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VOL. 76, NO. 2 JOURNAL OF GEOPHYSICAI, RESEARCH JANUARY 10, 1971

The Surface of Mars

g. Uneratered Terrains •

ROBERTP. SHARP,LAURENCE A. 8ODERBLOM BRUCEC. MURRAY, AND JAMESA. CUTTS Division of GeologicalSciences California Institute of Technology,Pasadena 91109

Mariner 6 and 7 photographs reveal two types of uncratered terrain on Mars. These are de- scriptively termed chaotic and featureless.Chaotic terrain is younger than cratered terrain and displays features strongly suggestiveof slump and collapse. The speculation is offered that it may be an expressionof geothermal developmentswithin Mars that only recently have begun to affect the surface. Featureless terrain, identified only within the large circular area Hellas, is devoid of any discernible topographic forms larger than the limit of resolution, about 500 meters. Mariner 7 data indicate that Hellas is a topographically low and structurally old basin. Smoothness of its floor could be the product of a recent event or of continuous processesthat obliterate craters. Local processesof high efficacy, unusual surface materials, or both, are prob- ably involved. Through its chaotic terrain the martian surface displays a development that does not seem to be recorded, at least in the form of preserved recognizable evidence, on the moon or earth.

Cratered terrain [Murray et al., 1971] is the this article are designed to show regional re- most extensive and enduring surface on Mars. lationships more than specificdetails. As such,it servesas a datum against which other CHAOTIC TERRAIN terrains can be evaluated. Two additional ter- rain types have been identified on Mariner 6 and Description. The areas of chaotic terrain 7 frames. They are regardedas distinctly younger recognized to date comprise 1.5 X 10økm 2 than cratered terrain, the product of more re- within Pyrrhae Regio and adjacent regions cently dynamic surface or near-surfaceprocesses, centeredat about 10øS, 325øE. Chaotic terrain and possiblyrelated to a maturing phase of in- consistsof a rough, irregular complex of short ternal planetary evolution. ridges, knobs, and irregularly shaped troughs To avoid genetic implications, these terrains and depressionsbest seen on individual frames are descriptively designated 'chaotic' and 'fea- 6N6, 6N8, and 6N14 and also shownon Figure 1. tureless'. Chaotic terrain is an irregular jumble The scale of individual features is in kilometers. of topographic forms, and featureless terrain is They display somethingof a northeasterly grain without recognizabletopographic configurations. (6N6), which is accentuated by lighting and We describethese terrains, advance speculations foreshortening. concerning their relative ages and origins, and Chaotic terrain appears nearly devoid of examine implications with respect to the evo- recognizable craters. Only three have been lution of Mars. tentatively identified, and all are of questionable Readers will find reference to the individual location. However, craters smaller than 5 km photo frames at the end of this section useful. may be hard to recognizewithin the jumble of The photomosaicsand compositesaccompanying chaotic terrain features. Other crudely circular depressionstherein are probably not craters. Some areas of probable chaotic terrain appear 1 Contribution 1882, Division of Geological Sciences, California Institute of Technology, on A frames as brighter than the surroundings, Pasadena. but chaotic terrain is not everywhere uniquely bright. Differences of brightness within chaotic Copyright • 1971 by the American Geophysical Union. terrain are as great in some instances as the

331 332 SHARP ET AL.

t •:- 290E 7F69 6N8 6'1"

3O0 E

6N14 Fig. 1. Mariner 6 A and B framesshowing chaotic terrain, global locationupper right. White rectangleson A frame 6N7 indicatelocation and overlapof B frames6N6 and 6N14 in west and location of 6N8 in east. Scale, 10ø latitude and longitude, roughly 590 km. 6N6, 158 X 108 km; 6N14, 236 X 96 km; 6N8, 125 X 96 km. Sun anglesabove horizonat lower right of B frames. differences between chaotic and cratered ter- distinctive topographic texture of chaotic ter- rains. rain over a greater area. Distribution. Chaotic terrain was initially By tracing contacts between chaotic and recognizedon B frames 6N6, 6N8, and 6N14, to cratered terrains from B frames onto the en- a total combined area of 12,500 kin*. Improved closing A frames and by extending these con- processingof A framessubsequently revealed the tacts on the basis of structural patterns, crater SURFACE OF MARs-UNCRATERED TEHRAINS 333 distribution, brightness contrasts, and char­ 1 % of the martian surface has been photographed acteristic regional trends, a map of possible adequately to reveal its presence. Irregular chaotic-terrain distribution in A frames 6N5, streaky patterns on Mariner 4 frame 2 hint at 6N7, and 6N9 has been constructed (Figure 2). the possible existence of chaotic terrain in the vicinity of 25°N. Recognized chaotic terrain This map shows approximately 1.5 X 106 km' of chaotic terrain, irregularly distributed in odd­ is centered in the mixed light and dark area shaped patches, displaying something of an Pyrrhae Regio and appears to extend at least elongation to the northeast that is accentuated modest distances westward into dark Aurorae by the obliqueness of view. The mapping pro­ Sinus, northeasterly into dark Margaritifer Sinus, cedure is subjective, .so Figure 2 is, at best, an and northward into light Chryse. The northern approximation. CO, pressures recorded by the tip of Margaritifer Sinus may be defined by a Mariner 6 infrared spectrometer [Herr et al., contact between chaotic terrain, to the north, 1970] suggest unusual topographic roughness in and cratered terrain (see SW part of 6N9). this region. Geometrical relationship to cratered terrain. The chaotic terrain recognized on Mariner Where the chaotic-cratered terrain contact is 1969 photos lies within an equatorial belt ex­ clearly shown, as on 6N7, 6N8, 6N9, and 6N14, tending between 15°S and 15°N. A conclusion it is planimetrically irregular but well defined. that this terrain is confined to equatorial lati­ In places (6N6, 6N8), the topography immedi­ tudes is premature, however, for much less than ately to the chaotic side displays features re-

I / 7°./ C2J Postulated Areas of 0.':,' : Chaotic Terra in / o t;:>- . Crater O�o�j ("j Obscure Crater

·0 • I fO at Equator: 59 km o 0 .0 0\00 / o .-----I o /10·S / __ 0 0 - -- _\..: 6N9/ 0' -- 'b. '..1

'610•5 <$. q, °0 Fig. 2. Interpretive map of chaotic-terrain distribution constructed from Mariner 1969 photos. Compare Figure 1 for location of B frames and other details.

. 334 S•^RP ET AL.

sembling arcuate break-away scarps, arcuate area immediately southeast of the Alpine blocks and ridges, and intervening irregularly Valley (Cassini quadrangle, south). Similarities shaped depressions. Extending into cratered exist between these lunar areas and martian terrain away from the contact are irregular, sub- chaotic terrain, but the dissimilarities are even parallel markings that look like huge cracks. greater.The distributionpattern, grossgeometry, Examples are south of the valley in 6N14 and and particularly the inset position of chaotic west of the contact in 6N8. terrain are not characteristic of an ejecta sheet. Wherever relationships are well displayed On earth some accumulations of volcanic (6N14, 6N8), areas of chaotic terrain appear material display highly irregular topographic lower than the adjoining cratered surface, and forms, which, on a small scale and as individual this is confirmedby C02 data [Herr et al., 1970]. features, resemblesome aspects of chaotic terrain. Furthermore, these data and radar observations In larger view, the volcanic phenomena are [Anonymous, 1970, p. 30] indicate that chaotic hardly comparable in distribution pattern, and terrain occupiesa regionally low area. they are not at all comparable in the topo- Relative age. Low elevation, contact con- graphically inset relationship. Marginal slump figuration, and the marginal features of chaotic blocks and extramarginal fractures are not terrain plus the surface markings (cracks?) on particularly characteristicof lava accumulations. adjoining cratered surfacessuggest that chaotic Large-scale collapse is, of course, commonly terrain formed at the expense of cratered ter- associated with volcanism [Williams, 1941; rain. If this is a valid conclusion,chaotic terrain Cotton, 1944], but the structures developed, must be younger. Further, the chaotic terrain caldera and volcano-tectonicdepressions, do not formed after the interval that reduced the re- have the geometricform or distribution displayed lief of fiat-floored craters, for such modification by areas of martian chaotic terrain. would surely have destroyed the fresh, sharp Complex dune sheets have topographic forms features of chaotic terrain. resembling somewhat the ridge and trough The paucity of craters in chaotic terrain pattern of some chaotic terrain (6N6), the suggeststhat it is a relatively late development Tifernine dunes of Algeria for example (P. D. in martian history. Insofar as craters larger than Lowman, personal communication, 1969). How- 15 km acrossare concerned,this is a reasonable ever, individual dunes within such complexes conclusion. For small, bowl-shaped craters are on a considerably smaller scale than the relationships are less clear. The number of ridges and troughs of chaotic terrain. Even when bowl-shapedcraters on surfacesadjoining chaotic allowancesare made for differencesin gravity, terrain in 6N8 and 6N14 suggeststhat the area atmosphericdensity, grain size, wind velocity, of a chaotic terrain shown on B frames should and similar factors,it seemsunlikely that martian display at least 6 or 7 bowl-shapedcraters. Only transverse dunes, if they exist, will be several three marginal examplesare identified. Consider- times larger than their earthly counterparts. ing the difiqculty of identifying small craters The arcuate breakaway scarps,rotated blocks, within areas of chaotic terrain, this difference, parallel ridges, closed depressions, jumbled by itself, is hardly enough to establish a young aspect, and surface cracks (?) associated with age for chaotic terrain, but coupled with the chaotic terrain are all features found in slump, probable youth of bowl-shapedcraters [Murray slide, or collapseareas on earth. The chaosvalley et al., 1971], it is strongly suggestive. of 6N14 looks like a feature that has extended Terrestrial and lunar analog. Understanding itself headward and sidewise into cratered of chaotic terrain is sought through comparison terrain, and the transition zone between cratered with possiblyanalogous phenomena on the moon and chaotic terrains shown on 6N8 displaysthe and earth. Thoughts turn naturally to meteorite characteristicsof a slump zoneon earth, although impacts and volcanic explosions,one or possibly the scaleis unusually large. both of which have formed areas of irregular It appearsthat nothing currently known on the hummocky terrain on the moon, as, for example, moon or earth is wholly analogous to martian in parts of the ejecta sheet along the southern chaoticterrain. However, many of its characteris- rim of Mare Imbrium (Montes Apenninus and tics are similar to features produced by or Mare Vaporum quadrangles) and in the rough associated with terrestrial collapse, slumps, SURFACE OF ]•ARS--[•NCIlATEIIED TEllRAINS 335

slides, or other forms of mass movement. The the associationwith a topographically low region problem is to find explanations for possible [.lnonymous, 1970; Herr et al., 1970] suggest large-scalemass movementson Mars occurring that climatological change might be a genetic after what appears to have been a considerable factor. If Mars had earlier developed large period of relative stability during which the bodies of segregatedground ice, climatic ame- ancient cratered terrain was greatly modified. lioration might cause decay of such bodies to Origin. Before consideringevents, processes, occur earliest in low areas within equatorial or influencesthat couldhave generatedsuch mass regions. Decay of segregatedbodies of ice, not movements, the nature of the materials involved just the thawing of perennially frozen ground, merits attention. The abundance and sizes of would be needed to produce the degree of craters [Murray et al., 1971] suggestthat the collapse required. Under current martian atmos- martian surfaceshould be mantled to a depth of pheric conditionsany melt water formed would severalhundred meters by a looseimpact rubble, quickly evaporate, if it approached the surface. and that rocks of the crust should be shattered The inset relationship of chaotic terrain to still greater depths. Rubble is used here to implies removal of material by some means designate a loose jumble of fragmented rock either downward, laterally outward, or upward. debrisof all particle sizes,which may not have A mechanism for upward removal of solid been transported any significant distance from materials coming readily to mind is deflation the original bedrock source. Under proper by wind. On earth, deflation has produced large conditions such rubble could undergo mass hollows, and slumping around the margins of movements, even on modest slopes. such depressions might produce features like Among processesof external origin that might those seen along the margins of chaotic terrain. cause mass movements, the most obvious is The large chaosvalley of 6N14 might conceivably meteorite impact. Vibrations generated by large be part of a deflationhollow modifiedby marginal impacts could presumably induce movement slumping.However, deflation on earth has never, within a mantle of looserubble restingon regional to the best of our knowledge,produced an area of slopes. The problems are to develop regional topography like that shown on 6N6. slopesand to provide enough large impacts to Among internal factors, volcanism is a priori form chaotic terrain at some considerable time one of the more attractive. Extrusion of volcanic after developmentof the ancient crateredsurface. material is a principal cause of large-scale Regional slopes can be formed by internal di- collapse on earth, commonly accompanied by astrophism, and it is a permissible speculation marginal slumping and ground cracking. How- that this has occurred.However, younger craters ever, the geometrical patterns and distribution on Mars all seem to be less than about 16 km in of chaotic-terrain areas are so unlike anything diameter, and it is debatable whether impacts known to result from volcanic collapse as to forming craters of this size are great enough to argue against this as a sole mechanism. However, cause slumping, sliding, or creep on the scale loose rubble might be set into movement on displayed by chaotic terrain. Although radar regional slopesif vibrated by volcanic explosions [Anonymous,1970; Rogerset al., 1970; Goldstein or fluidized by volcanic gasesrising surfaceward et al., 1970] and CO2-pressuredata [Belton and under pressure. Hunten, 1969; Wells, 1969] show that extensive A major problem with internal factors is to regional slopes exist on Mars, they are for the find a way of bringing them into play at some most part inclined at only a fraction of a degree significanttime after development and extensive [Herr et al., 1970]. Nothing is known about their modification of cratered terrain surfaces. In this age, and they may be as old as the primordially context the geothermal evolution of Mars cratered terrain itself. Mass movement on deserves consideration. Studies of this matter regional slopes motivated solely by meteorite (MacDonald [1962], Anderson and Phinney impacts has serious drawbacks as the principal [1967], Anderson[1969], and Hanks and Anderson cause of chaotic terrain. [1969], to cite just a few) are necessarilytheoret- Although further explorations of Mars may ical, highly speculative, and involve different show that chaotic terrain is not limited to equa- models, but they indicate that Mars may never torial regions, the strong development there and have attained a molten stage. This is contrary 336 to the conclusionsof Binder [1969b]. Be that as tureless terrain is devoid of recognizable topo- it may, one model suggeststhat a condition of graphic configurationsat the effective 500-meter partial melting accompaniedby differentiation, limit of resolution of 1969 Mariner photo- degassing, tectonism, and volcanism may be graphs. However, it may prove to be anything anticipated about 0.5 X 109years hence [Ander- but smooth on closer observation. Featureless sonand Phinney, 1967]. The legion of assumptions terrain displays some irregular, diffuse variations involved in these calculations is such that the in shading, possibly the result of gently undu- timing can only be approximate. lating topography or of albedo differences.To This raises the intriguing thought that Mars qualify as featureless,a region shouldbe devoid may just now be reaching that stage in its of discernible topographic forms over at least geothermal evolution in which some of the above 10,000 km 2, as small areas of intercrater surface discussed internal processes are coming into within regions of cratered terrain appear to be play. They could affect the surface through smooth even at B-frame resolution. melting and evaporation of segregated masses The bright circular region Hellas, roughly of ground ice, through volcanic collapse ac- 1700 km across and centered at about 70øE and companied by extrusion elsewhere, through 45øS, contains at least 1.6 X 106 km 2 and fiuidization by gaseousdischarge, or by tectonic possibly2.5 X 106 km • of contiguousfeatureless delevelingof the crust, all or any of which might terrain. Featureless terrain may be widely have helped to create chaotic terrain. This is distributed over the surface of Mars, but at admittedly pure speculationbased on a thermal present only the Hellas area is known to qualify. model of Mars not acceptableto all investigators A relationship between featurelessterrain and the [Binder, 1969b]. nature, origin, and behavior of Hellas is implied. Implications of chaotic terrain. The oldest, Hellas. Hellas is one of the largest and most most enduring surface on Mars is heavily nearly circular features on Mars (Figure 3). cratered [Murray et al., 1971]. Chaotic terrain is At times it is one of the brightest, occasionally clearly younger. Something unusual has hap- being mistaken for the south polar cap [Lowell, pened within areasof chaotic terrain to generate 1906]. The luminance of Hellas varies secularly distinctive topographic configurations at the as well as seasonallyand diurnally. It is usually expenseof cratered terrain. Chaotic terrain is brightest in the late afternoon, especially either the product of a recent happening, or it during southernwinters when the polar cap is at is the result of a continuing activity sufficient to maximum. At other times it fades into the destroy most craters and yet has remained backgroundof surroundingdarker areasirrespec- confinedgeographically. tive of diurnal or seasonal relationships. The Martian chaotic terrain exceeds in scale extreme brightnessof Hellas has been attributed anything similar known on earth, and it seems to clouds or frost [Lowell, 1906; Collinson, 1953; to have no recognizable analog on the lunar Michaux, 1967]. Its prevailing ground color is surface.Mars may be undergoinga development markedly pinkish [Lowell, 1906; Antoniadi, 1930]. either never experiencedby the moon or possibly Mariner 7 A frames cover about 65% of masked by formation of the lunar maria. If Hellas. They showits interior to be a monotonous earth experienceda similar development, the surface with just three clearly discernible surface evidence has been erased. craters at its western edge (7N27). Highly If the speculationthat martian chaoticterrain processed versions of 7N29 show one or two is an early expressionof geothermal maturing indeterminate markings that might be taken for of the planet has validity, Mars may be enter- craters closer to the center of Hellas. ing on a stage in its evolution that could profoundly alter its surface characteristicsand Transition zone,Hellas to Hellespontus. Hellas environment through deformation, degassing, is bordered on the west by Hellespontus, a and volcanism.As earlier hinted by Urey [1956], dark area of heavily cratered terrain (Figure 4). the most interestingtime for Mars may still be The Mariner 7 phototrack crossedthe transition ahead. zone between Hellas and Hellespontus at about 45øS. The western or Hellespontus edge of this FEATURELESS TERRAIN zoneis sharplydefined by a seriesof discontinous Definition, description,and distribution. overlapping scarps and narrow ridges, individ- SURFACE OF •IARs--UNcRATERED TERRAINS 337 338 SHARP ET AL. •IJIlFA('E OF 5[ARS--UN(qtATERED TEIm,•NS 339

ually 20-90 km long. The easternedge of the zone 5Iariner 7 A frames show only 15% of the is marked by an abrupt but a planimctrically Hellas border.Extral)olations from small samples irregular albedo contrast from dark within the are necessarilytenuous, but the circularity of zone to light on the adjacent Hellas surface Hellas suggeststhat it is a coherent structure (7N27). with consistent border characteristics. This The transition zone, 150 to 325 km wide, is interpretation is SUl)ported by COa-pressure characterizedby a number of narrow, irregularly data indicating highlands both to east and west linear ridges and a few scarps paralleling the and by far-encounter frames showing cratered borders. This zone has craters comparable in terrain along the south edge of Hellas at the abundanceto adjacentHellespontus, except that pole-cap margin (Figure 3). Thus Mariner 7 local areas (,7N26) are poor in small craters. No- data indicate that Hellas is a large circular where do the scarps or ridges crosscutflat-bot- depression, not just an albino freckle on the tomed craters; rather, the craters appear to surface of Mars. Infrared spectrometer data interrupt or deform the ridges. [Herr et al., 1970] suggest that the floor is rea- Relative elevation of Hellas and Hellespontus. sonably fiat, but ultraviolet spectrometry sug- Some observers have concluded that Hellas geststhat it riseseastward [Hord and Barth, 1971]. stands high because its variable brightness is Origin of Hellas. At least three modes of the result of clouds or frost [Collinson, 1953; origin merit consideration; impact, volcanic Tomba•gh, 1968]. However, recent arguments explosion,and subsidence.Nothing on Mariner 7 that frost on Mars may favor low rather than photosforces a choicebetween these possibilities high places, although not universally accepted or suggests other alternatives. Analogy with [Biader, 1969a], challenge that interpretation lunar features may favor impact. If the Hellas [Sagart and Pollack, 1966, 1965; O'Lcary and structure is the product of impact, it represents Rea, 1967]. Mariner 7 furnishes the following an occurrencelarger than anything known on the data indicating that Hellas is a depression. moon. The outer ring of Mare Imbrium, 1100 km across, is about 65% the diameter of Hellas. 1. The form of the transition zone, with a No terrestrial volcanic caldera or tectono- linear western border determined by scarps and volcanic depression [Williams, 1941; Cotton, ridges and an irregular eastern edge defined by 1944] approaches Hellas in size. However, a differencesin luminanee, suggestthat it is in- basin of this magnitude might result from iso- clined eastward, down toward Hellas. The static subsidence over a dense mass within the configuration of the contact between the tran- crust or near crustal interior [O'Learyet al., 1969]. sition zone and the Hellas surface (7N27) re- The low elevationof Hellas, roughly 2 km below semblesthat seenat the foot of slopesexperienc- the general level of the martian surface [Herr ing creep where they intersect a gentler surface. et al., 1970],is consistentwith this interpretation. 2. Insofar as is discernible, all scarps within Such a mass might be an undigestedinhomo- the transition zone face eastward, down toward geneity inherited from the planer's early accre- Hellas. tionary phase. 3. The ingoing radio occulation of Mariner 7 Age of Hellas. Whatever its origin, Hellas, occurred over the southern tip of Hellespontus as a structural feature, appearsto be very old. If at 55øSand 30øE. It indicatedan atmospheric it is of impact or volcanic explosiveorigin, all pressureof only 3.$ rob, showingthat this point signsof an ejecta rim and of ejecta sheetshave on Hellespontus is well above the average level been obscuredor eliminated.Indeed, they would of the martian surface [Kliorc ctal., 1969]. have to be older than the heavily cratered 4. COa-prcssure data from the Mariner 7 terrain of Hellespontus, which is regarded as infrared spectrometer[Herr ctal., 1970] indicate basically accretionary [Murray et al., 1971]. that Hellas lies 5.5 km lower than Hellespontus Further, the scarpsand ridges of the transition and that the transition slopeis inclined 0.7ø- zone, which appear to be structuresrelated to the 0.$ø eastward.Hellas was the lowestpart of the formation of Hellas, are older than the cratersof martian surface traversed by the Mariner 1969 the transition zone. They do not crosscutthe spacecraft, $km lower than the highest point craters; rather, they are distorted by them recorded. (7N25, 7N26, 7N27). Thus, Hellas as a structural 340 S•^•r •x ^•.

feature had its origin quite early in the history with the Hellas floor, or that they be effective of the planet. Although the Hellas structure is only there. very old, its present floor has experiencedmore Episodic events that might be considered recent activity that continues to obliterate include extrusions of volcanic ash or tuff, craters. fiuidization of surface rubble by emanating The cloud or haze problem. Mariner 6 and 7 volcanic gases,accentuation of surface creep by near-encounter photos show Mars to have been geothermal change, recent deterioration of largely if not completely free of optically thick frozen ground [Leightonand Murray, 1966], or an clouds along the flight path. The only features atmospheric cometary explosion. Continuing even remotely resemblingthick cloudslie over or processesmeriting consideration could include along the margins of the south polar cap. Even transport and deposition by wind, unusually they are suspectowing to the lack of shadowsand effective basal surges associated with impacts cloud-like structures[Leovy et al., 1971]. Nonethe- [Robertsand Carlson, 1962; Moore, 1967; Fisher less,the possibility shouldbe consideredthat the and Waters, 1970], continuous creep of unusual floor of Hellas was obscuredby ground clouds or efficacy,or somethingassociated with the unusual haze when photographed by Mariner 7. The brightening of Hellas. If basal surges were clarity of features in the transition zone indicates involved, Hellas should display at least a few that such a cloud would have to be right down on scattered craters. the Hellas floor. Hellas is exceptionally low, 2 km below the No significant difference in brightness is martian mean elevation [Herr et al., 1970], and detectable between the crater-free floor of it is located in far southern latitudes. The low Hellas and the marginal area where the three elevation might cause it to be effected by the craters are visible (7N27). If a ground fog or hypothecated internal warming of the planet haze were obscuringcraters, it must have had the earlier than other parts of the surface. This samephotometric properties as the ground itself. condition could activate some of the episodic A dust cloud is, perhaps,the most likely possibil- events listed above. Or the combination of low ity, but why should it be so sharply confined elevation and proximity to the pole-cap edge to the Hellas floors? Mariner 6 far-encounter might producewinds of unusualforce, frequency, frames (Figure 3) show an irregular bright and duration within Hellas (C. B. Leovy, streak extending eastward acrossthe face of the personalcommunication, 1970). planet at about 40%, passing over the north Whether or not special environmental condi- part of Hellas. This may be haze, but if so it was tions obtain in Hellas, there is clearly something not localized over Hellas. At the time of Mariner unusualabout its surface.Perhaps it is the nature 1969 flyby, ground-basedastronomical observa- of the surfacematerial. Much of Mars is presum- tions showed Hellas to be in a relatively somber ably mantled with an impact rubble or material state. derived therefrom. The surface mantle of Hellas These relationships combine to create doubt may have different physical properties, possibly that the lack of visible craters over most of the those of a lightweight material such as a fine- Hellas floor is due to obscuration by clouds or grained pumice, which could respond with haze. A ground surface devoid of discernible unusualfacility to processesof any type, episodic topographic forms is consideredmore likely and or continuing, at work on the martian surface. is assumed as a basis for further discussion. An inclination to regard Hellas simply as a Origin of the featureless surface of Hellas. dust-filled basin is partly dampened by the Some degree of crater formation has probably sharpnessof the contact with the transitional prevailed on Mars nearly up to the present. slope. The lack of craters within featureless terrain The origin of featureless terrain remains a could indicate a recent event that has swept highly speculative topic. A combination of sev- the surface clean or a continuing activity eral of the processesand conditionsenumerated capable of obliterating craters as rapidly as they may hold the answer. The thought that Hellas are formed. The ancient age of Hellas, as a may be mantled by a type of material markedly structural feature, requires that any such recent different from that covering other parts of the event or continuingprocess be nearly coextensive martian surface has the appeal of simplicity. Su•½F^c•: oF 5I^•½S--UN(q•^T•CRED TF.•^INS 341

In any case,it is clear that Hellas is the site of an be attributed principally to differences in ancient event, the creation of a 1700-kin circular constitution, internal evolution, and the existence structure; it is also the locale of some unusual of a martian atmosphere. Cratering has been the combination of surface processesand material principal process creating and modifying the that continue to obliterate craters at an enor- topographic features of both bodies, and the mously faster rate than on most other parts of extensive remnants of accretionary ('ratered the martian surface. surface on Mars establish it as more like the moon

COMMENTS PEt•TINENT TO THE MARINER 1971 than the earth. However, it is an intriguing MARS MISSIONS speculationthat in future consMars will become more like earth as it progressesto an advanced Chaotic and featureless terrain enhance the state of geothermal maturity. attractiveness of Mars as a subject for explora- tion by the Mariner orbiter flights of 1971. Acknowledgments. We are deeply indebted to Attempts should be made to obtain a more all personswhose combined efforts made the Mariner 1969 flights to Mars a success. With respec• to extended high-resolution coverage of chaotic the series of four articles on mar•ian surface features terrain with the objectives of: (1) demonstrat- published herein, we specifically acknowledge the ing that the individual features and areas identi- valuable aid of the following: G. E. Danielson, fied on B frames 6N6, 6N8, and 6N14 are parts S. A. Collins, J. J. van der Woude, T. C. Rindfieisch, of a singleterrain type and not isolatedunrelated J. A. Dunne, R. C. Dewar, and Patricia Conklin, all of the California Institute of Technology and phenomena, (2) determining the number of JPL. Our colleaguesof the Mariner TV team, M. E. craters within chaotic terrain as a possiblemeans Davies, A. H. Herriman, N.H. Horowitz, C. B. of giving someindication of its relative age, (3) Leovy, B. A. Smith, and A. T. Young have provided showingin more detail featureswithin the transi- counsel and information. Without the leadership tion zones between cratered and chaotic terrains and unending efforts of principal investigator, R. B. Leighton, the TV project would never have and more of the individualtopographic forms succeeded. within chaoticterrain, with the aim of contribut- The participationof Murray, Leighton, and ing to an understandingof its origin,(4) showing Sharp has been underwrittenby the California whether the extensionwestward of chaotic Instituteof Technology.Cutts has beenpartly supported by NASA-105-69836 and Soderblom by terrain(Figure 2) fromthe type area in 6N7 is NGL-05-002-003. valid, and (5) exploringother parts of the planet for otherregions of chaoticterrain. REFERENCES With respectto featurelessterrain, Mariner Anonymous,Radar Studies of Mars, Final Rcp., 1971 photography could be profitably exercised NASA Contract NAS 9-7330, 79 pp., MIT to show: (1) whether other large circular bright Lincoln Lab., 1970. areas on Mars harbor featureless surface, (2) Anderson, D. L., The evolu[ion of terrestrial-type planets, Appl. Opt., 3, 1271-1277, 1969. whether featureless terrain is limited to such Anderson, D. L., and R. A. Phinney, Early thermal circular areas or exists in other geometrical, history of the terrestrial planets, in Mantles structural, or topographic situations, (3) what of the Earth and Terrestrial Planets, edited by the structural and topographical relationships S. K. Runcorn, pp. 113-126, Interscience, New York, 1967. are to cratered terrain, and (4) further rela- Antoniaall, E. M., La Plan•te Mars, 239 pp., tionships that may throw light on the genesis Librairie Scientifique Hermann, 1930. of featureless terrain. Belton, M. J. S., and I). M. Hunten, Spectographic Both chaotic and featureless terrain merit detection of topographic features on Mars, Science, 166, 225-227, 1969. particular attention during the Mariner Mars Binder, A. B., Topography and surface features 1971 missionsbecause they appear to be among of Mars, Icarus, 11, 24-35, 1969a. the youngest and most dynamic parts of the Binder, A. B., Internal structure of Mars, J, martian surface. This makes them especially Geophys.Res., 7J, 3110-3118, 1969b. attractive as placesin which to searchfor unusual Collinson, E. H., The planet Mars, J. Brit. Astronom. Soc., 64, 4-17, 1953. surface environments. Cotton, C. A., Volcanoes as Landscape Forms, 416 CONCLUSION pp., Whitcombe and Tombs, London, 1944. Fisher, R. V., and A. C. Waters, Base-surge bed The uncratered terrains of Mars do not seem forms in maar volcanoes, Amer. J. Sci., 268, to be duplicated on the moon. This contrast can 157-180, 1970. 342 S-•r^•P •T A•.

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