VOL. 76, NO. 2 JOURNAL OF GEOPHYSICAL RESEARCH JANUARY 10, 1971 The Surface of Mars i. Cratered Terrains • BRUCEC. MURRAY,LAURENCE A. SODERBLOM, ROBERTP. SHARP,AND JAMESA. CUTTS Division of GeologicalSciences California Institute of Technology,Pasadena 91109 Mariner 6 and 7 pictures show that craters are the dominant landform on Mars and that their occurrenceis not correlated uniquely with latitude, elevation, or albedo markings. Two distinct morphological classesare recognized: small bowl-shaped and large flat-bottomed. The former showlittle evidenceof modifications,whereas the latter appear generally more modified than hmar upland cratersof comparablesize. A regionalmaria/uplands dichotomylike the moon has not yet been recognized on Mars. Crater modification on Mars has involved much greater horizontal redistribution of material than in the lunar uplands. It is possiblethat there are erosionalprocesses only infrequently active. Analysisof the natures and fluxesof bodiesthat have probably impacted the moon and Mars leads to the likelihood that most of the large flat-bottomed craters on Mars have survived from the final phasesof planetary accretion. Significant crater modification, how- ever, has taken place more recently on Mars. Inasmuch as the present small bowl-shaped craters evidence little modification, the postaccretion crater-modification processon Mars may have been primarily episodic rather than continuous. The size-frequency distribution of impacting bodiesthat produced the present small Martian bowl-shapedcraters differs from that responsible for post-mare primary impacts on the moon by a marked deficiency of large bodies. Survival of crater topography from the end of planetary accretion would make any hypothetical earthlike phasewith primitive oceansthere unlikely. The traditional view of Mars as an earthlike planetary neighbor in terms of its surface history is not supported by the picture data. Perhapsthe most excitingresult of the Mariner Mariner 6 and 7 television pictures as they bear 4 television experiment in 1965 was the discovery on the nature of the Martian surface.The present of cratered terrains on Mars, a generally unex- paper deals with the nature and significanceof pected addition to the annual development of cratered terrains. The second is concerned with the frost caps, seasonaldarkenings, and other uncratered terrains [Sharp et al., 1971a]. The presumed earthlike phenomena.The television third paper discussesphotographic observations pictures returned by Mariners 6 and 7 in 1969 relevant to the nature of light and dark markings greatly extended knowledgeof Martian cratered [Cutts et al., 1971], and the fourth describes terrains and showed two new uncratered terrains observationsand implications of surface features as well. Close-up pictures were obtained of the of the south polar cap [Sharp et al., 1971b]. We south polar cap and also of certain prominent shall refer to them as papers 1, 2, 3, and 4, light-dark boundaries. These and other results respectively. of the television experimentswere first discussed The picture data used are principally the in three preliminary reports published shortly 'maximum-discriminability' versions displayed after receipt of the picture data [Leightonet al., in the accompanying papers by Dunne et al. 1969a, b, c]. The objectiveof this paper and of the [1971] in this issue, supplemented by other three companionpapers is to describethe results versions of the near- and far-encounter photog- of subsequentanalysis and interpretation of the raphy. Image-processing techniques are dis- cussedin a separate paper in this issue [Rind- fieis& et al., 1971]. Picture location and notation i Contribution 1891, Division of Geological are summarized in the preceding article in this Sciences, California Institute of Technology, Pasadena. issueby Leighton and Murray. Craters are visible in 52 of the 55 near-en- Copyright • 1971 by the American Geophysical lJnion. counter frames (best resolution 0.3 km) and 313 314 M•URRAY ET AL. constitute the principal landform observed. At surface processesand history of Mars inferred this early stageof Martian exploration we define from the evidence of the terrains already recog- cratered terrains to be regions of the surface nized will provide a useful framework for more in which craters are the dominant, often the detailed information and knowledge to be ac- only, topographic forms recognizable at the quired by future spacemissions. resolution of the Mariner 6 and 7 pictures. In the following, the geographic distribution Intercrater areas are included in this definition. of cratered terrain is reviewed and possible As better pictures become available, more correlations,with elevation, latitude, and albedo sophisticatedcriteria may haveto be developed are considered.Morphology of local features and to categorizewhat may well be a variety of crater abundances is then treated. Martian }Martian terrains. We feel, nevertheless,that the observations are compared with those of the 4N14 Fig. la. Positions of Mariner 4 photographs4N7 through 4N14 plotted on Mariner 7 far- encounterphotograph 7F76 with a sub-spacecraftlongitude of 199øE.The ringed structure,Nix Olympica, is visible in the northern hemisphere. SURFACEOF MARs--CRATERED TERRAINS 315 4N9 Fig. lb. Crateredterrain displayedin Mariner 4 frames4N7 through4N14, taken across Zephyria,Mare Sirenurn,Mare Cimmerium,and Phaethontis.(See paper 3 for identification of namedfeatures.) Frames 4N8 and 4N13 containprominent light/dark boundaries.Individual frames are about 250 km on a side. Longitude, latitude, and solar elevation angle range from 186øE,13øS, 61 ø for 4N7 through209øE, 42øS, 29 ø for 4N14. Detailsare availablein Leighton ct al. [1967]. lunar-crateredterrains, leading to the conclusion or dark areas or with any particular elevation that many largeMartian craters,like thoseof the range, and it contains two distinct types of lunar uplands,have survivedsince the last phases craters: large fiat-bottomed and small bowl- of planetary accretion. The later history of shaped. In this section we summarize the Mars, however,appears to differ significantly characteristics of Martian cratered terrain and from that of the moon. Finally, the traditional compareit with the crateredterrain of the moon. view of Mars as once having experiencedearth- Geographicdistribution. Craters ranging in like conditions is recvaluated in light of the diameter from a few hundred meters to a few similarities of the Martian cratered terrain to the hundred kilometers are visible in the Mariner 6 lunar uplands. and 7 photographs.If this sampleis representa- tive, cratered terrain (including intercrater OBSERVATIONS OF MARTIAN CRATERED TERRAINS areas) constitutesat least 90% of the Martian Cratered terrain on Mars was first revealed surface. in televisionpictures returned by Mariner 4 in Somevery largecraters are recognizablein 1965. Mariners 6 and 7 extended the observations the far-encounter frames of Mariner 6 and 7, of cratered terrain sufiqciently to demonstrate especiallyin the dark area Syrtis Major (frame that it is the dominant Martian landscape;it is 7F87) and Mare Cimmerium(frame 7F82). The not uniquelycorrelated in occurrencewith light featureNix Olympica,long recognized as a bright 316 MURRAY ET AL. patch from earth-basedobservations, is seenat Figures3 and 4 of paper2 and Figures1 and 2 of higherresolution (frame 7F77) to be a multiple- paper 4). ring structureof maximumdimension exceeding Figureslb, 2, and 3 in this paperand Figure3 500 kms. Presumablythis is at least the remnant in paper4 are mosaicscomposed of the bestnear- of a very large crater. Circular features almost encounterphotography from Mariners4, 6, and 7. as large are faintly visiblein bright areaswithin They all show predominatelycratered terrain. frames 7F83 and 7F84. The polar cap edge Each of thesemosaics is discussedbriefly in the observed in the late far-encounter frames of both context of the regional associationsof Martian Mariner6 andMariner 7 is partlydelineated by cratered terrain. largec•aters up to 110 km in diameter(see Craters from 4 to 350 km in diameter are Fig. 2. Area of mosaicof frames6N9 through6N23, shownin outlineon the far-encounter frame7F67, i•mluding the dark areasMeridiani Sinus and SabaeusSinus and the light area l)eucalionisRegio. (Geographic names are locatedin paper3.) Solarelevation angles, which rangefrom 52ø downto 3 ø are shownat the lowerright-hand corner of the B frames.The outline of thesenarrow-angle frames is shownin whiteon the mosaic of A frames,along with latitude and lo•gitudegrid li•tes. Individual A framesare of the orderof 1000km in the longdirection; B framesare l/10 scaleof A frames.One degreeof latitude on Mars is 59 km. Maximum discrim- inability versionshave beenused that greatlyaccentuate topographic detail but distortand suppresssome albedo marking, especially the outlines of Meridiani Sinus. The crater counts usedi•t Figre'es4, 5, and6 arederived from frames 6N16 to 6N23and refer principally to the I)eucalionis Regio area. SURFACEOF •/•ARS--CRATEREDTERRAINS 317 318 M•RR^¾ ET AL. visible in Mariner 4 frames 4N5 through 4N14, By way of comparison, on the moon two distinct as is shown in Figures l a and lb. A prominent kinds of cratered terrain are evident that bear a- light/dark boundary,the northernboundary of simple relationship to height and light and dark Mare Sirenurn, is included in the pair of frames markings. On the moon, large, highly-modified 4N?/4NS. Crateredtopography does