The Hadean Earth and the Dawn of Life 118
Exercises and Problems 14
4.4 Geology and Habitability 123
MOVIE MADNESSAvatar 10
4.5 Climate Regulation and Change 134
✺
2 The Science of Life in the Universe 16 4.6 THE PROCESS OF SCIENCE IN ACTION ❉
Formation of the Moon 141
2.1 The Ancient Debate About Life Beyond Earth 17
Exercises and Problems 147
2.2 The Copernican Revolution 25
COSMIC CALCULATIONS 4.1Radiometric Dating 112
2.3 The Nature of Modern Science 33
2.4 THE PROCESS OF SCIENCE IN ACTION ✺❉ KEY GEOLOGICAL DEFINITIONS 117
The Fact and Theory of Gravity 41 MOVIE MADNESS Ice Age: The Dawn of the Dinosaurs 118
Exercises and Problems 46
5 The Nature of Life on Earth 150
COSMIC CALCULATIONS 2.1Kepler’s Third Law 30
5.1 Defining Life 151
SPECIAL TOPIC 2.1Geocentrism and the Church 32
5.2 Cells: The Basic Units of Life 162
COSMIC CONTEXTFigure 2.15. The Copernican Revolution 36
5.3 Metabolism: The Chemistry of Life 169
MOVIE MADNESSCinema Aliens 40
5.4 DNA and Heredity 173
3 The Universal Context of Life 50 5.5 Life at the Extreme 179
✺
3.1 The Universe and Life 51 5.6 THE PROCESS OF SCIENCE IN ACTION ❉
Evolution as Science 183
3.2 The Structure, Scale, and History of the Universe 53
Exercises and Problems 189
3.3 The Nature of Worlds 70
KEY BIOLOGICAL DEFINITIONS 154
3.4 A Universe of Matter and Energy 80
✺ SPECIAL TOPICCharles Darwin and the Theory of Evolution 160
3.5 THE PROCESS OF SCIENCE IN ACTION ❉
Changing Ideas About the Formation COSMIC CALCULATIONS 5.1The Dominant Form of Life
on Earth 167
of the Solar System 90
Exercises and Problems 97 MOVIE MADNESS War of the Worlds 180
Detailed Contents v
6 The Origin and Evolution 9 Life on Jovian Moons 293
of Life on Earth 192
9.1 The Moons of the Outer Solar System 294
6.1 Searching for Life’s Origins 193
9.2 Life on Jupiter’s Galilean Moons 302
6.2 The Origin of Life 199
9.3 Life Around Saturn, and Beyond 310
✺
6.3 The Evolution of Life 208 9.4 THE PROCESS OF SCIENCE IN ACTION ❉
6.4 Impacts and Extinctions 216 Chemical Energy for Life 317
6.5 Human Evolution 223 Exercises and Problems 323
✺
6.6 THE PROCESS OF SCIENCE IN ACTION ❉ COSMIC CALCULATIONS 9.1The Strength of the Tidal Force 299
Artificial Life 226
MOVIE MADNESS2010: The Year We Make Contact 308
Exercises and Problems 234
COSMIC CALCULATIONS 6.1Bacteria in a Bottle I: 10 The Nature and Evolution of
Lessons for Early Life 203 Habitability 326
MOVIE MADNESSArmageddon 221 10.1 The Concept of a Habitable Zone 327
COSMIC CALCULATIONS 6.2Bacteria in a Bottle II: 10.2 Venus: An Example in Potential Habitability 330
Lessons for the Human Race 227
10.3 Surface Habitability Factors and the
Habitable Zone 337
PART III 10.4 The Future of Life on Earth 342
✺
10.5 THE PROCESS OF SCIENCE IN ACTION ❉
LIFE IN THE SOLAR SYSTEM
Global Warming 346
Exercises and Problems 356
7 Searching for Life in Our Solar System 237
COSMIC CALCULATIONS 10.1Chances of Being in the Zone 342
7.1 Environmental Requirements for Life 238
SPECIAL TOPIC 10.1Five Billion Years 345
7.2 A Biological Tour of the Solar System:
MOVIE MADNESSThe Time Machine 347
The Inner Solar System 244
COSMIC CONTEXTFigure 10.12. Global Warming 352
7.3 A Biological Tour of the Solar System:
The Outer Solar System 247
✺
7.4 THE PROCESS OF SCIENCE IN ACTION ❉ PART IV
Spacecraft Exploration of the Solar System 252
LIFE AMONG THE STARS
Exercises and Problems 258
MOVIE MADNESS2001—A Space Odyssey 249 11 Habitability Outside the Solar System 359
COSMIC CALCULATIONS 7.1Newton’s Version of Kepler’s
11.1 Distant Suns 360
Third Law 251
11.2 Extrasolar Planets: Discoveries and Implications 368
8 Mars 260 11.3 The Possibility That Earth Is Rare 382
✺
8.1 Fantasies of Martian Civilization 261 11.4 THE PROCESS OF SCIENCE IN ACTION ❉
Classifying Stars 386
8.2 A Modern Portrait of Mars 263
Exercises and Problems 393
8.3 The Climate History of Mars 276
8.4 Searching for Life on Mars 279 SPECIAL TOPIC 11.1 The Names of Extrasolar Planets 369
✺
8.5 THE PROCESS OF SCIENCE IN ACTION ❉ COSMIC CALCULATIONS 11.1Finding Masses of Extrasolar
Planets 373
Martian Meteorites 286
Exercises and Problems 290 COSMIC CONTEXTFigure 11.16. Detecting Extrasolar
Planets 376
COSMIC CALCULATIONS 8.1The Surface Area–to–Volume
Ratio 277 MOVIE MADNESSStar Wars 384
MOVIE MADNESSMissions to Mars 280 COSMIC CONTEXTFigure 11.22. Reading an H-R Diagram 388
vi Detailed Contents
12 The Search for Extraterrestrial Epilogue: Contact—Implications
Intelligence 396 for the Search and Discovery 476
12.1 The Drake Equation 397 Is There Life Elsewhere? 477
12.2 The Question of Intelligence 403 Exercises and Problems 484
12.3 Searching for Intelligence 408 MOVIE MADNESS E.T. 483
✺
12.4 THE PROCESS OF SCIENCE IN ACTION ❉
UFOs and Aliens on Earth 424
APPENDIXES
Exercises and Problems 432
SPECIAL TOPIC 12.1 Frank Drake and His Equation 400
A Useful Numbers A-1
COSMIC CALCULATIONS 12.1The Distance Between Signaling
B Useful Formulas A-2
Societies 402
C A Few Mathematical Skills A-3
MOVIE MADNESSContact 414
D The Periodic Table of Elements A-12
COSMIC CALCULATIONS 12.2Sensitivity of SETI Searches 417
E Planetary Data A-13
13 Interstellar Travel
and the Fermi Paradox 435
Glossary G-1
13.1 The Challenge of Interstellar Travel 436
Photo Credits P-1
13.2 Designing Spacecraft for Interstellar Travel 442
Index I-1
13.3 The Fermi Paradox 454
✺
13.4 THE PROCESS OF SCIENCE IN ACTION ❉
Einstein’s Special Theory of Relativity 465
Exercises and Problems 472
COSMIC CALCULATIONS 13.1The Rocket Equation 439
COSMIC CALCULATIONS 13.2Time Dilation 449
MOVIE MADNESSStar Trek 453
Detailed Contents vii
Preface
To the Reader
Few questions have so inspired humans through the ages as the dented, large-scale scrutiny of the sky for signals from other civi-
mystery of whether we are alone in the universe. Many ancient lizations, spurring heightened interest in the search for extrater-
Greek philosophers were confident that intelligent beings could restrial intelligence (SETI). Perhaps most important, scientists
be found far beyond Earth. When the first telescopes were have found the interdisciplinary study of issues related to the
trained on the Moon in the seventeenth century, some eminent search for life beyond Earth to have intrinsic value, independent
astronomers interpreted lunar features as proof of an inhabited of whether the search is ultimately successful.
world. Only a century ago, belief in a civilization on Mars be- Given the intense research efforts being undertaken by the
came so widespread that the term martianbecame synonymous scientific community and the long-standing public fascination
withalien.But despite this historical interest in the possibility with the search for life, it should be no surprise that the study
of extraterrestrial life, until quite recently few scientists devoted of life in the universe—also known as astrobiology—has become
any effort to understanding the issues surrounding it, let alone one of the most publicly visible sciences. Colleges, too, have
to making a serious search for life. recognized the growing importance of this discipline, and many
In the past couple of decades, however, a remarkable con- have begun to institute astrobiology courses. This book aims to
vergence of biology, geology, astronomy, and other sciences has serve such courses by offering a comprehensive introduction to
suddenly placed the issue of extraterrestrial life at the forefront the broad science of life in the universe.
of research. Advances in our understanding of the origin of life Although this book is a text, it is designed to be of interest to
on Earth are helping us predict the conditions under which life anyonewith a desire to learn about the current state of research
might arise in other places. Discoveries of microbes thriving in astrobiology. No special scientific training or background is as-
under extreme conditions (at least by human standards) on Earth sumed, and all necessary scientific concepts are reviewed as they
have raised hopes that life might survive even in some of the arise. If you have a basic high school education and a willingness
harsh environments found elsewhere in our own solar system. to learn, you are capable of understanding every topic covered in
Proof that planets exist around other stars—first obtained in the this book. We wish you well in your efforts.
1990s—has given added impetus to the study of the conditions
that might allow for life in other star systems. Technological Jeffrey Bennett
advances are making it possible for us to engage in unprece- Seth Shostak
To Current or Prospective Instructors 1. For students who take only one or a few required sci-
ence courses, the interdisciplinary nature of the study
The rest of this preface is aimed primarily at current or prospec- of life in the universe offers a broader understanding of
tive instructors teaching courses on life in the universe. Stu- the range of scientific research than can a course in any
dents and general readers might still find it useful, because it single discipline.
explains some of the motivation behind the pedagogical fea- 2. Public fascination with UFOs and alien visitation offers
tures and organization of this book and may thereby help you a unique opportunity to use life in the universe courses
get the most from your reading. as vehicles for teaching about the nature of science and
how to distinguish true science from pseudoscience.
Why Teach a Course on Life in the Universe? 3. The science of life in the universe considers many of the
most profound questions we can ask, including: What is
By itself, the rapid rise of research interest in astrobiology might life? How did life begin on Earth? Are we alone? Could
not be enough to justify the creation of new courses for non- we colonize other planets or other star systems?
science majors. But the subject has at least three crucial features Students are nearly always interested in these questions,
that together make a strong case for adding it to the standard making it easy to motivate even those students who
science offerings: study science only because it is required.
viii Preface
These features probably also explain the growing number the possibilities for life elsewhere in our solar system. Chapter 7
of life in the universe courses being offered at colleges around discusses the environmental requirements for life and then of-
the world as well as at the high school level. It’s worth noting fers a brief tour of various worlds in our solar system, exploring
that, besides being fascinating to students, a course on life in their potential habitability. Chapters 8and9focus on the places
the universe can be a great experience for instructors. The inter- that seem most likely to offer possibilities for life: Mars (Chap-
disciplinary nature of the subject means that no matter what ter 8) and the jovian moons Europa, Ganymede, Callisto, Titan,
your specific scientific background, you are sure to learn some- Enceladus, and Triton (Chapter 9). Chapter 10 discusses how
thing new when you teach an astrobiology course at any level. habitability evolves over time in the solar system, with empha-
sis on comparing the past and present habitability of Venus and
Using This Book for Your Course Earth; this chapter also introduces the concept of a habitable
zone around a star, setting the stage for the discussion of life
As courses in astrobiology began to appear, instructors faced an beyond our solar system in Part IV.
immediate challenge: The interdisciplinary nature of the sub- Part IV. Life Among the Stars (Chapters 11–13). This final
ject made it difficult to decide where emphasis should be placed. set of chapters deals with the question of life beyond our solar
Over time, however, a general consensus emerged in favor of a system. Chapter 11 focuses on the types of stars that seem
rough balance between the different disciplines that contribute suitable as “suns” for habitable planets, and then discusses the
to the study of life in the universe. This book was written to methods of detection and results of recent discoveries of ex-
serve that consensus course, and the success of our first two trasolar planets; it also covers the question of whether we
editions gives us confidence that we achieved that goal. We should expect Earth-like planets to be rare or common.
have maintained the same interdisciplinary approach for this Chapter 12 covers the search for extraterrestrial intelligence
third edition, while also responding to feedback from the many (SETI). Chapter 13 discusses the challenge of and prospects
users of the prior editions and updating the book with new de- for interstellar travel, and then uses these ideas to investigate
velopments in the science. With an interdisciplinary course goal the Fermi paradox (“Where is everybody?”), the potential so-
in mind, we now turn our attention to a few details that should lutions to the paradox, and the implications of the considered
help instructors use this book effectively. solutions.
COURSE TYPES This book is designed primarily for use in
PACE OF COURSE COVERAGE Although the chapters are
courses for nonscience majors, such as core course require-
not all of equal length, it should be possible to cover them at an
ments in natural science or elective follow-up courses for stu-
average rate of approximately one chapter per week in a typi-
dents who lack the preparation needed for more technical
cal 3-hours-per-week college course. Thus, the 13 chapters in
offerings in astrobiology. It can also be used at the senior high
this book should provide about the right amount of material for
school level, especially for integrated science courses that seek
a typical one-semester college course. If you are teaching a one-
to break down the traditional boundaries separating individual
quarter course, you might need to be selective in your cover-
science disciplines.
age, perhaps dropping some topics entirely. If you are teaching
a yearlong course, as might be the case at the high school level,
OVERALL STRUCTURE We’ve developed this book with a
you can spread out the material to cover it at an average rate of
four-part structure that matches the content of most courses on
about one chapter every 2 weeks.
life in the universe. The table of contents gives more detail; a
brief outline of the structure follows:
Part I. Introducing Life in the Universe (Chapters 1–3). New for the Third Edition
Chapter 1offers a brief overview of the topic of life in the uni-
verse and why this science has moved to the forefront of re- Astrobiology is a fast-moving field, and there have been many
search. Chapter 2discusses the nature of science based on the new developments since we wrote the second edition. You will
assumption that this is many students’ first real exposure to how therefore find many sections of the book almost entirely rewrit-
scientific thinking differs from other modes of thinking. Chap- ten, though we have retained the basic organization of the text.
ter 3presents fundamental astronomical and physical concepts Here, briefly, is a list of some of the most important changes and
necessary for understanding the rest of the course material. updates we have made:
Part II. Life on Earth (Chapters 4–6). This is the first of three
• We have significantly expanded our coverage of light and
parts devoted to in-depth study of astrobiology issues. Here we
spectroscopy; see, for example, new Figures 3.31 to 3.33
discuss the current state of knowledge about life on Earth.
and the associated narrative.
Chapter 4 discusses the geological conditions that have made
Earth habitable. Chapter 5explores the nature of life on Earth. • We have revised our discussion of the Hadean Earth based
Chapter 6 discusses current ideas about the origin and subse- on new research indicating that large impacts of the heavy
quent evolution of life on Earth. bombardment are less likely to have been sterilizing than
Part III. Life in the Solar System (Chapters 7–10). We next previously thought. We have similarly updated our discus-
use what we’ve learned about life on Earth in Part II to explore sions of snowball Earth episodes.
Preface ix