Life in the Universe: Expectations and Constraints

259 Pages·2008·4.894 MB·English

by  Dirk Schulze-Makuch, Louis N. Irwin (auth.), Dirk Schulze-Makuch, Louis N. Irwin (eds.)

Life in the Universe
Advances in Astrobiology and Biogeophysics
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·
Dirk Schulze-Makuch Louis N. Irwin
Life in the Universe
Expectations and Constraints
Second Edition
123
DirkSchulze-Makuch LouisN.Irwin
WashingtonStateUniversity UniversityofTexasatElPaso
SchoolofEarthand Dept.BiologicalSciences
EnvironmentalSciences 500WestUniversityAvenue
Pullman,WA99164-2812 ElPaso,TX79968-0519
USA USA
[email protected] [email protected]
ISBN:978-3-540-76816-6 e-ISBN:978-3-540-76817-3
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Toaspeciesoflifeonawater-richplanetin
anotherwiseunremarkableSolarSystemin
anoutlyingpartofthegalaxyinan
unexceptional partoftheuniverse,just
beginning toexploretheworldbeyondthe
homeonwhichitbegantoevolveabout
4billionyearsago.
ThroughthecuriouseyesofachildweexploretheUniverse
Preface
Life on other worlds has been a source of speculation since ancient times. Exo-
biology and its derivative form, Astrobiology, now form a significant part of the
scientificprogramforallthespaceexploringnations.RussiaandtheUnitedStates
ofAmerica,thepioneersinspace,havebeenjoinedbytheEuropeanSpaceAgency,
both independently and collaboratively, with a host of new missions. In Asia, the
Japanese have entered the exploration of space, and the Chinese have launched a
human into Earth orbit. The International Space Station and the Cassini-Huygens
Mission to Saturn testify to the vigor and maturity of the international effort in
space, much of which is geared toward the study of life in space and on other
worlds.
With this upsurge of interest has come a host of books, mostly for a broad au-
dience,andoftenforpopularconsumption.Nearlyallhavebeenenthusiasticabout
the possibility of life on other worlds. But scientific depth has often been sacri-
ficed in favor of the laudable goal of engaging a broad audience. This book em-
braces the conviction that life is not restricted to our planet. But, our objective is
toanalyzeincriticalscientificdetail,thefundamental,commonlyheldassumptions
aboutlifebeyondEarth-particularlythoserelatingtotheprobablecosmicprefer-
ence for carbon-based life, the overwhelming focus on water as the preferred sol-
ventforlife,andtherelativemeritsofdifferentformsofenergyforthesustenance
oflife.
Sincethefirsteditionofthisbookwaspublished,rovershavecombedthesurface
of Mars for ever growing evidence of oceans on the Red Planet, a lander has con-
firmed a hydrocarbon-soaked sandy basin fed by liquid-cut channels through hills
onTitan,anorbiterhasseennewgeysersonseveralofSaturn’sfrozensatellites,and
theingeniousobservationaltoolsofnumeroustalentedastronomershavedisclosed
over 300 new planets beyond our Solar System. None of these discoveries has di-
minished our conviction that life is likely on other worlds somewhere, perhaps as
weknowitonEarth,andperhapsnot.Onthecontrary,wehaveeveryevidencethat
the universe we see is physically and chemically consistent, and compatible with
lifeinavarietyofforms-someperhapsbeyondourimagination.
MissionstoMarsareinthepipelineeverytwoyearsfortheforeseeablefuture;
another mission to the icy moons of Jupiter is on the drawing board; and sample
returnmissionstoVenusareevenbeingdiscussed.Thesebilliondollarprojectswill
vii
viii Preface
allrelyonremotedetectiontechnologies.Itiscriticallyimportantthatwesearchin-
telligentlyandcomprehensivelyfortheappropriatebiosignaturesandgeoindicators
thatcouldmarkorsuggestthepresenceoflife.Thisbookiswrittenwithaparticu-
laremphasisonthescientificrationaleforwhatweshouldbeseeking,andhowwe
shouldbelookingforit.
Wehavealsotriedtowriteabookthattreatsbiologywithgreatersophistication
thanmostmonographsinastrobiology.Certainassumptionscommonlyrecurinthe
astrobiological literature: Life originates quickly once it has the opportunity to do
so, but takes a long time to evolve to multicellularity. Most extraterrestrial life is
probablymicrobial.Darwinianevolutionisarequirementforlife.Theevolutionof
intelligenceisanimprobableevent.Someoftheseassertionsarevalid;othershave
littlefoundation.Wehavedrawnonevolutionaryandecologicaltheorytocritically
evaluate these and related issues, and have documented our arguments so that the
skepticalreadercanpursuethemtotheirsource.
Ourfirsteditionwaswrittenwiththeviewthatarigorousscientificapproachto
the possibility of life on other worlds could substantiate many of the more vague
and intuitive notions held by scientists and the public alike, while placing rational
constraintsonmanyoftheunderlyingassumptionswithinthegrowingfieldofas-
trobiology.Whilegratifyingreviewsofthefirsteditionindicatethatoureffortwas
largelyasuccess,werecognizedfromtheonsetsomegapsandweaknessesinour
treatment that a second edition would enable us to remedy. Toward that end, we
have added a substantial amount of content, while updating our understanding of
theuniverse,fromournearestneighborsintheSolarSystem,tothefurthestreaches
ofourobservationalcapabilities.
This new edition provides a more sophisticated treatment of the definition of
life- anessential platformforallthatfollows.Our original aim,todefine aliving
system in terms of its irreducible features and in a form that would be useful in
guidingitsdetectionwhereveritmightbefound,hasbeenretained.Asmallamount
of wording has been added to the definition for clarity, but we continue to decline
thetemptationtodefinelifeasanabstractconceptdrawnmerelyfromacollection
ofcharacteristics.
AbetterunderstandingofhowlifearoseonEarthwouldprovideinvaluablein-
sightsintotheplausibilitythatlivingsystemscouldariseunderagivensetofcon-
ditionsonotherworlds.Whilesidesteppingthequestionoftheoriginoflifeinthe
first edition in deference to the breadth and quality of work by others in that area,
we have concluded that some consideration of this topic is essential for a treatise
inastrobiology,inasmuchastheoriginoflifeispartofthedefiningscopeofthe
field.Webynomeanshaveendeavoredtobecomprehensive,buthavetriedtogive
afairsynopsisofthecriticalissues,includingourtakeonwhatconsensusmayexist
concerningtheoriginoflifeatthepresenttime.
To our treatment of lessons from the history of life on Earth, we have added
asignificantlyexpanded reviewoftheextremophilicorganismsthatliveunderthe
harshestconditionsonourplanet.ConditionsthatareharshonEarth,ofcourse,may
be normative on other worlds. The desiccated surface of Mars, the acidic clouds
of Venus, the volcanic convulsions of Io, and the frigid subsurface of Europa are
Preface ix
examples of possible habitats in our neighborhood of the Solar System where or-
ganisms could persist with a semblance of the features revealed by the study of
extremophilesonEarth.
The fate and future of living systems is supposed to be another of the defining
subjects in the field of astrobiology. While popular treatments of the topic can be
found,aseriousscientificattempttomodelgenericscenariosforthelifecycleoflife
itselfarealmosttotallymissing.Wehavenowaddedachapteronthistopic,which
has not only philosophical import, but vital relevance to our survival as a species
and the persistence of life on Earth. Astrobiology, after all, is not about alien life
alone.
Inthisedition,wealsodecidedtoexamineingreatercriticaldetailtheresultsof
theVikingMission,whichstandsasanearlyheroicifflawedattempttodetectlife
on Mars, and the intense scrutiny twenty some years later of the alleged evidence
forpastlifeonMarspresentedbythemeteoriteALH84001.Thesetwoprojectsare
uniquely instructive because they represent direct attempts to deduce evidence for
alienlife.Inadditiontotheirhistoricalvalue,theypointoutthechallengethatwill
likelyconfrontusinassessingtheevidencethatwecanrealisticallyexpecttocollect
foratleastthenear-termfuture.
Finally,wehaveaddedmaterialoncosmicbiogeography-revisingslightlyour
plausibilityratingsforlifeonseveralbodiesinourSolarSystem-andupdatedthe
chapter on optimizing space exploration and the search for alien life in the light
of current and foreseeable technology. Included is an emphatic restatement of our
beliefthatroboticandhumanexplorationofthecosmosarebothscientificallynec-
essaryandinherentinournature.
The first edition of this book had its origins in a student seminar, and the pub-
lished version has been tested again by student use at both the undergraduate and
graduate levels. We thank all our students, as before, for their valuable contri-
bution. Likewise, we acknowledge with gratitude the helpful feedback and com-
mentsofmanycolleagues,includingespeciallyfruitfuldiscussionsandinputfrom
Penelope Boston, Alfonso Davila, James Dohm, Alberto Faire´n, Wolfgang Fink,
DavidGrinspoon,KevinPlaxco,AntjeRusch,PhillipRust,andHelgaStan-Lotter.
Dirk Schulze-Makuch would like to thank his wife, Joanna, and his children
Nikolas, Alexander, Alicia, and Kristian for their patience and understanding. He
also thanks all his students and colleagues for their willingness to bounce around
ideas.LouisIrwinlikewiseappreciatestheconstantencouragementandsupportof
hiswife,Carol,andthegraduatestudentswhomhehadneglectedonceagaininthe
courseofcompletionofthis2ndedition.
ElPaso,TX DirkSchulze-Makuch
LouisN.Irwin
Contents
1 Introduction................................................... 1
2 DefinitionofLife............................................... 7
2.1 ProblemswithCommonAssumptionsabouttheNatureofLife .... 7
2.2 HistoricalViewsontheDefinitionofLife ...................... 12
2.3 ModernDefinitionsofLife................................... 13
2.4 ThermodynamicCriteria..................................... 14
2.5 BioinformaticCriteria....................................... 15
2.6 EvolutionaryCriteria ....................................... 16
2.7 LifeasaGlobalEntity ...................................... 16
2.8 LifeasaCollectionofAttributes.............................. 17
2.9 AUtilitarianandGenericView ............................... 17
2.9.1 Self-organizing and Bounded Environments
inThermodynamicDisequilibrium...................... 18
2.9.2 TransformationofEnergytoMaintainaLowEntropy
StateandPerformWork............................... 20
2.9.3 InformationEncodingandTransmission................. 21
2.10 ImplicationsfortheRemoteDetectionofLife................... 22
2.11 ChapterSummary .......................................... 24
3 OriginofLife.................................................. 25
3.1 ScenariosfortheOriginofLife............................... 25
3.1.1 ALukewarmMarineOriginofLife..................... 27
3.1.2 ABenthicThermophilicOriginofLife .................. 28
3.1.3 AnIceWaterOriginofLife ........................... 28
3.1.4 AComprehensiveScenariofortheOriginofLife ......... 29
3.2 InferencesfortheFirstCellularMembranes .................... 32
3.3 InferenceswithRegardtotheFirstMetabolism ................. 33
3.4 InferencesfortheFirstReplicationMechanism ................. 35
3.5 OtherInferences ........................................... 37
3.5.1 Size ............................................... 37
3.5.2 EnvironmentalConditions............................. 37
3.5.3 Medium............................................ 38
xi
xii Contents
3.5.4 MineralsandSubstrates............................... 39
3.5.5 ImplicationsforthePossibilityofLifeonOtherWorlds .... 40
3.6 ChapterSummary .......................................... 41
4 LessonsfromtheHistoryofLifeonEarth ........................ 43
4.1 ABriefHistoryofLifeonEarth .............................. 43
4.2 LessonsfromtheHistoryofLifeonEarth ...................... 45
4.2.1 LifeArisesRelativelyQuicklyunderConduciveConditions 45
4.2.2 LifeTendstoStaySmallandSimple.................... 46
4.2.3 Most Organisms Remain Relatively Unchanged
overtheirEvolutionaryLifeSpans...................... 47
4.2.4 EvolutionisAcceleratedbyEnvironmentalChanges....... 49
4.2.5 ComplexityInevitablyIncreasesbutastheException
ratherthantheRule .................................. 50
4.2.6 BiodiversityisPromotedbyHeterogeneousEnvironments.. 51
4.2.7 IndividualsareFragile,butLifeisHardy ................ 52
4.3 AdaptationstoExtremeEnvironments ......................... 52
4.3.1 TemperatureExtremes................................ 54
4.3.2 ExtremepH-Conditions............................... 56
4.3.3 LowAvailabilityofWater ............................. 57
4.3.4 LowOxygenAvailability.............................. 59
4.3.5 Pressure............................................ 60
4.3.6 Radiation........................................... 61
4.3.7 LowNutrientAvailabilityandChemicalExtremes ........ 62
4.4 QuestionsUnansweredbytheHistoryofLifeonEarth ........... 63
4.5 ChapterSummary .......................................... 64
5 EnergySourcesandLife ........................................ 65
5.1 LifeAsWeKnowIt ........................................ 65
5.1.1 Oxidation-ReductionChemistryasanEnergySourceforLife 66
5.1.2 LightasanEnergySourceforLife...................... 68
5.2 LifeAsWeDon’tKnowIt................................... 68
5.2.1 ElectromagneticWaves(otherthanVisibleLight) ......... 69
5.2.2 ThermalEnergy ..................................... 70
5.2.3 KineticEnergy ...................................... 73
5.2.4 OsmoticorIonicGradients ............................ 74
5.2.5 MagneticFields ..................................... 77
5.2.6 GravitationalForces.................................. 81
5.2.7 TectonicStress ...................................... 82
5.2.8 PressureGradients ................................... 82
5.2.9 SpinConfigurations .................................. 83
5.2.10 Radioactivity........................................ 84
5.3 TheQuestionofEntropy,Uniformity,andOrigin ................ 85
5.4 SurveyofEnergySourcesinourSolarSystem .................. 86
5.5 ChapterSummary .......................................... 88