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Martian Fluidized Crater Morphology Variations with Crater Size, Latitude

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Martian Fluidized Crater Morphology Variations with Crater Size, Latitude VOL. 84, NO. BI4 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 30, 1979 Martian Fluidized Crater Morphology: Variations With Crater Size, Latitude, Altitude, and Target Material PETER MOUGINIS-MARK Department of GeologicalSciences, Brown University,Providence, Rhode Island 02912 On the basisof morphology of the exterior depositsof 1558 fresh Martian impact craters,6 crater types are defined, and the incidence of each crater type on 10 different geological units is documented.It is shown that severalcrater types are preferentially associatedwith specifictarget materials:radial textured cratersare found primarily on Tharsis and Elysium lavas,a type of crater here called 'pancakecraters' on fractured terrain, old lavas, and channel materials. The occurrenceof secondarycraters is also strongly terrain dependent.Three times as many craters on young lavas have secondarycraters as compared to those craters on ridged and cratered plains materials, and 10 times as many have secondarycraters when comparedto primary craterson ancient terrain materials.The maximum radial extent of fluidized ejecta blankets is demonstratedto be a function of both crater altitude and latitude. The most extensiveejecta units are found at low altitudesand high latitudes,while the least mobile ejecta is locatedat high eleva- tions and closeto the equator. Only pancake cratersexhibit any pronouncedlatitudinal variation in their distribution. These cratersare almost exclusivelylocated poleward of latitudes40øN and 40øS. For the majority of the samplecraters (N = 1333), there is no suchsystematic latitudinal variation in crater oc- currence. INTRODUCTION nuity, or the preservationof floor materialsanalagous to simi- The formation of ejecta faciessurrounding fluidized craters lar depositsseen within freshlunar and Mercurian craters[Ar- on Mars has been attributed to a surfaceflow phenomenon,as thur et al., 1963; Wood et al., 1977]. These Martian fluidized a consequenceof ejecta fluidization by subsurfacevolatiles craters were subdivided into six types on the basis of the within the target material [Head and Roth, 1976; Carr et al., classification scheme described below. Diameter, location on 1977;Mouginis-Mark, 1977].Atmospheric deceleration of fine the planet, elevation above 6. l-mbar Mars datum, the maxi- ejecta may also representan aspectof the multiphasedejecta mum range of the continuousejecta facies,and the type of emplacementsequence for certain Martian craterslarger than target material were recorded for each crater. In addition, 30 km in diameter [Schultz and Gault, 1979]. Although Allen wherethey were observed,the presenceof a centralpeak, ter- [1978] has observedthat rampart cratersoccur on nearly all races, or scallops[Cintala et al., 1977] within the crater and major geological units on Mars, Johansen[1978] concludes obvious secondary craters beyond the rim were also docu- mented. that specificcrater morphologiesillustrate a strong correlation with latitude: the incidenceof craterswith fluidized ejecta in- The majority of the terrain typesand all of the crater eleva- creasesat higher latitudes. This investigation was initiated to tions were taken from the geologicalmap of Scott and Carr assessthe influence of crater latitude, target material and ele- [1978]. The name 'fracturedterrain' was adoptedfrom Guest vation on the morphology of the ejecta facies and crater inte- et al. [1977] to describethe polygonal fractured ground in riors as a function of crater diameter. easternAcidalia Planitia (45øN, 10øW). The unit name 'an- cient terrain' was usedto includethe crateredplateau material DATA BASE and the hilly and crateredmaterial of Scottand Carr [1978]. Using techniquesalready developed for the collection of Craters occurring close (within 2 crater diameters) to the crater morphological data [Arvidson, 1974; Arvidson et al., mouths of fluvial channelswere included in the group of 1974; Cintala et al., 1976a], a data base containing 1558 flui- 'channelmaterial craters'in an attemptto incorporatecraters dized craterswas constructedfrom Viking orbiter imageswith excavated in channel outwash materials, inferred to be the a resolution between 40 and 220 m per picture element. The same type of target material as that for craters formed within the channels. coverage of the images utilized within the analysis was re- strictedby thoseframes currently available (January 1979) at CLASSIFICATION OF CRATER TYPES Brown University. These images correspondto photographs taken on revolutions 1-210 and 580-690 of both orbiters. The Various attemptshave already been made to devisea classi- resultant distribution of the sample craters, which shows a fication scheme for Martian fluidized craters on the basis of preferential clustering of craters at the candidate Viking ejecta facies morphology [Head and Roth, 1976; Mouginis- lander sites,is illustrated in Figure 1. This crater distribution Mark, 1977; Johansen, 1978]. None of these schemes,how- has been qualitatively augmentedby inspectionof subsequent ever, are sufficientlyadaptable to incorporateall forms of flui- Viking imagesand is consideredhere to reflect the basic char- dized ejecta, nor are they adequately explicit to permit consis- acteristicsof crater interior morphology and areal extent of tent identificationof similar cratersby different investigators. the ejecta deposits. In an attempt to rectify this situationthe morphologicalclassi- Only morphologically fresh craters were included in the fication schemeillustrated in Figure 2 was adoptedhere. sample. Fresh craters were defined to be those that illustrate Six different types of fluidized craters are identifiable. Two fine surface textures on their ejecta deposits,rim crest conti- types, exhibiting single (type 1) and double (type 2) ejecta facies(Figures 2a and 2b, respectively)surrounding the parent Copyright¸ 1979by the AmericanGeophysical Union. craterhave beendescribed by Carr et al. [1977]and Mouginis- Paper number 9B 1041. 8011 0148-0027/79/009B- 104 1$01.00 8012 MOUGINIS-MARK: SECOND MARS COLLOQUIUM MOUGINIS-MARK: SECOND MARS COLLOQUIUM 8013 Fig. 2. (a-J) All the fluidizedcraters included in this investigationare dividedinto the six cratertypes displayed here. The morphologicalcriteria upon which this subdivisionwas made are describedin the text. Frame numbers,locations, targetmaterial, and elevationsfor all the cratersshown in this and successivefigures are summarizedin Table A I. Mark [1977]. The ejecta materials around type I craters and tal ridges on their outermostejecta facies and together consti- the outer ejecta faciesof the type 2 cratersare lobate deposits tute the fluidized craters previously described as rampart cra- that illustrate a surface flow emplacement mechanism. In con- ters [McCauley, 1973; Carr et al., 1977]. trast, the inner ejecta facies of type 2 craterspossess a convex Type 4 craters (Figure 2d) represent the radially textured distal edge.A number of cratersdisplay multiple ejectafacies craters of Carr et al. [1977]. This crater type is characterized (Figure 2c) that have a similar morphologyto the lobate de- by strong radial pattern of groovesand ridges on the ejecta positsof crater types I and 2. Such multiple lobate cratersare blanket, superimposedupon sheetsor small 'plates' of ejecta. defined here as type 3. Crater types l, 2, and 3 all possessdis- Unlike crater types l, 2, and 3, there is no distal ridge present 8014 MOUGINIS-MARK:SECOND MARS COLLOQUIUM 900 LATITUDINAL DISTRIBUTION OF CRATER TYPES <• 800 Figure4 showsthe latitudinal variation of each crater type, • 700. expressedas a percentageof all craterssampled within any o• 6 O0 10ø latitudinal band. This technique was utilized to avoidany samplingbias towards the mid-latitudes as a consequenceof m•500 the distributionof Viking imagesavailable for this investiga- zm400 tion. In order to presentreasonable statistics, only latitudinal bands where more than 30 craters were measured are illus- •z 6 2 trated. All fluidized cratersincorporated in this data setare lo- • 200 cated between78øN and 73øS. It is apparent,however, from images that became available after this analysiswas com- pleted that many more fluidized cratersexist at each latitude, I0 20 30 40 50 60 70 so this distributionshould not be interpretedto be a compre- CRATER DIAMETER,kin hensiveglobal analysis. Fig. 3. Cumulative frequency curvesfor all six types of fluidized Johansen[1978] has reported that different types of flui- crater.Number at the end of eachcurve refers to cratertype. Sample dized cratersdisplay a stronglatitudinal variationin their dis- bins were set at 2-km intervals. Not shown are five type 5 craters be- tribution, which is attributed to a poleward concentrationof tween 65 and 104 km in diameter. subsurfacewater. Such an observationis not supportedby this analysis. With the exceptionof the pancakecraters (type 6), onthe ejecta blanket of type4 craters.Although not identified thereis no stronglatitudinal variation in the distributionof asfluidized craters by Carret al. [1977],this analysis indicates the craterclasses. Between 50øN and 70øS, 40-60% of all cra- thatthe radially textured craters may have an ejecta morphol- tersat eachlatitude are type 1, while 15-30% are type 2. Be- ogythat is transitionalin complexityfor cratersin the size tween40øN and 50øS, crater types 3, 4, and5 eachconstitute range10-30 km in diameterthat are excavated in geological1-9% of thepopulation, while the most frequent occurrence of unitsinferred to be coherentlavas surrounding the Tharsis type 4 cratersisobserved
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