Abstract

Would it be possible to interpret the dynamics of the northern polar ice cap of Mars with the same criteria as for terrestrial ice sheets? The aim of this paper is to illustrate the problems posed by such an approach. We present a detailed study of the MOC narrow angle image E01-01773, taken at 83.5°N/241.4°W in the upper part of a 300 km-long depression situated at the edge of the northern polar ice cap. On Mars, every depression resembling a valley is a “chasma”, and as this one has no official name, we suggest to call it “West Olympia Chasma” (WOC). By analyzing the satellite image, we have been able to highlight some unusual features of martian glacial geomorphology and to compare them with topographies and sediments known in terrestrial glacial environments. In particular:

– the scarp cutting the edge of the ice cap above the chasma is formed of thin alternations of ice and aerosols and is bordered by numerous small and dissymetrical ridges that run parallel to it. On Earth, these ridges would be interpreted as a succession of terminal moraines; in fact, they consist of gravitational debris accumulated under the nearly motionless ice cliff;

– at the edge of the ice, where signs of melt water action would be clearly visible in terrestrial environments, there is practically no evidence of such a process on Mars. However, two intriguing channels, several tens of meters wide and about 1 km-long, suggest the occurrence of temporary water flow;

– these channels originate at the base of an ice tongue that appears to have flowed recently over the scarp towards the floor of the chasma, in the same manner as surging glaciers on Earth, whereas Mars ice seems motionless;

– the sandy sediments covered by the ice cap do not seem to be ground moraines, as would be the case on Earth, but eolian sand brought by winds when the ice cap did not exist.

At present, the most effective erosion agents are the winds channelled into the chasma and the alternations of condensation/sublimation. Their combined action explains the dissymetry between both sides of the chasma. The winds are enhanced by local topography and contribute to the movement of dunes towards the outlet of the chasma.

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