Evolution of the Permanent North Polar Cap during the North Summer Season
Résumé
The OMEGA VIS/IR imaging spectrometer has observed the North polar cap of Mars since the spring equinox from distances of 6000 km to 8000 km, following the recess of the seasonal cap. Since science operations resumed after the conjunction in late September, the outlines of the ice rich regions have remained stable. The North polar cap appears very similar in 2004 when compared to the situation in 1976 (Viking), with only a few regions exhibiting drastic changes in ice coverage. A comprehensive coverage has been obtained typically every week for the central regions (> 80° N), with resolutions improving from ~ 4 km in October to 2 km or less in late November and December. Large albedo variations are observed in the visible range, as already reported from Viking and MGS observations. The high repeat rate of OMEGA observations show that significant changes can occur in less than one week, and that regions may exhibit decreases in albedo while others get brighter, with ice variegation at scales of less than 10 km. The unique spectrometric capability of OMEGA has made it possible to simultaneously monitor the evolution of ice grain size and that of its dust content from the intensity of the ice absorption bands between 1 and 4 μm. We show that the observed albedo variations result from a complex combination of the full range of processes previously considered as possible explanations: changes in grain size due to frost sublimation or surface ice thermal processing, dust deposition or removal during ice sublimation, increase of macroscale surface roughness by sublimation of interstitial ice. This complex evolution presents intriguing similarities with processes at work on terrestrial ice caps.