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 morphologyof 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 elevations 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 occurrence.

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 documented. 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 'ancient 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 thecrater classes. 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 the population, while the most frequent occurrence of unitsinferred to be coherentlavas surrounding the Tharsis type 4 cratersisobserved at latitudes where young lavas exist. volcanoes.Craters smaller than 10 km do not displaythis Polewardof 40øNand 40øS the type 6 cratersbecome much radialtexture, while craters larger than about 30 km in diame- moreprevalent. At theselatitudes, over 40% of all craters ter possessa more complex morphology incorporating mas- measuredwere of thisclass, while pancake craters are virtu- siveflow units with transverse ridging, pitted material, smooth ally absent (zero, except between 0 and 10øN, there they rep- materialsuperimposed upon the lobate ejecta, and secondary resent 3.5% of thetotal) from the equatorial latitudes. As will craters.Complex craters with these diverse ejecta materials be demonstrated in the next section, such a strikingdistribu- havebeen identified by Shultzand Gault [1979] and Mouginis- tion of pancakecraters cannot be attributedto thecharacter- Mark andHead [1979] and are describedas type5 cratersistics of anyspecific target material, so that in thisparticular (Figure 2e) in this analysis. The final crater group, type 6, is one describedhere as a 100- 'pancake' crater (Figure 2f). Such craters,typically lessthan 5 km in diameter, were defined to be morphologically simple bowl-shaped craters that possessa single continuous ejecta facies that appears to have been emplaced by a surface flow mechanism.The distal edgeof the pancakecrater ejecta material is characterized by a convex slope rather than the distal m 30- ridge associatedwith rampart craters. Frequently, the ejecta material has traveled a proportionally greater radial distance from the parent crater in comparisonto ejectaassociated with other typesof fluidized craters[Mouginis-Mark, 1978].A min- imum ejecta range equivalent to 3 crater radii from the center <1:_110- of the parent crater was required for the identification of a crater as a member of this type. In many instancesthis ejecta range for pancake craterswas found to exceed6 crater radii. Pancake craters are similar to the pedestal craters identified by Guestet al. [1977, Figure 4] but form only a subsetof the Z '/ /*-'V \\ 1:',, pedestalcraters described by Arvidsonet al. [1976] and Head I1:: 3. and Roth [1976].

SIZE DISTRIBUTION OF CRATERS Figure 3 illustrates the cumulative frequency of each type \// )i",. : """"" \ of crater as a function of diameter. It is clear that type 1 cra- "'.._,.\# ,, ,. , / ',t 5..- ters (N = 834) are predominantly smaller than 20 kin, while type 2 craters(N = 368) may be larger than 30 km. Type 3, 4, io ,•o :•o 4 •'o do do SOUTH NORTH and 5 craters (N = 50, 24, and 55, respectively)are nearly all LATITUDE larger than 10-km diameter and may be greater than 50-km N = diameter. In contrast,all type 6 craters(N = 226), the pancake Fig. 4. Latitudinal distribution of samplecraters. Each crater type class, are smaller than 8 km, and 95.6% are less than 5 km in is expressedas a percentageof all cratersobserved in any 10ø latitudi- diameter. nal band. Number of cratersat each latitude is given below abscissa. MOUGINIS-MARK: SECOND MARS COLLOQUIUM 8015

o• 80-

Fig. 5. Distribution of samplecraters as a function of target material. The frequencyof occurrenceof each crater type is expressedas a percentageof all cratersobserved on that geologicalunit. The definitionsof units not describedby Scott and Carr [1978]are givenin the text. Each terrain type is representedby at least30 craters.

case,crater distribution may be a latitudinal effect reflectinga (12.9%) on channelmaterial and are mostfrequently found on tendencyfor more mobile ejectato occurat high latitudes. ridged plains (25.1%). Other typesof cratersexhibit a strongerterrain affinity than DISTRIBUTION OF FLUIDIZED CRATERS WITH types 1 and 2. Expressedas a percentageof all craters on a RESPECT TO TARGET LITHOLOGY specific material, multiple-facies craters (type 3) are most Head [1976] and Cintala et al. [1977] have suggestedthat common (5.6%) on ridged plains and are lessthan 2% of the probable variations in strength,density, and other character- sampleon fractured terrain, Tharsis, and Elysium lavas. Radi- isticsof the target material may influencethe morphology of ally textured craters (type 4) are totally absent from fractured Mercurian and lunar impact craters. Such a correlation be- and ancient terrain, smoothplains, old lavas, and channel ma- tween crater morphology (crater type) and local terrain may terials. In contrast, they are most common on young lavas be expectedto be particularly important on Mars [Wood et al., (5.4% on Tharsisand 9.6% on Elysium) and rolling plains ma- 1978], owing to the probable high spatial variability in rock terials (6.5%). Complex craters (type 5) are only absent from strength and possible subsurfacevolatile content associated channelsand smooth plains and are most numerouson lavas, with fresh lavas, cratered and ridged plains, and the heavily ridged and rolling plains materials,and ancient terrain. Type cratered materials that constitute the ancient terrain. In an at- 5 craters represent 5-8% of the number of craters on each of tempt to investigatethe importanceof local geologyon the in- thesegeological units. cidence of each crater type, the target materials associated In the previous sectionthe pronouncedasymmetry in the with the samplecraters were categorizedinto 10 main geologi- distribution of pancake craters(type 6) was attributed to lati- cal units, adapted from the mapping of Scott and Cart [1978] tudinal effectsrather than physicalcharacteristics of the target and regional studiesof Guestet al. [1977]. A total of 1523 cra- material. Figure 5 demonstratesthat pancakecraters represent ters were observed on these 10 materials; the remainder of the the largestpercentage of craterson channel materials (54.8%) population lay on chaotic and knobby materials or were very and are also frequentlyfound on the fracturedterrain (46.2%), close to the rim of canyons. In order to present reasonable old lavas (28.7%), and smooth plains material (18.7%). All samplingstatistics the 35 cratersin theselast three categories theseunits representgood candidatesfor higher than average have been omitted from this part of the analysis. subsurfacevolatiles: many of the channelsare thought to be The distribution of each crater type on the different materi- the product of fluvial erosion [Baker, 1978], while all other als is shownin Figure 5, expressedas a percentageof all cra- terrains occur poleward of 40øN or 40øS, where near-surface ters observedon that unit. Reflectingtheir large total number, volatilesmay be concentrated[Fanale, 1976]. Conversely, geo- type 1 cratersconstitute 32-70% of all craterson any terrain. logical units that could be predicted to have low volatile con- They are most common (69.9%) on Tharsis lavas and com- tents, suchas recent volcanicsand terrains at high elevations, prise more than 60% on Elysium lavas, ridged plains, and an- possessvery few pancake craters.No pancake craterswere ob- cient terrain. Type I craters are least numerous (32.3%) on served on the ridged plains material or Tharsis or Elysium channel materials. Type 2 craters are also relatively scarce lavas, suggestingthat target materials predictedto have high 8016 MOUGINIS-MARK.' SECOND MARS COLLOQUIUM

Fig. 6. Comparisonof fluidizedcraters excavated in differentgeological units. Figure 6a depictsold lavas,Figure 6b showsridged plains material, and Figure 6c showsTharsis lavas. The smallestcraters on each unit that has a central peak (CP), secondarycraters (S), radial texture (RE), and complexejecta (CE) are indicatedwhere they exist. Scalebars represent 50 km in all three images. physical strength and low volatile content may preclude the different: more extensiveejecta is seen on the old lavas, while formation of these craters. central peaks occur within craters 6 km in diameter on the The predominance of radially textured craters (type 4) on ridged plains materials but do not occur until craters are 18 Tharsis and Elysium lavas, together with a virtual absenceof km in diameter on old lavas. Craters on Tharsis lavas (Figure type 3 craters (and fewer type 2), may indicate two alternate 6c) display exterior morphologiesmore complex than craters size-related trends in crater morphology associatedwith the formed in other target materials. Secondary craters (S) occur physical characteristicsof the target material. Figure 6 dis- around a 13-km crater, and a pronounced radial texture is plays three representativegroups of fluidized craters formed present on the ejecta of a 21-km crater. The near absenceof on different target materials. The old lavas shown in Figure 6a type 3 craters from this unit and the frequently observedradi- have ridges similar to those observedon the ridged plains ma- ally textured ejecta surroundingcraters larger than 15 km sug- terial (Figure 6b). However, the areal extent of the ejecta and gest that these two types of crater may be mutually exclusive the incidence of central peaks (CP) on the two units are quite and reflect the mechanical strength and/or volatile content of

8o w t-

w ,•, 60-

TERRAIN TYPES

z o ALL CRATERS, N=1469. ww •.o• ' ß LAVAS, N=139. o CRATERED PLAINS, N=302. Z•o- w ß RIDGED PLAINS, N=264. ß ANCIENT TERRAIN,N=249. w .

CRATER DIAMETER, km Fig. 7. Frequencyof central peak occurrencein samplecraters for different target materials.¾alu½s are expressedas the percentageof all craterswith the given diameter that have centralpeaks. Each data point representsat least:•0 craters. MOUGINIS-MARK:SECOND MARS COLLOQUIUM 8017

,O•o c• 80-

L,J 60

TERRAIN TYPES Om 40- o ALL CRATERS, N=1469. ß LAVAS, N = 13 9. o CRATERED PLAINS, N=302. •' o ß RIDGED PLAINS, N=264. o :: 20- ß ANCIENT TERRAIN,N=249.

CRATER DIAMETER, km Fig.8. Incidenceofterraces and scallops within fluidized craters on different geological units. Values are the percentages of cratersin eachsize range on the differentterrains that showevidence of wall failure. the target material at the time of crater formation. Alterna- VARIATIONS IN CRATER MORPHOLOGY tively, an elevation effect might be evident because the WITH TARGET MATERIAL Tharsis lavas are at an altitude of 5.5 km and the old lavas lie at an elevationof 2.5 km [Scottand Carr, 1978].This transi- The interior morphologyof Martian cratershas been inves- tion from one crater type to another can also be extended to tigated by several authors, on the basis of Mariner 9 data complex craters(type 5). Complex ejecta (CE) is observed [Cordellet al., 1974;Smith, 1976;Cintala et al., 1976b]and on aroundthe 35-km crateron the Tharsislavas, is only present a limited data base from Viking images[Carr et al., 1977; on the 65-km crater on ridged plains, and is absentfrom cra- Mouginis-Mark, 1977; Woodet al., 1978].Mariner 9 data in- ters formed on the old lavas. dicated that central peaksare more frequentlyobserved in

o5O

•40- -Jz TERRAIN TYPES •-0 o30- o ALL CRATERS, N=1469. <•Ld ß LAVAS, N=139. u_ c220- o CRATERED PLAINS, N=302. ß RIDGED PLAINS, N=264. co m 10- ß ANCIENT TERRAIN,N=249.

zO Ld O- s iS CRATER DIAMETER, kin Fig.9. Percentageofall craters on different target materials that have observed secondary craters beyond the crater rim. 8018 MOUGINIS-MARK: SECONDMARS COLLOQUIUM

80-

2-4(N=ii27)

n-J •40- 4-6(N:$27)

I.lUJ

,,, •20- +6(N:65)

60 4'0 io io 4:0 SOUTH NORTH LAIIIUDœ

SAMPLE SIZE

Fig. 10. Variabilityof ejectamobility with craterlatitude. For eachcrater the maximumejecta range from the center of the primaryis normalizedby the craterradius to givethe ejectarange ratio ER. The fourcurves illustrate the incidence of craterswith eachER valueas a percentageof all cratersin any 10ø latitudebin. At least30 cratersare representedat each latitude;N is the number of cratersat all latitudeswith eachER value.

Martian craters than within lunar cratersof comparable size were set at 5-km intervals,and each data point displayedrep- and degreeof preservation.Onset of Martian centralpeaks resentsat least 20 craters in that diameter range on that par- occurs at crater diameter of about 5 km, while the onset diam- ticular terrain. eter of terracesis comparablefor Martian and lunar craters. Centralpeaks. For the total populationof cratersthe fre- Cordellet al. [1974]reported that Martian centralpeaked cra- quency of peak occurrences(Figure 7) is intermediarybe- ters may be more prevalentin polar regionsthan near the tween the resultsof Carr et al. [1977] and Wood et al. [1978]. equator. The smallestcrater with a peak was 1.5 km in diameteron the Improvedresolution provided by Viking imagesaffords fracturedplains at 41øN, 10øW.With increasingdiameter the considerable revisions of the earlier Mariner 9 observations to percentageof craterswith centralpeaks rises from 3-9% be- be made[Wood et al., 1978].The database presented here al- tween zero and 5-km diameter (N = 456) to 75.7%between 30 lowsa more extensiveanalysis of fluidizedcrater morphology and 35 km (N = 33). On Tharsisand Elysiumlavas, 10%of on a near-globalscale. The presenceof centralpeaks (includ- craters smaller than 5 km have peaks (N = 20), while this ing peakswith summitpits and peakrings), wall terraces(in- value decreasesto 5.3% (N = 75) for crateredplains and 3.2% cludingscallops; Cintala et al. [1977]),and secondarycraters (N = 62) for ancientterrain. For the rangeof craterdiameters were recordedwhere observedas a function of physiographic and targetmaterials investigated here, the incidenceof central provincefor all 1558craters studied here. structureswas found to vary by 10-30 percentagepoints from Illustrated in Figures 7 and 8 are the percentageoccur- the geologicalunit with the most peaksto the material with rencesof peaksand terracesfor differenttarget materials. In the fewest central structures. each case it is assumedthat the crater is excavated in a target Terraces. The incidence of terraces and scallopswithin of uniform physicaland mechanicalproperties. Buried frag- fluidized craters(Figure 8) displaysa distributionsimilar to mental material,eolian depositscovered by lavas,or a vari- that of centralpeaks. The smallestcrater observed with wall able thicknessof groundice/subsurface water may make this failure was 3.0 km in diameter and was excavated in smooth an oversimplificationof reality,but this situationis not con- plainsmaterial at 44øS,254øW. However,Figure 8 illustrates sideredin this analysis.Only the more frequentlyoccurring that cratersexcavated in lavas systematicallypossess a higher geologicalunits are shownin Figures7 and 8 (Tharsisand frequencyof wall failure cratersthan target materialsinter- Elysiumlavas are combinedinto a singleunit), althoughall pretedto be lessconsolidated, particularly the ancientterrain. cratersare included in the whole samplecurve. Sample bins A separationof 28 percentagepoints is observedin the in- MOUGINIS-MARK:SECOND MARS COLLOQUIUM 8019

occurrenceof secondarycraters around larger-diameter primary craters. i- •80- 2-4(N= 1127) MOBILITY OF EJECTA DEPOSITS Experimentalstudies of crateringevents into viscous-liquid targetsby Gault and Greeley[1978] indicatedthat decreasing 3:60-

<3: target viscositypromoted postdepositionalflow of the ejecta and an increasein the radial extent of the continuous ejecta deposits.Although Gaultand Greeley[1978] caution the appli- •:40- cability of extrapolating small-scaleexperiments to craters tens of kilometersin diameter, the relationshipbetween ejecta viscosityand maximumrange may alsoapply to the emplacement of fluidized Martian ejecta. From analysisof Mariner 9 •20- images,when the range of the continuousejecta depositswas normalized to the size of the parent crater, both Head and , , 4-6 (N=327) Roth [1976] and Arvidsonet al. [1976] noted a slight increase O- 2(N= 36) in the ejectarange ratio with increasingcrater latitude, consis- 0 -- +6(N=65) tentwith the hypothesis of Fanale [1976] that subsurface volatiles are concentrated toward the polar regions of Mars. A HEIGHT ABOVE MARS DATUM (km) similar increasein ejecta mobility, independent of latitude, was also reportedby Mouginis-Mark [1978] for progressively SIZE I II cu larger sizesof craters,which may have been relatedto the ex- Fig. 11. Variability of ejectamobility with crateraltitude. ER val- cavation of volatile-rich material at greater depths beneath ues are calculatedin the samemanner as the data shown in Figure 10. the surface. Elevations are taken from the map of Scott and Carr [1978] and are Ejecta mobility as a function of crater latitude and altitude subdivided into 1-km elevation bins. Each altitude is representedby at least 30 craters. canbe expressedwith the availabledata. For eachcrater the maximum range of the ejecta deposit from the center of the cidence of terraces in craters 10-15 km in diameter excavated primary craterwas normalized by the radiusof the parent cra- in lavas(81.8%, N = 22)and ancient terrain (54.2%, N = 59). ter (a ratio henceforthreferred to as 'ER'). ER ratios were Secondarycraters. Howard [1974] has identifiedsecondary then subdividedinto four samplebins (<2, 2-4, 4-6, and >6), cratersto be ubiquitous around fresh lunar craterslarger than allowing the percentageof all craters at that latitude or alti- I km in diameter.In contrast,the fluidized nature of crater tude to be calculatedfor each samplebin. ejecta blanketgapparently precludes the formation (or preser- Figure 10 displaysthe variability of ER with crater latitude. vation) of SecondaryCraters around many Martian craters.Al- Between 40 and 85% of all cratersat any latitude have ER be- though secondarycraters have been observedaround certain tween2 and 4, with the greatestpercentages (>80%) occurring rampart craters [Carr et al., 1977; Schultz and Gault, 1979; close to the equator. A similar equatorial concentrationof Mouginis-Mark and Head, 1979], the physical characteristics low-mobilityejecta (ER < 2) is alsoobserved: zero at 50ø- (mechanicalstrength, mean particle size,and volatile content) 70øS and 50ø-60øN, and 17.5% at 0ø-I0øN. Conversely,the of the Martian surface may exert a considerableinfluence on mostmobile ejectablankets are observedin the mid-latitudes the production of ejecta blocks sufficientlylarge to produce to high latitudes; 10%of all cratersbetween 60 ø and 70øS and secondarycraters. Alternatively, the surfaceflow emplace- 16% between 50ø and 60øN have ER values greater than 6, ment of ejecta material effectivelyburies all the near-field bal- whileno craters with ER this high occur between 0ø-30øS and listicejecta for moS{ craters formed in fragmentaltargets. 20ø-30øN. Craters with ER between 4 and 6 are also most The incidenceof secondarycraters around fluidized craters common (greater than 35% at any latitude) poleward of 60øS on differentgeological units was recordedwithin this morpho- and40øN. It is thereforeconcluded that crater latitude has an logical analysis.For craters between 5 and 10 km in diameter influence on the mobility of the ejecta, indicating that if the (N -- 478), 11.5%of the samplewere found to have secondary conclusionsof Gault and Greeley[1978] are correct, lower ef- craters.This percentageincreased to 21.2% (N -- 33) for cra- fective viscosityejecta flows occur preferentially at high lati- ters 25-30 km in diameter. For all craters smaller than 30 km tudes, perhaps owing to a greater concentrationof subsurface in diameter(N = 1469),only 8.4% possessed secondaries. The volatiiesin thoseregions [Fanale, 1976]. smallestprimary crater (3.3-kin diameter) with secondarycra- On the basis of the elevations provided by Scott and Carr tersoccurs on the ridgedplains at 20øN,58øW. [1978] it has also been possibleto correlate ER values with Figure 9 presentsfor each size range of parent crater the crater elevation. Comparing the percentage of craters with percentageof fluidized craterson each terrain with secondary eachER valueto theheight of thecrater above the 6. l-mbar craters. Apparently, there is a strong correlation between tar- reference datum on Mars, Figure 11 indicates that elevation get material and the preservationof secondarycraters. For also exerts an influence on the mobility of crater ejecta. At 2 primary craterslarger than 5 km in diameter, secondarycra- km below the mean surface,9.7% of all craters(N - 31) have ters are approximately 3 times more common on Tharsis and ER values greater than 6, and 54.8% have ER between 4 and Elysium lavas than the averagevalue and 10 times more com- 6. With increasingaltitude no cratershigher than 4 km (N -- mon than on ancient terrain. Values for cratered plains are 395) have ER greaterthan 6, and only 11.5%of all cratersbe- consistently4-8 percentagepoints higher than ridged plains tween 7 and 8 km (N-- 26) have ER between 4 and 6. Also and 10-15 points higher than the ancient terrain values. For evident from Figure 11 is a progressiveincrease in this per- each target material, there is an increasein the frequencyof centageof craterswith ER < 4 with greater elevations. 8020 MOUGINIS-MARK: SECOND MARS COLLOQUIUM

Fig. 12. (a-d) The effectsof altitude on the morphologyof fresh Martian craters.All four cratersillustrated are 16-18 km in diameter and are excavated in Thatsis lavas. The range of elevations is from 200 m below mean Mars datum (Figure 12a)to 22 km abovethe 6. l-mbar referencelevel (Figure 12d).See text for full discussion.Scale bars are 30 km in all pictures.

This decreasein ejecta mobility is strikingly illustrated in ters, terraces, and a small peak are observed, and there is a Figure 12,which comparesfour craters,all 14-16 km in diam- radial pattern superimposedupon the continuousejecta de- eter, formed on Tharsis lavas for a variety of elevations. Fig- posit.With increasingelevation the ER value falls to 3.04 at 6 ure 12a showsa rampart crater at a level of 200 m below mean km (Figure 12c), and the continuousfluidized ejecta deposits datum. The ER value for this crater is 4.24. Morphologically, have been replaced by a pronouncedradial texture and a ser- the crater displaysa typical fluidized ejecta blanket, with dis- rated distal edge. Well-developed terracesand a central peak tal rampartsand an absenceof secondarycraters. The interior are also evident. The extreme caseof a fresh impact crater at a morphologyis relatively simple:a flat floor with no evidence high elevation (22 km) is shown in Figure 12d. This crater oc- of major wall failure. At a height of 2 km (Figure 12b), ER = curs close to the summit of Olympus Mons and is more char- 3.58, and crater morphology is more complex; secondarycra- acteristic of a lunar crater than a fluidized crater. There are no MOUGINIS-MARK: SECOND MARS COLLOQUIUM 8021

continuousejecta faciescomparable to the rampart lobes, and can be made: many secondary craters occur close to the crater rim. There 1. With the exception of pancake craters (type 6), all are alsoextensive scallops on the crater floor and wall. classes of fluidized craters are found all over the Martian sur- Several alternate explanations are possible to account for face. As noted by Allen [1978], there is no obvious latitudinal this decrease in the ER ratios and the more lunarlike mor- control for the incidence of these craters. phology of cratersat high elevations.Target texture may be 2. The occurrence of different classesof craters, the in- highly variable on the flanks of Olympus Mons. Lava flows cidence of central peaks and terraces,and the preservationof may be interlayered with ash depositsat unknown elevations secondarycraters can be strongly influenced by the inferred on the volcano, and this would significantly affect the in situ character of the target material. Craters formed on recent particle sizeprior to the impact event and hencethe ejecta size lavas more frequently possesssecondary craters, central distribution (P. H. Schultz, personal communication, 1979). peaks,and terracesthan primary cratersfound on other geo- Atmospheric decelerationof this fine ejecta could therefore logical units. Conversely,craters on ancient terrain have a less operate preferentially in areaswhere ash layers have been ex- complex morphology than comparable-sized craters excavated,which would in turn influencethe morphologyof the cavated in different materials.Radially textured craters (type ejecta blanket. Subsurfacevolatiles have been suggestedas the 4) are most common on lavas and rolling plains material, fluidizing medium for rampart crater ejecta emplacement while few cratersdisplaying multiple ejectafacies (type 3 cra- [Head and Roth, 1976; Carr et al., 1977; Mouginis-Mark, ters) are observedon theseunits. The gradational character of 1977], and it would appear reasonablefor thesevolatiles to ac- type I cratersinto type 4 and 5 with increasingdiameter sug- cumulate preferentially at lower altitudes. Hence the ejecta geststhat crater size is the determining factor in controlling viscosityof craters formed at low elevations would be less crater morphologyfor any singletarget material. than ejecta at greater heights,increasing low-level ER values. 3. The mobility of the fluidized ejecta is influencedby Alternatively, atmospheric effects may be responsible for both crater latitude and altitude. The maximum ejecta range ejecta fluidization. An air-layer support mechanism,analo- occursat high latitudes and low elevations.For laboratory- gous to that proposedfor some terrestrial landslides[Shreve, scale experiments,Gault and Greeley [1978] attribute ejecta 1968], may be significantfor ejecta mobilization. At an alti- mobility to the viscosityof the ejecta and hence target mate- tude of 22 km (2.75 Martian scale heights; Seiff and Kirk rial. If meaningful extrapolationscan be made from theseex- [1977]) the influence of the atmospherewill be less pro- perimentsto the Martian environment,this impliesthat if the nouncedthan at Mars datum, possiblyreducing the fluidiza- crater ejectawere fluidizedby subsurfacevolatiles, the ratio of tion of the ejecta by the absenceof atmosphereentrainment maximum ejectarange to crater radius(ER) might provide an within the ejecta cloud. indicator of the relative target volatile content at the time of crater formation.If this were to be the case,then a technique SUMMARY AND IMPLICATIONS FOR FUTURE ANALYSISOF FLUIDIZED CRATERS to differentiate between the extent of ejecta fiuidization by targetvolatiles [Head and Roth, 1976;Carr et al., 1977;Moug- This paper representsa preliminary analysis of the large inis-Mark, 1977] and atmosphericeffects [Schultz and Gault, volume of morphological data derived from approximately 1979]would exist. A comparisonof equal-diametercraters ex- two thirds of the suitable images acquired by the Viking cavatedin materialsinferred to have similar properties,at the spacecraftduring their first 700 orbits. Work is currently un- sameelevation but at differentlatitudes, should produce equal der way to extend this investigationto include more craters ER values if atmosphericeffects dominate. Varying ER ratios photographedat medium resolutionin order to improve the as a function of latitude would indicate that subsurface vola- global distribution of the data base. With the recognitionthat tiles have the main influenceof fluidizedcrater ejecta.Investi- the samplepreented here is preferentiallylocated in regions gations designed to resolve the discordanceof ideas are cur- with high-resOlUtionphotography, the followingconclusions rently being conducted.

TABLE A1. Viking ImagesSupport Data Figure Viking Elevation,J- 1 Resolu- in Text Frame Position* km Terrains tion,* m 2a 545A45 22øN, 175øW 0.7 smoothplains 147 2b 60A53 49øN, 349øW 0.3 crateredplains 53 2c 87A11 26øS, 255øW 3.8 ridged plains 229 2d 612A24 35øN, 239øW -0.5 rolling plains 83 2e 22A54 24øN, 52øW 0.5 crateredplains 43 (near canyon) 2f 581A10 41øS, 342øW 4.2 ancientterrain 155 6a 94A63 57øS, 323 øW 2.5 old volcanics 242 6b 87A20 14øS,246øW 3.7 ridged plains 234 6c 56A40 29øS, 121øW 5.5 Tharis lavas 216 12a 7B23 49øN, 114øW -0.2 Tharsis lavas 95 12b 7B55 45øN, 113øW 2.0 Tharsis Lavas 99 •12c 56A42 27øS, 119øW 6.0 Tharsislavas 216 12d 46B 13 19øN, 131øW 22.0 Tharsis lavas 91

*Center point of picture. •-Data from Scott and Carr [1978]. $Taken from mappingof Scottand Carr [1978]and regionalstudy of Guestet al. [1977]. 8022 MOUGINIS-MARK:SECOND MARS COLLOQUIUM

Acknowledgments. The author wishes to thank Ed Robinson for ters formed in viscous-liquidtarget: Analogs for martian rampart producingthe computersoftware needed to collect and processthe craters?,Icarus, 34, 486-495, 1978. datapresented here. Mark Cintala, Peter Schultz, Carlton Allen, and Guest, J. E., P.S. Butterworth,and R. Greeley, Geologicalobserva- two anonymousreviewers provided thought-provoking reviews of an tionsin the Cydoniaregion of Marsfrom Viking, J. Geophys.Res., earlier manuscript.David Haas and Mary Sommer helped with the 82, 4111-4120, 1977. compilationof the photographicdata base,and Margaret Cummings, Head, J. W., The significanceof substratecharacteristics in determin- Nancy Christy, and Lisa Ranalli typedthe manuscript.This work was ing morphology and morphometry of lunar craters,Proc. Lunar supportedby NASA grantsNGR 40-002-008and NGR 40-002-116. Sci. Conf 7th, 2913-2929, 1976. Head, J. W., and R. 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