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Published January 10, 1971 | Published
Journal Article Open

The surface of Mars 4. South polar cap


The south polar cap of Mars occupies a 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 elsewhere on 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 classical polar 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 suggestion that 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 suggests that slopes of topographic features therein are mostly gentle, on the order of a few degrees. The frost cover of the pole-cap interior may range widely in thickness, obscuring parts of some craters and seemingly enhancing topographic visibility elsewhere, possibly through variations in thickness and 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 depressions within the polecap 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 seems likely that this region has been occupied repeatedly by perennial masses of CO_2 ice, formed and maintained during those phases of the martian precessional cycle that resulted in short cool summers in the southern hemisphere. Such ice masses may 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 CO_2 ice of still greater thickness may exist locally, for example, in the 'etch pit' area.

Additional Information

Copyright 1971 by the American Geophysical Union. (Received August 10, 1970; revised September 7, 1970.) We are deeply indebted to all persons whose combined efforts made the Mariner 1969 flights to Mars a success. With respect to the series of four articles on Martian surface features published in this issue, we specifically acknowledge the valuable aid of the following: G. E. Danielson, S. A. Collins, J. J. van der Woude, T. C. Rindfleiseh, J. A. Dunne, R. C. Dewar, and Patricia Conklin, all of Caltech and California Institute of Technology JPL. Our colleagues of the Mariner TV team, M. E. Davies, A. H. Herriman, N.H. Horowitz, C. B. Leovy, B. A. Smith, and A. T. Young have provided counsel and information. The participation of three authors (Murray, Leighton, and Sharp) has been underwritten by the California Institute of Technology. Cutts has been partly supported by NASA-105-69836 and Soderblom by NGL-05-002-003. Contribution 1893, Division of Geological Sciences, California Institute of Technology, Pasadena 91109.

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