ICE OR LIQUID WATER in the HARTIAN REOOLITH? T'iorpholo01c INDICATORS from RAMPART CRATERS; P.D

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ICE OR LIQUID WATER IN THE HARTIAN REOOLITH? t'IORPHOLO01C INDICATORS FROM RAMPART CRATERS; P.d. Mouginis-Mark, Planetary Geosciences Division, Hawaii Institute of Oeophystcs, University of Hawaii, Honolulu, HI 96822*

At therecentLunarand PlanetaryInstitute'sSpecialMECA Sessionon geomorphic indicatorsofmartianvolatiles(March 1986), itbecameclearthatconsiderabledifferencesof opinionexistregardingthephysicalstate(or even totalabsence)of thevolatileswithinthe martianregoliththatwere responsiblefortheformationof therampart craterlobateejecta flows.Possibleformsthaithisfluidizingmedium mighttakeincludewater iceor liquidwater withinthetargetmaterial( 1-4), or atmosphericgasesinteractingwithsuitableparticlesizes withintheejectacurtain(5).Becauserampart cratersaredistributedplanet-wide,andoccur inregionalsettingswhere bothliquidwateror watericemightbe expectedtooccur(basedon thedistributionofchannelsandperiglaciallandforms)ithaslongbeen realisedthatthecorrect Interpretationof cratermorphology would providethecapabilityto document the spatlal distributionand physicolstateof near-surfacevolatilesaround the planet(cf. I-4). Recognitionof the correctvolatilephase would have implicationsnot onlyfor the mode of formationof craterejectadeposits,but alsoforothergeomorphicprocessessuch as channel network formationand terrain softening.This abstractreviews previouslypublished Interpretationson thissubject,and considerssome of the morphologicfeaturesthatmay be recognizablefrom the VikingOrbiter images,in an attemptto help resolvethisunknown conditionoftheregolithandprovideconstraintson theglobaldistributionofvolatileson Mars.

Global Observations: Early analysis of martian craters (6) drew attention to the numerous craters that possess central peaks with summit pits. Thesepits are not associatedwith all fresh rampart craters of a given diameter, and led to the suggestion that the pits were formed by explosivedecompression of strata containing subsurface volatiles. Sucha situation suggestedthat in additionto the effects of the volatiles that created the lobate ejecta deposits,volatiles of another phase ( possible liquid water as opposedto water ice) might be present. Circumstantial evidence from the analysis of craters (7) and volcanic landforms (8) in the Elysium Planitta region may support this interpretation, since an unusally large number of craters in this region possess pitted central peaks, possibly indicating that liquid water existed close to the surface as a result of a higher than usual thermal gradient. Johansen(3) and recently Kragel (9) have suggested that the presence or absence of a distal ridge on an ejecta flow may be an indicator of water (ridge present) or Ice (ridge absent) within the target material. Although there is a predominance of ridged craters at low latitudes and ridga-less craters at high latitudes (9) no corroborating models for ejecta emplacement, or theoretical or experimental data, were presented by these investigators to support their hypothesis that this difference in morphology was indeedassociated with the physical state of water within the target. Ejecta lobe thickness appears to be limited by somephysical aspect of the deposit, inferred to be the maximum shear strength of the fluidized medium ( I 0). No global study has bean made of this variation in the upper limit for ejecta thickness, but the value appears to bequite uniform. For example, studiesof craters on ridged plains materials between 40°S and 30°N and found no appreciable difference in the surface area (and, by inference, ejecta thickness) for a range of altitudes between+9 km to -2 km ( 11). This situation was felt to imply that the viscosity (and, hence, degree of ejecta fluidization) may have been constantfor cratering events over a wide geographic area for on appreciable amount of martian history.

* Currently at NASAHeadquarters,CodeEEL, WashingtonDC 82

WATER OR ICEON MAR6? {)E POOR QUALITY Mouginis-Mark,P.J.

There are few closecorrelationsbetween cratermorphologiesand targetmaterials,but craterswithtwoconcentricejectalobesar_most numerous inAcidaliaandUtopiaPIanitia(two areaswhich hove been independentlyidentifieda_ probableareaswith periglaciallendforms; 12).Therealsoappearstobe littlevariationincratermorphologyas a functionofage(andthus possibleevolvingatmosphericconditionson Mars). Cratersthatare apparentlyvery young (inferredtobe youngduetothepreservationofejectarays)stillpossessthesame basicform of ejectamorphologyasoldercraterson thesame terrains(13). Becausethere does not appear to be any strongmorphologicaldifferencebetween the individuallobesassociatedwithcratersintheS - I0 km diameterrangeandcratersinthe30 - 35 km diameterrange,itislikelythatthefluidizingmedium andviscosityofthesedepositswas similar.Such an ideais borneout by the linearrelationshipbetweenajectaareavs.crater diametercurves(for.craters6 - 35 km in diameter;tO).Thisobservationin turn would implythatno appreciablechangeinvolatilestateor concentrationexistedatthetimeofcrater formationforejectaoriginatingatshallowdepths(smalldiametercraters)andgreaterdepths (largecraterdiameters).

ExperimentalModels: Wohletzand Sheridan(14) suited thatterraceddepositsseenon certainejectablankets resultedfrom surgesin theemplacementoftheejectaas targetwater explosivelyvaporized duringtheimpactevent.However.,theirmodelsdidnotconsidertheenergyrequirementsto vaporizewatericeasopposedtoliquidwater.Craterrampartswere thoughttoform when ejecta surgeslostthefluidizingvaporsandtransportedparticlesware depositedan masse. No rigorousattemptsat laboratorysimulationsof impacteventsintotargetsdesignedto simulatespecificmartianconditionshaveso far beenattempted.Initialexperimentsusingthe Ames Oun haveneverthelessshown thatthefinalcratermorphology,thebreak-upoftheejecta curtainintodiscretefragments,and themorphologyofthesecondarydepositsdependupon the viscosityofthetargetmedium (15).

CraterMorohol_L: While littlepositiveinformationon volatilestatecan be gainedfrom regionaltrendsinthe distributionof cratermorphologies,a few high resolution(20 meter per pixelor better) YikingOrbiterimagesprovidethecapabilityto searchforsmallmorphologicalfeaturesthat might distinguishbetween water _ iceexistingwithinthe targetmaterial.For example, grooveson theejectalobesof"thecratersBamburg (55 km dia.)and Arandas(25 km die.) indicatethat immediatelyafterlobeemplacement,but priorto cessationof ajeciacurtain deposition,the lobeshad establishedsufficientphysicalstrengthto preservethese"scour marks" intheirsurfaceduringthe passageof thelaterejectamaterials( 12, 16).A similar effectsisalsoobservedwhere pre-existingobstacleshavecreatedpressureridgeswithinthe ejectalobeon the crater-wardsideof the obstacle.Itappears unlikelythata very fluid, water-richejectacouldretainthes_featuresaftertheirformation,suggestingthaticemightbe a more acceptabletargetvolatile. Few examplesof small-scaleoutflowof watercan be seen in even thehighestresolution (betterthan20 meterper pixel)YikingOrbiterimages.5orneoftherare exceptionsare the channelnetworkson therim of SchiaparelliBasin(17), channelson the innerwallsand abnormallysmoothterrainwithintheejectablanketofBamburg (16) andpreviously 83 i WATERORICEONMARS? Mouginis-Mark,P.J.

undocumented channels on the inner walls of crater Cerruli. Were liquid water to be more commonlypresent within ejecta blankets at the time of their emplacement, depositsassociated with seepageof water from the ejecta lobesare expectedto be more frequently observable in the Viking images.

Summary and5Deculattons: The absence of remobilized materials as a consequenceof water sapping from the emplaced ejecte, and the formation (and preservation) of scour marks and pressure ridges within the still-forming ejecta blanket, suggestthat the ejecta possessedappreciable mechanical strength at the time of emplacement. Numerical models would be neededto determine the effects of different volumeric amounts of liquid water versus water ice entrained within the ejecta, but these observations appear at this time to favor groundice as the physical statefor the fluidizing material responsible for creating the rampart crater lobes. Boyce (2) suggestedthat in some regions of Mars, crater morphology may reflect a layer of water-rich material underlying an ice-rich permafrost. Such a situation might be responsible for the formation of the twin lobed craters, and thus mark the point where near-surfece ice overlays liquid water. If this hypothesis is valid, it could explain the apparent anomalywhereby the inner, more viscous ejecta lobe was empleced prior to the outer more fluid lobe ( i 2). It is possible that a reanalysis of the highest resolution Viking Orbiter images could resolve this two-phase model for the regolith if suitable craters were imaged at 10 - 20 meters per pixel. Clearly, however, these observations of ejecte morphology' pertain to a only a few examples of martian craters, rather than comprise a general set of properties for the entire crater population. While these well imaged craters provide an insight into the physical state of the target materials, image resolution may still be insufficent to identify key landforms. As a result, it is concluded that making the morphology and geochemistry of fresh martian Impact craters one of the prime targets for the ultra-high resolution camera and YIMS experiments to be flown on the Mars Observer may be the most appropriate method for identifying the state of volatilas within the regolith at ',he time of impact.

Referenc_s: !) Carr,M. H etal(1977) d.Oeophys.Ras.v.82: p.4055-'I065. 2) Boyce,J.M.(1979) NASATM80339,Rpts. Plan.Oeol.Pr_. 1978-1979, p. 114. 3) Johansen,L.A.(1979) NASATMSO339, Rpts.Plan.Oeol.Pr_. 1978-1979. p. 123. 4) Mouginis-Mark,P.J.(1979) ,J.Seophy@,Res.v.84_p.8011-8022. 5) ,Schultz,P.H.andD E.8ault(1979) J_8eophys.Rm. v_84_ p.7669- 7687. 6) Wood,C.A.etal(1978) PLP,SC9th,p.:3691-3709. 7) Hale-Erlich,W.S.(1986) LPSXVII:p.303-304. 8) Mouginis-Mark,P.J.(1985) Icarus_v.64, p.265-284. 9) Kargel,J.S.(1986) LPS XVII_.p.410-411. 10) Mutch,P. andA.Woronow (1980) _p. 259-268. 11) Mouginls-Mark,P.J.andE.Cloutls( 1983).LP_._ p.532-533. 12) Mouginis-Mark,P.J.( 1981 ) Icarusv.45: p.60- 76. 13) Mouginis-MarkP.J. etal (1980) LPSXI,p. 762-764. 14) Wohletz,K.H.andM. F.,Sheridan(1983) Icarusv.56, p. 15-37. 15) Oault,D. E.andR.Oreeley(1978)_, p.486-495. 16) Mouginis-Mark,P.d. (1979) _ p.2651-2668. 17) Mouginis-Mark,P.J.etal(1980). proc.Conf.Multi-Ri_ Basins,Schultz& Merrill Eds.,Pergamon,NY,p. 155- 172.