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A BRIEF INTRODUCTION
Kevin W. Plaxco and Michael Gross
The Johns Hopkins University Press Baltimore
© 2006 The Johns Hopkins University Press
All rights reserved.Published 2006
Printed in the United States ofAmerica on acid-free paper
9 8 7 6 5 4 3 2 1
The Johns Hopkins University Press
2715 North Charles Street
Baltimore,Maryland 21218-4363
www.press.jhu.edu
Library ofCongress Cataloging-in-Publication Data
Plaxco,Kevin W.,1965–
Astrobiology :a briefintroduction /Kevin W.Plaxco and Michael
Gross.
p.cm.
Includes bibliographical references and index.
ISBN 0-8018-8366-0 (hardcover :alk.paper) —
ISBN 0-8018-8367-9 (pbk.:alk.paper)
1.Life—Origin. 2.Exobiology. I.Gross,Michael,1963– II.Title.
QH325.P57 2006
576.8′39—dc22 2005027406
A catalog record for this book is available from the British Library.
CONTENTS
Preface vii
Acknowledgments xi
1 What Is Life? 1
2 Origins ofa Habitable Universe 14
3 Origins ofa Habitable Planet 38
4 Primordial Soup 61
5 The Spark ofLife 92
6 From Molecules to Cells 117
7 A Concise History ofLife on Earth 142
8 Life on the Edge 171
9 Habitable Worlds in the Solar System and Beyond 192
10 The Search for Extraterrestrial Life 218
Index 251
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PREFACE
Once upon a time,there was a scientific discipline called exobiology,the
science of extraterrestrial life.Cynics said that it was the only research
field with precisely no subject material to study.And thus,after a brief
period of fame—thanks partly to the pioneering research and public
outreach of Carl Sagan—exobiology fell out of fashion because it was
too narrowly defined and focused on things that we did not,and indeed
could not,know anything about.
Astrobiology, in contrast, removes the distinction between life on
our planet and life elsewhere.It focuses on the more fundamental,and
more tractable,question of the relationship between life and the Uni-
verse.* Not surprisingly,astrobiology tends to focus on life on Earth—
Terrestrial life is,after all,the only example we have on hand—but it at-
tempts to understand this single example in the broadest contexts ofthe
Universe.Using our vast (but incomplete) knowledge ofthe Terrestrial
biosphere,astrobiology addresses three broad questions about life in the
Universe:
(cid:1) What had to happen to allow the Universe to support life?
(cid:1) How did the origins and evolution of life transpire on Earth,and
how differently might they be transpiring elsewhere?
(cid:1) Where else might life have arisen in our Universe,what might it be
like,and how can we find it?†
Much ofthe worth ofastrobiology lies in the fact that,by most stan-
dards,these are perhaps the most fundamental questions addressed by
science today;they address the most profound issues regarding who we
are and where we came from. Still more worth comes from the truly
interdisciplinary approach that these questions demand; astrobiology
*Note that throughout this book we use Universe,Solar System,Sun,andMoon (and
Lunar) for our particular universe,solar system,star,and moon;similarly,we use Ter-
restrialto refer to something ofthe Earth,terrestrialto refer to any rocky planet—
though by extraterrestrialwe mean specifically not ofour Earth.
†And,in homage to the late Douglas Adams,author ofThe Hitchhiker’s Guide to the
Galaxy,“Will it buy me a drink?”
viii Preface
encompasses a variety of usually distinct scientific disciplines,ranging
from cosmology,astrophysics,astronomy,geology,and chemistry to,of
course,biology itself.
In this book we outline the current status ofastrobiology with only
the absolutely necessary amount of detail.We begin, in chapter 1, by
defining the object ofour study,life.While we can easily distinguish liv-
ing from nonliving systems here on Earth,setting up a definition that
encompasses all living systems in the Universe and unambiguously sets
them apart from all nonliving systems requires careful consideration.
Next (chapter 2) we need to investigate how the stage was set for life
to arise in our Universe. How did the Universe come into being, and
which of the crucial factors in its origins and early history distinguish
it from other possible universes that may not be able to harbor any life
at all?
From the vastness ofthe Universe we zoom inward,in chapter 3,to
explore the tiny blue dot that is our home planet and ask similar ques-
tions about it.Why did the third rock from the Sun turn into a living
planet,while its neighbors did not? Which conditions are necessary for
a planet—any planet,anywhere in the Universe—not only to become
and remain habitable but also to give rise to life and thence a rich and
diverse biosphere?
Zooming in by many more orders ofmagnitude,chapter 4 looks at
the molecular world present at the surface of the young Earth and in-
vestigates which chemical conditions and which potential chemical path-
ways could have set the stage for life to originate here.
So the Universe,the Earth,and the molecules are all set and ready to
go.But how did it actually happen? How did a habitable planet turn into
an inhabited planet? The short answer is we don’t know.Still,there are
partial answers to some ofour questions and constraints regarding oth-
ers,giving us a chance to spin some speculative scenarios in chapter 5.
Following the history oflife on our planet chronologically,in chap-
ter 6 we move on from the first spark ofself-replicating,evolving life to
the first cells and organisms.Again,we know very little about what re-
ally happened,but our current knowledge allows us to put constraints
on what may have happened here and risk an educated guess or two
about how it might have to happen anywhere.
The veil begins to lift somewhat when we come to the last common
ancestor of today’s organisms, a bacterium that was already quite
evolved and had DNA,RNA,and hundreds of distinct proteins.From
that point onward molecular phylogeny and,increasingly,paleontology
can help us to decipher the history oflife on Earth as a proxy for life in
our Universe,as we outline in chapter 7.
Preface ix
Is there life elsewhere in the Universe? Today,researchers are pon-
dering this question with a much more concrete and,in some regards,
more optimistic outlook than they did thirty years ago,when the Viking
missions failed to detect life on Mars.This newfound optimism is based
on the accumulating examples of organisms thriving in what we hu-
mans would consider extremely hostile conditions,such as at high pres-
sures,in water well above its nominal boiling point,or in extremely salty,
acidic,or alkaline brines.The study ofthese extremophiles,which we de-
tail in chapter 8,has had serious repercussions:the discovery of life in
many seemingly uninhabitable environments on Earth has radically ex-
panded our perceptions ofpossible habitats on other planets.The search
for life elsewhere in the Universe has thus become intimately connected
to the search for new habitats on Earth,a consideration that constitutes
an important element ofthe definition ofastrobiology,setting it apart
from the earlier,“extraterrestrial only”definition ofexobiology.
Having explored the origins and the limits oflife on Earth,in chap-
ter 9 we expand our view beyond our home planet and ask:if life can
thrive around hydrothermal vents in the deepest depths ofour oceans,
in the driest,coldest valleys of the Antarctic,and even deep within the
Earth’s crust, then which other places in the Solar System previously
deemed inhospitable might actually be inhabited?
Ultimately,speculation on where,or even whether,extraterrestrial
life exists frustrates the scientific mind,unless it can be followed up with
actual exploration ofthe potential habitats concerned.Thus,in chapter
10,we conclude our briefoverview ofastrobiology with a survey ofpast,
present,and future searches for our cosmic neighbors.
It is our hope that,by the end ofthis book,both cynics and enthu-
siast alike will be convinced that,unlike exobiology,astrobiology has a
subject matter to study; its subject matter is nothing less than an un-
derstanding ofour place in the Universe.
