NASA Technical Reports Server (NTRS) 20170000745: The Ice-Covered Lakes Hypothesis in Gale Crater: Implications for the Early Hesperian Climate

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THE ICE-COVERED LAKES HYPOTHESIS IN GALE CRATER: IMPLICA-
TIONS FOR THE EARLY HESPERIAN CLIMATE.
AlexandreM.Kling,NasaAmes/USRA,RobertM.Haberle,ChristopherP.McKay,ThomasF.Bristow,Nasa
AmesResearchCenter,FrancesRivera-Hernandez,UCDavis,USA.
Introduction 10,000 years if not resupplied by precipitation. Simi-
larly,forsurfacepressurevaluesbelow100mbar,which
Recent geological discoveries from the Mars Science isourpreferredenvironmentalcontext,anytemperature
Laboratory(MSL),includingstreamandlakesedimen- T >247K willalsoresultintheevaporationofthelake
tarydeposits,provideevidencethatGalecratermayhave in less than 10,000 years. The calculation is conser-
intermittentlyhostedafluvio-lacustineenvironmentdur- vative as we assumed a high humidity content of the
ing the Hesperian, with individual lakes lasting for a atmosphere(RH=60%)andnoconvectiveevaporation.
periodoftenstohundredsofthousandsofyears[1]. Es-
timatesoftheCO2contentoftheatmosphereatthetime At273K,weestimatedthatEarth-likerainfallrates
the Gale sediments formedare far less than neededby in the order of 0.1-1 m/yr are required to resupply
anyclimatemodeltowarmearlyMars[2],giventhelow the lake, depending on the average wind speed (not
solarenergyinputavailableatMars3.5Gya. We have shown). The absence of geological evidence for the
thereforeexploredthepossibilitythatthelakesinGale overall weathering that would be associated with such
duringtheHesperianwereperenniallycoveredwithice rainfallratesduringtheHesperianmotivatedtheinves-
usingtheAntarcticlakesasanalogs. tigationofa"coldandwet"scenarioforGale.
TheAntarcticlakesanalogs
Environmentalrequirementforliquidlakes
TwodifferenttypesofAntarcticlakesmaybeadequate
We estimated the lifespans of individual lakes for dif-
analogsforancientGale: the DryValleysandthe lake
ferentpressuresandtemperaturesofinterest,including
Untersee. Bothlakesexperienceannualmeantempera-
temperatures which are below the freezing point and
ture below freezing,respectively -20o and -10.6o [4],
forthoseanicylakewouldsublimate. Thelakesareas-
[5] and still maintain liquid water underneath the ice
sumedtobe100mdeep,[1]andthelifespansarederived
cover. Theareaof a perenniallycoveredlakeis setby
fromtheevaporationrates. [3]
thebalanceofannualmeltwaterinputandablationfrom
the surface of the ice cover. The thickness of the ice
coverissetbytheenergyandmassbalancesoftheice
3 bar Time to evaporate a 100 m deep lake, RH= 60 %, u=0 m/s 1e+07 yr cover,massbeingremovedbyablationatthesurfaceand
addedtothebottombyfreezing. [4]FortheDryValleys,
1 bar 1e+06 yr 1e+06 yr tghleacmiaelltawndatedrriivsepnrobvyidtehdebdyegsreeaesodnaaylsmaeblotvoeffareneezairnbgy.
500 mbar ForthelakeUntersee,themeltwaterisprovidedbysub-
100000 yr
aqueousmeltingofa glacialdamflowingintothe lake
Pressure100 mbar 10000 yr athnedbdortivtoemnboyftthheesicuenlcigohvterw[hMicchKapyeneettarla.,teinsapnredp.w].arWmes
50 mbar 100000 yr 1000 yr decided to apply the lake Untersee model for Gale as
itdoesnotrequireliquidwatertoflowatthesurfaceat
10000 yr
1000 yr any time of the year and computed ice thicknesses of
100 yr 100 yr
perennially-coveredlakesatGalecraterasafunctionof
10 mbar Water the mean annual temperature and the ice transparency
boiling
6 mba2r20 225 230 235 240 245 250 255 260 265 270 275 280 285 290 10 yr [4],[McKayetal.,inprep.]
Lake surface temperature [K] Time [Earth Years]
Ourbestestimatefortheannualmeansurfacetem-
peratureatGale3.5Gyais 230K.Thisvaluewasderived
Figure1: Individual lake’slifetimeasafunctionofpressure from the surface energy balance assuming present-day
andtemperature obliquityandasimpleparametrizationforthedownward
IRradiationwasusedto mimicagreenhousewarming
Figure1showsthatevenformulti-baratmospheres, effect. The downwardIR flux was arbitrarychosen as
a liquid lake (T > 273K) will evaporate in less than 15%ofthepeaknoontimeinsulation(present-dayvalue
REFERENCES
Mean thickness (contours) and half thickness variation (shades) of the ice cover [m] Possibly,iftheiceisthinenoughtoletsedimentspene-
P=100 m2.0bar, RH= 60 %, u=10 m/s, fgeo= 0.04 W/m2, albd= 0.5, 0% dust cover, f below 750nm =40%, Fwater= 1.4D5Z W= /12m (m2ax(Z)-min(Z)) [m] tratetheice,geologicalfeaturesassociatedwithaqueous
1.45 m
11..89 1.5m 1.35 m environmentsarestillpossibleinaperennially-covered
1.7 1.25 m lakeonacold,yetwetplanet.
Extinction path length [m] 01111111........90123456 50m 40m 30m 20m 10m9m8m7m6m5m4.5m4m3.5m3m2.5m 2m 000011......678901555555 mmmmmm p[tdhh7reae]evp,teaoIeafbsdnismosteasotnas,mbjcbeoeeoevreuoinldglfidmeeocenoebircltrsoacteeagtosriiinvonocgenafgdlloloaaamffcptetieaaGharstleuaatftlrieceeecaosse[t,l-1udoac]rrto.ecfvslOGriesmonarusel-actedgthweolcaeiaeandkgnnggelvaewibmcrseoio,oonwrdinkmnheethilneecaniirhcss-t
00..78 0.55 m beendonetoinvestigatethesedimentationratesandpro-
0.6 0.45 m cesses for ice-coveredlakes (particles melting through
( 1.092.5 1c5m /yr)( 3.32 2c0m /yr)( 5.72 2c5m /yr)( 9.62 3c0m /yr)( 15.283 c5m /yr)( 25.244 c0m /yr)( 40.234 c5m /yr)( 62.285 c0m /yr)( 96.225 c5m /yr) 0.35 m the ice, ice cracks, subglacial flow...) to assess if they
Annual mean temperature [K] and (Avg. sublimation rate [cm\Earth yr])
could possibly be consistent with the sedimentary de-
Figure 2: Ice thickness as a function of the extinction positsobservedatGale[Rivera-Hernandezetal.,inprep].
pathlengthandmeanannualsurfacetemperature(contours). Finally,themainadvantageoftheice-coveredlake
Thicknessvariationisshowninshades modeland the main reason we pursuedit, is thatit re-
laxestherequirementforalong-livedactivehydrologi-
calcycleinvolvingrainfallandrunoff,whichnoclimate
iscloseto4%[6]). modelisabletoproducegivenknownconstraintsonthe
earlyMarsenvironment.
Results
Figure 2 shows that at 230K, the ice thickness ranges References
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parabletotherangeofthosefortheAntarcticlakes(3-6 PredictingCombinedForcesplusFreeConvection1990:
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m),andarenotimplausible. Andtheyarenotsothick
thatsedimentscannotpenetratetheice.
[4] C.P.Mckayetal,Thicknessoficeonperenniallyfrozen
lakes1985: Nature,3133(561-562)
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dry"scenariosforearlyMarsmayhavecomefromthe iceExposedbyNewimpactsintheMartianMid-latitudes
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