Persistent or repeated surface habitability on Mars
during the Late Hesperian - Amazonian
EdwinS.Kite1,*,JonathanSneed1,DavidP.Mayer1,SharonA.Wilson2
1.UniversityofChicago(*[email protected])
2.SmithsonianInstitution.
Abstract.
LargealluvialfandepositsonMarsrecordthemostrecentundisputedhabitablewindowof
surfaceconditions(≲3.5Ga,LateHesperian–Amazonian).Wefindnetsedimentationrate
<(4-8)μm/yrinthealluvial-fandeposits,usingthefrequencyofcratersthatareinterbedded
withalluvial-fandeposits.Consideringonlytheobservedinterbeddedcraterssetsalower
boundof>20Myronthetotaltimeintervalspannedbyalluvial-fanaggradation,>103-fold
longerthanpreviouslowerlimits.Amorerealisticapproachthatcorrectsforcratersfully
entombedinthefandepositsraisesthelowerboundto>(100-300)Myr.Severalfactorsnot
includedinourcalculationswouldfurtherincreasethelowerbound.Thelowerboundrules
outfan-formationbyabriefclimateanomaly.Therefore,duringtheLateHesperian–
AmazonianonMars,persistentorrepeatedprocessespermittedhabitablesurface
conditions.
1.Introduction.
LargealluvialfansonMarsrecordoneormoreriver-supportingclimateson≲3.5GaMars
(e.g.Moore&Howard2005,Grant&Wilson2012,Kiteetal.2015,Williamsetal.2013)(Fig.
1).Thisclimatepermittedprecipitation-sourcedrunoffproductionof>0.1mm/hrthatfed
riverswithdischargeupto102m3/s(Palucisetal.2014,Morganetal.2014).Thelarge
alluvialfanscorrespondtotheyoungest(Grant&Wilson2011)unambiguousevidencefor
Marssurfacehabitability.Althoughsmall(<10km2)alluvialfanswith>10°slopesformed<5
MyaonMars,theseneednotrecordahabitableenvironment(e.g.Williams&Malin2008),
andmightnotbetheresultofliquidwater(Pilorget&Forget2016).Bycontrast,large(>10
km2)alluvialfanswith≲2°slopesformedfromaqueousflows(Williamsetal.2013),and
thesefansaremoredeeplyerodedandmorecratered,sotheyareolder.Inthispaperwe
focusonthelargealluvialfans.Didtheyresultfromasingleanomalousburstofwet
conditions,suchasmightresultfromanimpact(Williams&Malin2008)orvolcanic
eruption?Or,dothefansrecordpersistentorrepeatedwetconditions,forexampleasthe
resultofasustainedwarmerclimateregime?Betterconstraintsonthetimespanofalluvial
fanformationwouldconstrainmodelsofLateHesperian–Amazonianclimate.
Previousworkonthedurationoftheintervaloffanbuild-upusedsedimentologytoestimate
thetimeoverwhichsedimenttransportoccurred(e.g.,Armitage2011,Williamsetal.2011,
Palucisetal.2014).Thesesedimentologicmethodsrequireassumptionsaboutflow
intermittencyorsediment:waterratio,which(foralmostallMarsalluvialfans)arepoorly
constrained.Therefore,thelowerlimitsobtainedfromsedimentologicalmethodsareshort–
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forexample,~3600years(Morganetal.2014).Moreover,brief(1-100yr)aggradation
intervalshavebeenproposedfordeltasonMarsthataresimilartothealluvialfansinage
andvolume(Kleinhansetal.2010,Hauberetal.2013).Anotherapproachtoestimatingthe
intervalofalluvial-fanbuild-upistomeasurethedensityofcraterssuperimposedon
differentfans,andusethespreadofcrater-retentionagesforthefansurfacesasaproxyfor
therangeoffan-formationages.Thismethodisnotreliable,becausecrater-retentionages
forareas<103km2arenotreliable(Warneretal.2015).Therefore,thetimespanof
habitableclimatesintheLateHesperian-Amazonianremainsanopenquestion.
Figure1.LargealluvialfansonMars(28°S333°E).The≲3.5Gaageofthefansisshownby
theirlowcraterdensity.Aeolianerosionexposeslayersandchannelswithinthedeposit.
2
A:
B:
Figure2.Idealizedcross-sectionthroughanalluvialfandeposit.Craterdensitywithin
alluvialfansmaybeestimatedusingthefrequencyofvisibleinterbeddedcratersatthe
exposedsurface.CaseA:Ifpostfluvialerosionwasmodest,somecratersthatformedlatein
thehistoryofalluvial-fanaggradation(filledsemicircles)areonlypartiallyburiedandare
visibletoday(thoughoutnumberedbypostfluvialcraters;outlines).CaseB:Ifpostfluvial
erosionwassevere,thearealdensityofexposedembeddedcratersofagivendiameteris
stillproportionaltothevolumetricdensityofthosecraters.Ineithercase,assumingsteady
aggradation,acountofinterbeddedcratersconstrainsthefanaggradationrate.
Togetamoreaccurateestimateoftheintervaloverwhichalluvialfansformed,weusedthe
embedded-cratermethod(Hartmann1974,Kiteetal.2013a).Thismethodworksasfollows.
Craterdensityonaquiescentplanetarysurfaceisproportionaltoexposureduration.This
methodmaybeextendedtothreedimensions:thetotalnumberofcratersinterbedded
withinasedimentarydepositisproportionaltothetimespannedbyactivesedimentation
(includinganyhiatuses)(Fig.2).Thegreaterthevolumetricdensityofcraters,thelongerthe
timespanofdeposition.Ofcourse,manyormostoftheinterbeddedcratersmaybe
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completelyburied(Fig.2);thereforeacompletecountofinterbeddedcratersisusually
impossible.However,ifanimpactoccursneartheendofactivesedimentation,thenthe
correspondingcratermaybeonlypartiallyburied.Smallercratersaremorereadilyburied,
andlargercratersrequiremoresedimenttobecompletelyobscured.Thetimeneededto
accumulatethepopulationofvisiblyembedded(synfluvial)cratersis
τraw,D=ND/(fDa) [1]
whereDistheminimumcraterdiameterofinteresttobeconsidered,τraw,Distheminimum
timerequiredtobuilduptheobservedembedded-craterpopulation,NDisthenumberof
observedembeddedcraters,fDisthepastcraterflux(#/km2/yr),andaisthecountarea
(km2).Prefluvialcraters(whichareoverlainbyalluvialfandeposits,butthatformedbefore
thestartoffluvialdeposition;Irwinetal.2015)areexcluded.Thisproceduregivesastrict
lowerlimitontheintervaloffanformation;itdoesnotaccountforcratersthatarefully
entombedwithinthedeposit.Tocorrectforfullyentombedcraters,wecanassumesteady
aggradationanddividefanthicknessZbybest-fitaggradationrateWtogetdurationoffan
formationτsteady:
WD≈1.33Dφ/τD [2]
τsteady,D=Z/WD
[3]
Thenumeratorin[2]correspondstotherequiredburialdepthforobliteration.Theamount
ofburialthatisrequiredisconstrainedbythegeometryofsmallimpactcraters(Melosh
1989,Wattersetal.2015).φistheobliterationdepthfractionforagivencrater,expressed
relativetocraterdiameter.(Inthispaper,wedefineacrateras“obliterated”ifitcanno
longerbeidentifiedinahigh-resolutionopticalimage;Kite&Mayer2016).Thefactorof
1.33correctsforthefactthat,foranyminimum-diameterD,themediandiameterinacount
willexceedD–thus,Dφisanunderestimateoftherequiredburialdepth.Thiscorrection
dependsonthecraterproductionfunctionused.Thecorrectionisrelativelysmall(1.3×–
1.5×)inoursizerangeofinterest,becausecraterfrequencyfallsoffsteeplywithincreasing
diameter,andwerepresentitbyafixedfactor.Post-depositionalerosionofalluvialfan
depositsdoesnotaffectthisprocedure.Arandomlyorientedcutthroughthecratercontainingvolumeintersectseachcraterwithaprobabilityproportionaltothatcrater’ssize;
thesampleofcraterspartiallyexhumedatanerosionalsurfaceisbiasedtowardslarger
impacts.Justaswithpartialburial,thenumberofcratersthatareexposedisproportionalto
thevolumetriccraterdensity(andinverselyproportionaltoaggradationrate).Thus,the
volumetricdensityofinterbeddedcratersmaybeestimatedfromsurfacecountsbothon
pristinefansurfacesandforfansthataredeeplyeroded(Kiteetal.2013a).Therefore,
embedded-cratercountscanbeusedasaMarsfluvialprocessspeedometer.
2.Methodsandresults.
Inordertosetalowerboundonthetimespanofalluvial-fanformation,wesearched1.7×
104km2ofpreviously-cataloguedfans(correspondingtomostofthesurfaceareaoflarge
alluvialfansonMars;Wilsonetal.2012)using6m-per-pixelCTXimagestoscoutfor
candidateembeddedcraters.Candidatecratersshowpossibleevidenceofinterbeddingwith
4
paleochannelsorotherfandeposits.Eachcandidatefeaturewasreviewedbythreeofthe
authors(E.S.K,D.P.M.,andJ.S.)forfinalclassification.Whereavailable,25cm-per-pixel
HiRISEimagesandanaglyphs,plusCTXDigitalTerrainModels(DTMs)wereusedtoinspect
candidatesflaggedintheinitialCTXsurvey.HiRISEanaglyphsprovedparticularlyvaluable
fordetectingcraterrims.Eachcandidatefeaturewascategorizedasqualitylevel1,2,3
(representingdecreasingconfidencethatthecraterwasembedded),oritwasdiscarded.A
totalof25embeddedcraterswerefoundat<30°latitude(D=0.08-5.0km;Fig.3,
SupplementaryInformation).Theseembeddedcraterswerethenclassified(usuallywiththe
aidof24-m-per-pixelCTXDTMs)as“synfluvial,”“uncertain”,or“prefluvial.”Thesecraters
constituteatinyfractionofthetotalnumberofcratersonthesurfacesofthefans(Grant&
Wilson2012).
Figure3.EmbeddedcraterswithinalluvialfansonMarsfromourcatalog(Supplementary
Information).
AsupplementaryHiRISE-onlysurvey(570km2)wascarriedouttocheckforresolution
effects.13embeddedcraterswerefound(D=0.05–0.22km;SupplementaryInformation).
HiRISEembedded-craterdensitiesND/aforD<0.2kmarethesame(withinPoissonerror)as
CTXembedded-craterdensitiesforD>0.2km.Therefore,theHiRISEcheckprovidesno
evidencethatourconclusionswouldbechangedbyaHiRISEre-surveyofthe1.7×104km2
areacoveredbyourCTXsurvey.
Diametermeasurementerrorwasestimatedbyblindlyremeasuringcratersandfoundtobe
negligiblecomparedtoothererrors.Possiblecratershrinkageorexpansionduring
degradationisignored.
Thecontributionoffalsepositivestoourcatalogislikelysmall.Althoughpolygonalfaulting
inEarthmarinesedimentscanproducecrater-likeconcentriclayering(Tewkseburyetal.
2014),thisisunlikelyforMarsalluvial-fandeposits.Forexample,theembeddedcratersare
isolated(notspace-filling),andfrequentlyshowpreservedrims.Ontheotherhand,there
arecertainlyfalsenegativesinoursurveyarea:re-surveyofacraterofinterestfound
severaladditionalcandidateswithscores≤3onpanelinspection.Therefore,ourcrater
densitiesarelowerlimits.
WeestimatedalluvialfanthicknessesbydifferencingCTXDTMprofilesacrossfansand
analogousprofilesacrosspartsofthesamefan-hostingcraters(n=13)thatlackedfans.We
foundmaximumfanthicknessof1.1km,withthicknesses~1kmcommon.
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3.Analysis.
Theusualprocedureforestimatingcrater-countingerroristousePoissonstatistics(Michael
etal.2016).Theresultsofthisprocedureareshownbythebluelinesandblueerrorbarsin
Fig.4.Togeneratetheseresults,weassumedafixedcraterflux(Michaeletal.2013),no
changeinatmosphericscreeningfromtoday’sMars,astrong-rocktargetstrength,anda
fixedobliterationdepthfractionφ=0.1.
Thetrueerrorislargerthanthisbecauseofuncertaintyin(1)truecraterflux(Johnsonetal.
2016),(2)targetstrength,(3)filteringbyapotentiallythickerpastatmosphere,(4)thetime
offormationofthealluvialfans,and(5)theamountofburialorerosion–expressedasa
fractionofthecrater’sdiameter–thatisneededtopreventthecraterfrombeingdetectedat
CTXresolution.Therefore,weadoptedconservativepriorprobabilitiesonparameters(1)-
(5)inaMonteCarlosimulationofourlowerboundthatalsoincludesPoissonerror(details
aregivenintheSupplementaryInformation).Specifically,weassumed(1)afactor-of-4
uncertaintyincraterflux(log-uniformuncertaintybetween0.5×and2×theMichaeletal.
2013fluxes);(2)log-uniformuncertaintyintargetstrengthbetweenlimitsof65kPaand10
MPa(Dundasetal.2010);(3)log-uniformuncertaintyinpaleo-atmosphericpressure
betweenlimitsof6mbarand1000mbarforasimplemodelofatmosphericfiltering;(4)a
uniformuncertaintybetweenfanformation2.0Ga(low-endfancraterretentionage)and3.6
Ga(ageofthelargecraterswhichhostalluvialfans);and(5)alog-uniformpriorfor
obliterationdepthfraction(expressedasafractionofcraterdiameter)from0.05(rimburial;
Melosh1989)and0.2(originalcraterdepth;Wattersetal.2015).Foreachof103Monte
Carlotrials,theeffectofPoissonerroriscalculatedanalytically.Giventheobservationsand
therandomly-sampledparameters,eachMonteCarlotrialyieldsananalyticprobabilityfor
eachcandidateage(oreachcandidateaggradationrate)ineachsizebin.Theseprobabilities
aresummedover103MonteCarlotrialsandnormalized.Resultsareshownbythegray
errorbandsandblackstarsinFig.4.
Wereportonlylowerlimitsbecausewedonotknowourfalsenegativerate,anditis
possiblethatmanyembeddedcratersareeasilyidentifiableasimpactcratersinCTX
imagery,buthaveanexpressionthatisindistinguishablefrompostfluvialcratersatCTX
scale.
Thebest-fitlowerlimit(Fig.4a)increaseswithincreasingdiameter,asexpected.Bins≥1.4
kmcontainonly1crater,andthePoissonuncertaintyconstitutesmostofthetotal
uncertaintyinthelowerlimit.Forthesmallerdiameters,thecounting-statisticserroris
smallcomparedtothetotaluncertaintyinthelowerlimit,butsystematicundercountingof
embeddedcratersismostlikelyforcratersthataresmallerandthusmoreeasilyburiedand
modified.Thelargest>1km-diameterbincontains2embeddedcratersandyieldsa2-sigma
lowerlimitof>17Myr,whichweroundto>20Myr.Usingthesingle~5kmcraterfoundin
oursurveygivesa>54Myrlowerbound.
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(a)
(b)
Figure4.(a).Minimumtime-spanofsedimentationbasedonobservedsynfluvialcraters.
Blueerrorbarsbracketthe90%confidenceintervalsonlowerlimit(byPoissonestimation).
FullMonteCarlofitcorrespondstothegrayband.Blackzoneisexcludedwith>95%
confidence.Whitezoneisexcludedin<5%oftrials.Blackasteriskscorrespondtothe
medianoutcomeoftheMonteCarloprocedure.Reddashedlinesassumeaconstant
aggradationrateandthatburialby10%ofacrater’sdiameterissufficienttoobscurethe
craterfromorbiter-imagesurveys.(b)Correspondingaggradationrateestimatesforalluvial
fans,binnedbydiameter.Errorbarsarethesameasin(a).
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Turningtotherateplot(Fig.4b),constantaggradationratesof<(4-8)μm/yearmatchour
data.Forsediment:waterratioof10-4,thiscorrespondstorunoffof<(4-8)cm/yr.Different
craterdiametersprobedifferentdepthrangesandthusaggradationrateoverdifferent
timescales,butourdatadonotprovidestrongevidenceforachangeinaggradationratewith
timescale(Jerolmack&Sadler2007).Dividingtypicalfanthicknessesbythisrategives125250Myr,whichweroundto100-300Myr.Forbothplots,theMonteCarloproceduregives
limitsthataremoreuncertain(graybandversusblueerrorbarsinFig.4)andslightlymore
permissive(blackasterisksversusbluelinesinFig.4).
The>(100-300)Myrageestimatefromsteadyaggradation(Fig.4b)isamorerealistic
estimateofthetruefan-formingintervalthanthe>20Myrtimespanfromobserved
synfluvialcraters(Fig.4a).Thatisbecause,iftheobservedembeddedcratersrepresentthe
totalembedded-craterpopulation,thenfanaggradationmusthavestartedveryfastand
decreasedsharplyneartheendoffanbuild-up.Suchanaccumulationhistorywouldfavor
small-craterpreservationrelativetolarge-craterpreservation–theoppositeofwhatis
observed.Furthermore,afterfluvialdepositionstopped,manyofthealluvialfandeposits
wereerodedbyaeolianprocesses(e.g.Fig.1).Becausepostfluvialerosionwoulddestroy
someembeddedcraters,theobservedembeddedcratersareveryunlikelytorepresentthe
totalpopulationofcratersthatformedembeddedwithinthealluvialfans.
Resultsareplottedexcludingcratersforwhichsynfluvialversusprefluvialstatuscouldnot
bedetermined,butincludingcratersofqualityscore3.Thesechoiceshavelittleeffectonour
conclusions.Thatisbecausethequality-3embeddedcraters,andthecraterswhose
synfluvialstatusisuncertain,correspondtosmalldiameterbinsforwhichourcountsare
probablyincomplete.
4.Discussion.
4.1.Factorsnottakenintoaccountwouldincreaseourlowerlimit.
Byfittingasingletime-span(andseparatelyfittingasingleerosionrate)tothefandeposits,
weimplicitlyassumethattheprobabilityoffindinganembeddedcraterisspatiallyuniform.
However,theobservedspatialdistributionofembeddedcratersistooclumpytobe
consistentwithaspatiallyuniformprobability.Forexample,onesite(Crater“W”;Kraaletal.
2008)has20%oftheembeddedcraters(5outof25)eventhoughthefanareaatthatsiteis
only500km2,3%ofthetotal.Thisclusterisunlikelytobeduetochance(assuming
crateringisaPoissonprocess):ifthefanareaisdividedinto(17000km2)/(500km2)=34
equal-areasites,thentheexpectednumberofcraterspersiteisλ=25/34=0.73.The
probabilityoffinding5ormoreembeddedcratersinatleastonesiteisthenonly
1–(1–Σx=5∞f(x|0.73))34=3%,wherefisthePoissonprobabilitydistributionfunction.To
beconsistentwithdataatthe50%level,wemustincreaseλ(equivalently,fanage)by200%.
Thismightcorrespondtouniformfanagewithspatiallynon-uniformdetectabilityof
embeddedcraters.AlternativelyCrater“W”mightrecordanomalouslyslowaggradation.In
eithercase,ourbestestimateofthetimespanoffan-formingclimatesis200%longerthanin
ourlowerlimit.
8
Spatialstaggeringoffanaggradationwouldincreaseourlowerlimit.Time-varyingorbital
forcingwouldfavorsnowmelt(e.g.Kiteetal.2013b)indifferentplacesatdifferenttimes.
Localizedprecipitationwouldnotbegloballycorrelated(Williams&Malin2008,Kiteetal.
2011).
Ifthealluvialfandepositsunderwenterosionduringtheperiodofnetaggradation,thenthis
woulddestroysomeembeddedcraters.Therefore,iferosionoccurredduringfan
aggradation,ourupperlimitwouldincreasefurther.
MarscraterfluxesareextrapolatedfromLunarradiogenicagesandcorrespondingcrater
densities.Thosecraterdensitieshavebeenarguedtobeincorrect(Robbins2014).We
comparedRobbins’chronologyfunctiontotheNeukum(2001)chronologyfunctionatDmin=
1km.Anagerangeof2.0–3.6Gyr(Neukum)mapsto1.4–3.0Ga(Robbins).Theflux
uncertaintyisreducedfrom(1-12)×modern(Neukum)to(1.5–2.2)×modern(Robbins).
BecausehighaggradationratesinourMonteCarlorunsalwayscorrespondtohighcrater
fluxes,includingRobbins’chronologywouldcauseourerrorbarstoshrinkandthusraise
ourlowerlimit.
4.2.Implicationsforpaleohydrologyandclimate.
D<100membeddedcratersplaceanupperlimitonpaleoatmosphericpressure(Kiteetal.
2014).SinceD<50mimpactcratersareextremelyrareonEarth(atmosphericcolumn
density104kg/m3),theexistenceofD<100membeddedcratersinthealluvialfansonMars
suggestsatmosphericcolumndensity<2×104kg/m3,i.e.P<1bararoundthetimeofalluvial
fanaggradation.
Previousanalysesoftheintervaloverwhichalluvialfansformedhavedividedfanvolumeby
theinferredfluvialsedimenttransportflux(e.g.Jerolmacketal.2004).Thisdurationisa
lowerboundontheintervaloverwhichalluvialfansformed,becausenotallyearsneed
producerunoff.Ourlowerboundexceedssedimentologicallowerboundsby>1000×.Many
alluvialfansare~1kmthick.Supposeafan:alcovearearatioof0.5.Typicalwater:sediment
ratiosonEarthare103:1.2000kmofwaterat0.5msnowmelt/yeargives4Myr.However
thesecalculationsarehighlyuncertain.Forexample,iftheamountofsnowmeltislimitedby
asnowsupplyrateof10cm/yr,thenthetimerequiredrisesto20Myr,equaltoourstrict
lowerlimitonthetotaltimespanofalluvialfanformation.Thereforeourdatadonotrequire
intermittency.However,givenquasi-periodicorbitalvariability,fine-tuningofMars’
hydrologicalcycleisrequiredtoproducesmallamountsofrunoffeveryyear,especiallyfor
ourpreferredlowerlimitof>(100-300)Myr.Broadly,theveryslownetaggradationratesin
areasofsteeprelief(Fig.1)suggestintermittency.
Intermittencyinalluvial-fan-formingclimateisfurthersuggestedbycombiningourdata
withotherconstraints.Thepaucityofmineralogicevidenceforin-situalterationoffan
deposits(McKeownetal.2013),thepresenceofhydratedsilica(possiblyopal;Carteretal.
2012),andthepersistenceofolivinewhichdissolvesat0°CandpH=5.5in<5Myr(Stopar
etal.2006),whencombinedwiththe>20Myrspanofsurfaceliquidwaterrequiredbyour
9
data,suggestthatclimateconditionswerecoldandthatintermittencyfurtherreducedliquid
waterinteractionwithsoil.Coldconditionsarealsosuggestedbysedimentary-deposit
mineralogyatGale(McLennanetal.2014,Siebachetal.2017).Intermittencyisalso
suggestedbymultiplepulsesoffanformationatHolden(Irwinetal.2008),Gale(Paluciset
al.2016),andMelasChasma(Williams&Weitz2014).
Insummary,thedataexcludeanyexplanationthatproducesasingleburstofhabitabilityof
<20Myrduration.Forexample,thedataexclude triggeringbythethermalpulsecausedby
theimpactsthatformedthelargecraterswhichhostthealluvialfans.Thedatadisfavor
fluvialsedimenttransporteveryyearfor>20Myr. Amongotherpossibilities,thedatapermit
a long-livedhabitableenvironment(snowmeltorrainfall);achaostrigger(Bakeretal.
1991);orobliquity-pacedfluvialintermittency(Kiteetal.2013b).
Figure5.GeomorphichistoryofMars(Howard2007,Fassett&Head2011).FSV=Fresh
ShallowValleys(Wilsonetal.2016).RSL=RecurringSlopeLineae,FSV=FreshShallow
Valleys.“*”symboldenotesquestionablestatusofgullies,whicharemodifiedbyCO2ice.PrevalleynetworkfluvialsedimenttransportisfromIrwinetal.(2013).
4.3.Implicationsforhabitability.
Thetimespanofaggradationofthealluvialfandepositsthatwehavecalculatedisaproxy
forthetimespanofspatiallyassociatedpaleolakes.Thosepaleolakesincludecandidate
playadepositsatthetoesofalluvialfans(e.g.Morganetal.2014),a>100mdeeppaleolake
inCrater“P”(Kraaletal.2008)suggestedbyacommonfan-frontal-scarpelevationof-2700
m(inourCTXDTMs),andtheEberswaldepaleolake(whichsharesadrainagedividewith
alluvialfansatHolden;Irwinetal.2015).Inaddition,riversandlakesoccurredduringthe
LateHesperian-AmazonianinVallesMarineris(e.g.Mangoldetal.2004,Williams&Weitz
2014)andArabiaTerra(Wilsonetal.2016).Lakedepositshavegoodbiosignaturerecovery
potential(Summonsetal.2011),andbiosignaturerecoveryfromProterozoiclakedeposits
10
isroutine(Petersetal.2005).Ontheotherhand,biosignatureswouldhavebeendestroyed
iflakewaterswereoxidizing.
5.Conclusions.
ExplainingyoungalluvialfansonMarsisachallengetoclimatemodels.Todeterminethe
timespanofalluvialfanformingconditions,wecountedembeddedcraters.Wefoundahigh
densityofembeddedcraters,whichrequiresthattheriver-permittingclimate(s)spanned
>20Myr.Ifaggradationwassteadyat<(4-8)μm/yr,whichisconsistentwithourdata,then
fanbuild-uprequired>(100-300)Myr(Table1). Thedatamakethechallengeofexplaining
thealluvialfansmoresevere,becausetheyexcludeasingleshort-livedanomalyasthecause
ofthealluvialfans.
Acknowledgements.
WethankAlanHoward,MarisaPalucis,BeckyWilliams,RossIrwin,Jean-PierreWilliams,Bill
Dietrich,andBrianHynek.
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