The Surface of Mars 4. South Polar

VOL. 76, NO. 2 JOURNAL OF GEOPHYSICAL RESEARCH JANUARY 10, 1971 The Surface of Mars 4. SouthPolar Cap ROBEaTP. SHARP,BRucE C. MunnA¾, ROBEaTB. LAUrEnCE A. SODE•BLOM •D J•MES A. Cu•s Division of GeologicalSciences and Division of Physics,Mathematics and Astronomy California Institute of Technology,Pasadena 91109 The south polar cap of Mars occupiesa region of cratered terrain. Immediately outside the shrinking cap craters appear no more modified than those in areas farther north that are not annually frost covered. Craters showing through the frost mantle are locally as abundant, as elsewhereon Mars. Only in a central region close to the pole are craters sparse. Both far- and near-encounter views reveal a highly irregular pole-cap edge. Photos of the same sector taken six days apart are near duplicates,suggesting that the irregularity is primarily ground controlled. No evidence of the classicalpolar collar is seen. Within the marginal zone, frost is preserved largely in crater bottoms and on slopes inclined away from the sun. Preferential retention in low spots supports the earlier suggestionthat the Mountains of Mitchel may actually be depressions.An argument based on insolation as the prime factor in frost wastage and the narrow width of the marginal zone suggeststhat slopesof topographicfeatures therein are mostly gentle, on the order of a few degrees.The frost cover of the pole-capinterior may range widely in thickness,obscuring parts of some craters and seeminglyenhancing topographic visibility elsewhere, possiblythrough variations in thicknessand reflectivity. Unusually bright areas on the cap surface, and differences in luminance between bright rims and the more somber floors of craters and other depressions,may be due in large part to differences in related frost textures and to the local history of evaporation and sublimation. Irregularly angular depressionswithin the pole- cap frost termed 'etch pits' may be the product of differential ablation or the undermining by wind of a slabby surficial crust. Encircling the south pole is a region of subdued relief with a paucity of craters, which displays enigmatic quasi-linear markings believed to be ground features. Although no satisfactory explanation of these markings has been formulated, it seemslikely that this region has been occupiedrepeatedly by perennial massesof CO2 ice, formed and maintained during those phasesof the martian precessionalcycle that resulted in short cool summersin the southern hemisphere. Such ice massesmay play a role in producing the unusual features of the central polar region. Physical relationships suggest a local maximum frost thickness as great as tens of meters. The possibility should be kept in mind that remnants of perennial CO2 ice of still greater thickness may exist locally, for example, in the 'etch pit' area. The south polar cap of Mars was first seenby relationships of the polar cap as revealed in man more than 300 years ago, and its annual these photos are described.Implications of these waxing and waning phases have been observed observationswith respectto the physical nature, for over 200 years by earth-based optical astron- distribution, thickness, and behavior of the omers [Slipher, 1962]. In 1969 Mariners 6 and 7, pole-cap frost are examined. The origin of through numerous far-encounter photographs features seemingly unique to the cap is explored. (Figures 1 and 2), and Mariner 7, through ten The near-encounterphotomosaic (Figure 3) is near-encounter frames (Figure 3), gave the one of the more informative and striking products closestand most satisfyingviews of this Martian of the Mariner 1969 TV experiment. This feature. near-encounterpolar cap photography involved The characteristic physical features and an in-flight reprogramming of the on-board computer directing a slue of the instrument • Contribution 1893, Division of Geological platform to afford the sciencesystems a view of Sciences, California Institute of Technology, Pasadena 91109. the polar cap. Locations of features or areas on polar-region Copyright • 1971 by the American Geophysical lfnion. photos are more effectively expressedin terms 357 358 of right or left than east or west. Therefore over areas of various shapesand sizes up to readers referring to individual photos in this 300 km across.Individual features appear at section should make sure that the orientation fixed positions and with correspondingsize is consistent with that of the corresponding and shape on successivefar-encounter frames photoin the mosaics(Figures 3 and 4). (Figure 1) taken about 60 rain apart. These featurespresumably represent real differencesin FAR-ENcOUNTERVIEWS reflectivityon the cap's surface or a differencein Far-encounter frames show parts of the the atmosphereover it. pole-capedge and interior not coveredby near- encounterphotos, aswell as the regional configu- NEAR-ENcouNTERV•;ws rations of the pole-capmargin, and they also Near-encounterviews of the polar cap were record possibletemporal variations in frost takenfrom spacecraft positions north of 45%, so cover, associatedatmospheric phenomena, or that obliquityis high (400-48ø from vertical), photometricfunctions. and slant distanceswere large (4925-6340 kin). Far-encounter pictures show considerable irregularityinthe pole-cap edge on a 50-200km Featureso[the Marginal Zone scale.The relief of underlyingcratered terrain is Treatment of the pole-capmargin is facilitated clearly one cause,and other topographicrelief by division into three parts: the extramarginal is inferredto pla•ya part. Cratersexceeding 100 area, the outer marginalzone, and the inner km in diameter are visible just within the pole- marginal zone. These areas are transitory, cap margin on frames taken from distancesin moving inward with the shrinking cap. Along excessof 500,000km (6F36, Figure 1). the 330ø longitudeline (Figure 4), the extra- All far-encounterviews of the polar cap show marginalarea extendssouth to 57%, the outer fuzzinessand lack of definition toward the marginalzone lies between57 ø and 58.5ø , and morningterminator on the west; comparethe the inner marginalzone is between58.5 ø and eastand westedges of the planetarydisc (Figures 60.5ø . 1 and 2). A localizedmorning haze might account Extramarginal area. This area comprises for this fuzziness and for the limb darkening the bare ground immediately beyond the outer over the polar cap appearingprincipally on its limit of recognizablefrost patchesat the time of morning side. However, Leow et al. [1971] viewing (earth date, August 5, 1969) but lying suggestthat the limb darkening is due to a within the region covered each year by the photometricfunction of the planetary surface extendedpolar cap. Most of the extramarginal and not to a generalatmospheric haze. area displays craters (7Nll, 7N13, Figure 3) Comparison of successiveviews (Figure 1) seeminglycomparable in abundanceand charac- suggeststhat changesin the size, shape, and ter to other well-cratered regions on Mars. density of irregular, wispy projectionsfrom the However,the westernthird of 7Nll seemsdevoid pole-cap edge may occur from hour to hour. of craters.Whether this area is truly featureless, However,photos taken 1 rain apart .(Figure 2) as suggestedby B frame 7N10, or is simply show differences about as great, and pairs of obscuredby local clouds or haze is not known. views of correspondingsectors of the pole-cap The possibilityof a localizedhaze is supported margin taken six days apart show no greater by a light patch at this location on a true- changes (Figure 2). This suggeststhat the intensity version of 7Nll (Figure 4), although seeming variations may be the product of Leow et al. [1971] suggestthat this lightness differencesin lighting, viewing angles, and could be the product of a surfacephotometric instrumental response.Indeed, six-day photo function sensitive to the solar incident angle. l•airs show a consistency in pole-cap edge The remainder of the extramarginal zone configuration that hardly seems compatible visible on 7Nll and 7N13 (Figure 3) exhibits with anything as ephemeral as clouds or mist, craters seemingly no different from craters unless they are strongly controlled by ground farther north along the Mariner 7 path outside features. the area of annual pole-cap coverage (7N9, The interior of the cap displays an irregular 7N7). No unusual topographic forms are seen mottling involving modest differencesin albedo in the extramarginal zone. SURFACE OF •IARS--SOUTH POLAR CAP 359 .. Fig. 1. Mariner 6 far-encounterviews of south polar cap, enlarged to a common scale. Each successiveframe representsan elapsedtime ranging between 56 and 65 minutes and a plantetary rotation between 14ø and 16ø. Crater at A in 6F37 about 110 km across.Small dark spot,s are geometricreference marks on vidicon tube. A, B, and C are simply marginal irregularities selected as referencepoints. Approximate longitudes: A, 209øE; B, 240øE; C, 133øE. 360 SHARP ET AL. Outer marginal zone. This zone is character- The outer marginal zone displays some ized by preservation of disconnected frost scattered, irregular, dense, white patches (see patches. In maximum discriminable photo right center, 7Nll), suggestive of optically versions (Figure 3) it lies within a spurious thick clouds. However, these patches do not dark band produced by an automatic gain display shadowsor have the outlinesor structures control (AGC) in the spacecraft photographic normally expected of uplifted or convective system. As the scan trace in the TV system clouds [Leovy

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