How to Build a Habitable Planet

291 Pages·1985·19.294 MB·English

by  Wallace S. Broecker

-HOW TO BUILD
al / fe

HABITABLE /PLANET

WALLACE} S|BROECKER
Howto Build a
Habitable Planet
Wallace S. Broecker
This bookis distributedthrough the
Lamont-DohertyEarth Observatory
ofColumbia University
Address all inquiries to:

ELDIGIO PRESS
LAMONT-DOHERTYEARTH OBSERVATORY
PALISADES, NY 10964
First Printing - January [987
Second Printing - April 1998
ThirdPrinting- September 1998
Fourth Printing - February 2002
Copyright ©1985 by Wallace S. Broecker
Allrights reserved. Nopart of this bookmaybe reproduced by
any means without the written permission of the author.
Printed in the United States of America
Acknowledgements
The layout and typesetting for this book was done by
Computer Artistry
Tucson, Arizona
The cover design was done by
Scott Broecker
Contents
. The Setting: The Big Bang and Galaxy Formation 7
. The Raw Material: Synthesis ofElements in Stars 31
. The Heavy Construction: The Formation ofPlanets 67
. The Schedule: The Timing ofRaw-Material
Production and Heavy Construction 97
. Interior Modifications: Segregation into Core,
Mantle, Crust, Ocean, andAtmosphere 129
. Contending with the Neighbors: Moons,
Asteroids, and Comets 165
. Making It Comfortable: Running Water and
Temperature Control 197
. Storing Up Resources: The Ingredients ofa
Civilization 229
. Maintenance: Mankind at the Helm 259
Glossary 283
Credits 289
Index 291
To the late Paul Gast, who pioneered the trace-element
approachto planetary geochemistry. From the time he was

assigned byWheatonCollegeasmysophomore"bigbrother,’
untilhisdeath in 1973, Paul wasa friendandinspiration. He
rescued me from becoming an actuary and turned my
energies toward geochemistry. I owe much to him.
Foreword
This book is an outgrowth of an undergraduate course I
taught forstudents from ColumbiaCollege and Barnard Col-
lege. My approachwasto trace the developmentofthe Earth
from its roots in the Big Bang to its future in man’s hands.
At each stage I depict the important observational evidence
and attempt to show how it has been linked together into
hypotheses. I attempt to show that while in some casesthis
evidenceconstrainsustoawidelyaccepted single hypothesis,
in others the evidence is incomplete and permits a range of
competing explanations. It was my hopethatin this way I
could bring mystudents to see that scienceis far fromstatic.
Rather, itisa seriesofongoingstudieswherealltheevidence,
assumptions, and hypothesesare continually being reexam-
ined and where new information is regularly being added.
Manyreaders will surely ask why this book was not
published throughthe the usualchannels. Beyond myliking
forthe unconventional, there isan important financial reason.
Foreach booksold $2.50will be returned to the Department
of Geological Sciences as repayment of typing and drafting
costs. After a year of negotiations with various publishers I
found this to be the only way I could recover these costs.
Thanks to computer layout schemesandlaserprinters, it is
nowpossible to circumventtheveryhighoverhead associated
with conventional publishing.
I would like to thank Vicky Costello who patiently typed
and retyped the manydrafts of this manuscript, and Patty
Catanzaro who drafted and redrafted themanyfigures.Ialso
thank the ten or so scientists who read through the
manuscript and madevaluable suggestions. Finally, I thank
the students in Geology 1011xduringthe years 1981 to 1985.
They were the guineapigs as well as the inspiration for this
project.
Whirlpool Galaxy
Chapter One
The Setting:
The Big Bang and Galaxy Formation
Earth is a minor memberofa systemofplanets in orbit around
astarwe call theSun. TheSun isone ofabout100billionstars that
make up ourMilky Way. The lightfromthis myriadofstars allows
observers in neighboringgalaxies todefineourgalaxy’sspiralform.
Thegalaxy is the basic unitinto which universe matter issubdivided.
Likeits billionsoffellowgalaxies, oursisspeedingouton thewings
ofagreatexplosionthatgavebirth to the universe. That these ma-
jorpieces ofthe universe areflyingaway from each otheris reveal-
edby ashift toward redinthespectra ofthe light reaching us from
distantgalaxies. The close correlation betweenthe magnitude ofthis
shiftandthedistanceofthegalaxy tellsus thatabout15billionyears
ago all thegalaxies must have beenin oneplace. The catastrophic
beginning ofthe universe is heralded by a dullglow ofbackground
light. Thisglow istheremnantofthegreatflash thatoccurredwhen
thedebrisfromtheexplosioncooledto thepointwhere theelectrons
couldbecapturedintoorbitsaroundthe hydrogenandheliumnuclei.
Contained in the galaxies lying within the range ofour telescopes
are about 100billionbillion stars, A sizable numberofthese stars
are thought to have planetary systems. While planets with Barth's
highlyfavorable environmentare surely exceptions, it is difficult to
believe thatEarthis unique. Otherequallysuitable habitatsforlife
must exist.
8 Howto Build a Habitable Planet

Introduction
Whyare wehere? Are otherslike us to be found elsewhere?
Difficult questions indeed! Despite millennia of thought the
answersavailable todayarelittle more satisfyingthanthose that
were available toour distant ancestors. Newinsightshave been
morethancounterbalanced bynew puzzles. Religion tellsuswe
were created by God. Sciencetells us we were created by
chance. Yet neither theologians norscientists are quite happy
with these pat answers. Both probefor greater insight. Are we
God‘s only charges? Has chance operated successfully
elsewhere? Perhapssomeday God will revealhimselfor chance
will be duplicated in the laboratory. In the meantime, humans
will continue to puzzle and probe.
While weas yet have no way to know whetherliving beings
who matchor exceed our ability to appreciate, to reason, and
toexploit reside elsewhere in the universe, scienceisproviding
some very valuable constraints on such speculations. Two
approachesare being taken. Oneisgeological. It has to do with
thelikelihood thathabitatssuitabletolifehave comeinto being
elsewhere. Theotheris biological. Ithas todo withthelikelihood
that given a suitable environment,life will evolve. Although, to
date, only a few steps have been taken along these long roads,
important progress has been made.In this book I attempt to
summarize the success of the geologic approach.
In trying to assess the likelihood that other settings suitable
for intelligent life exist, we must firsthave an ideaofhowmany
planets and moonsthere are in the universe. This proves to be
an awkward question,because to date even planets associated
with our neareststellar neighborsare undetectable. Since they
do not glow, we cannotsee them. They aretoo tiny to create
shadowsor to measurably perturb their host star's path. New-
ly developed sensors mounted on the soon-to-be-launched space
telescope will, it is hoped, give us our first view of distant
planets.
Thus, the only planets we are sure about are thosecircling
our Sun. Excluding those objects that are less than 100 kilo-
metersin diameter (which, as weshall see, are clearly incapable
The Setting 9

of spawningintelligentlife}, there are nine planets and 40 or
so moons. These objectsgive everyappearanceofbeing the by-
products of the Sun's formation. This leads most astronomers
to believe that, at least for stars in the size class of the Sun,
planetsmaybetherule rather thanthe exception.Ifthisspecula-
tion is correct, then there may be as many or moreplanets as
there are stars! The total would be a staggering 10?°* objects.
To comprehend this number, think of groups with 100 billion
objects each. Therewould be billion such groups. One might
say at this point there is no need to speculate further; with so
manyplanets, there must be myriads ofthem on which condi-
tions favorable to intelligent life exist. Before wejump to this
conclusion, however, let us hold open the possibility that our
Earth is so unusual that there is only one chance in 10?! ofit
being duplicated. In this case, there would be only one chance
inten that another Earth exists. While notpromisingto be able
toprovide any useful evidence regardingthe actualmagnitude
of this probability, the evidence in the chapters thatfollow leads
us to believe that Earth conditions are not so improbable as to
make our planet unique. There must be hosts of planets that
have the critical prerequisites.
Table 1-1. Shorthand system for
expressing very large and very small numbers
1,000,000,000 1 x 10°
1,000,000 1 x 106
1,000 { x 108
1 1 x 10°
0.001 1x 1073
0.000001 1x 10~°¢
In order to assess the likelihood that Earth conditions are
duplicated elsewhere in the universe, we must go back to the
beginning. Even the very early events in the universeleft their
mark on the Earth.

“Ifyouarenotfamiliarwiththisnotation,pleasestudytheexamplesinTable 1-1.