View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Caltech Authors - Main 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