The morphology of ejecta blankets around certain Martian craters carries implications for volatiles in either the Martian crust or the atmosphere or both. The evidence that rampart crater ejecta can be used to infer the physical condition of volatiles in target rocks at the time of impact is reviewed. It was concluded that ice, and not liquid water, was the main volatile state, although rare examples also suggest the presence of liquid water at the time the craters were formed

Mouginismark, P. J.

English

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108 0 6
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*
AttherecentLunarand PlanetaryInstitute'sSpecialMECA Sessionongeomorphic
indicatorsofmartianvolatiles(March 1986), itbecameclearthatconsiderabledifferencesof
opinionexistregardingthephysicalstate(or even totalabsence)ofthevolatileswithinthe
martianregoliththatwere responsiblefortheformationoftherampartcraterlobatejecta
flows.Possibleformsthaithisfluidizingmedium mighttakeincludewater iceor liquidwater
withinthetargetmaterial(1-4), oratmosphericgasesinteractingwithsuitableparticlesizes
withintheejectacurtain(5).Becauserampartcratersaredistributedplanet-wide,andoccur
inregionalsettingswhere bothliquidwateror watericemightbe expectedtooccur(basedon
thedistributionfchannelsandperiglaciallandforms)ithaslongbeenrealisedthatthecorrect
Interpretationf 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
formationofcraterejectadeposits,but alsoforothergeomorphicprocessessuchas channel
network formationand terrainsoftening.This abstractreviews previouslypublished
Interpretationson thissubject,and considerssome ofthe morphologicfeaturesthatmay be
recognizablefrom the VikingOrbiterimages,in an attemptto help resolvethisunknown
conditionoftheregolithandprovideconstraintsontheglobaldistributionfvolatileson 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
https://ntrs.nasa.gov/search.jsp?R=19890001435 2020-03-20T06:07:29+00:00Z
82
WATER OR ICEON MAR6?
Mouginis-Mark,P.J.
{)E POOR QUALITY
There are few closecorrelationsbetweencratermorphologiesand targetmaterials,but
craterswithtwoconcentricejectalobesar_mostnumerous inAcidaliandUtopiaPIanitia(two
areaswhich hovebeenindependentlyidentifieda_ probableareaswithperiglaciallendforms;
12).Therealsoappearstobelittlevariationincratermorphologyasafunctionofage(andthus
possiblevolvingatmosphericconditionson Mars).Cratersthatare apparentlyveryyoung
(inferredtobeyoungduetothepreservationofejectarays)stillpossessthesame basicform of
ejectamorphologyasoldercraterson thesameterrains(13).
Becausethere does not appear to be any strongmorphologicaldifferencebetweenthe
individuallobesassociatedwithcratersintheS - I0 km diameterrangeandcratersinthe30 -
35 km diameterrange,itislikelythatthefluidizingmedium andviscosityofthesedepositswas
similar.Such an ideaisborneout by the linearrelationshipbetweenajectaareavs.crater
diametercurves(for.craters6 - 35 km indiameter;tO).Thisobservationin turnwould
implythatnoappreciablechangeinvolatilestateorconcentrationexistedatthetimeofcrater
formationforejectaoriginatingatshallowdepths(smalldiametercraters)andgreaterdepths
(largecraterdiameters).
ExperimentalModels:
Wohletzand Sheridan(14) suited thatterracedepositseenon certainejectablankets
resultedfrom surgesin theemplacementoftheejectaas targetwater explosivelyvaporized
duringtheimpactevent.However.,theirmodelsdidnotconsidertheenergyrequirementsto
vaporizewatericeasopposedtoliquidwater.Craterrampartswerethoughtoformwhen ejecta
surgeslostthefluidizingvaporsandtransportedparticleswaredepositedan masse.
No rigorousattemptsat laboratorysimulationsof impacteventsintotargetsdesignedto
simulatespecificmartianconditionshavesofar beenattempted.Initialexperimentsusingthe
Ames Oun haveneverthelessshown thatthefinalcratermorphology,thebreak-upoftheejecta
curtainintodiscretefragments,and themorphologyofthesecondarydepositsdependuponthe
viscosityofthetargetmedium (15).
CraterMorohol_L:
While littlepositiveinformationon volatilestatecan begainedfromregionaltrendsinthe
distributionofcratermorphologies,a few highresolution(20 meter per pixelor better)
YikingOrbiterimagesprovidethecapabilitytosearchforsmallmorphologicalfeaturesthat
might distinguishbetweenwater _ iceexistingwithinthe targetmaterial.For example,
grooveson theejectalobesof"thecratersBamburg (55 km dia.)and Arandas(25 km die.)
indicatethatimmediatelyafterlobeemplacement,but priorto cessationof ajeciacurtain
deposition,the lobeshad establishedsufficientphysicalstrengthto preservethese"scour
marks" intheirsurfaceduringthe passageofthelaterejectamaterials(12, 16).A similar
effectsisalsoobservedwhere pre-existingobstacleshavecreatedpressureridgeswithinthe
ejectalobeon the crater-wardsideof the obstacle.Itappearsunlikelythata very fluid,
water-richejectacouldretainthes_featuresaftertheirformation,suggestingthaticemightbe
a moreacceptabletargetvolatile.
Few examplesof small-scaleoutflowofwatercan be seenineven thehighestresolution
(betterthan20 meterper pixel)YikingOrbiterimages.5orneoftherareexceptionsare the
channelnetworkson therim ofSchiaparelliBasin(17), channelson the innerwallsand
abnormallysmoothterrainwithintheejectablanketofBamburg (16) andpreviously
WATERORICEONMARS?
Mouginis-Mark,P.J.
i
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 ground ice 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.
83


Ice or liquid water in the Martian regolith. Morphologic indicators from rampart craters

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