Summer observations of the Martian north polar residual cap by the High Resolution Stereo Camera (HRSC) in 2004/2005 and 2006

Abstract

Monitoring the polar caps is impor- tant for understanding the current climate on Mars. The northern residual cap (NRC) is an important source of atmospheric water vapour and it is unclear if there is a net sublimation or deposition of water va- pour under present climatic conditions. The recession of the seasonal polar cap in spring uncovers the bright NRC, which is composed of H2O ice. First obser- vations suggested interannual changes of the NRC in extent. Bass et al showed that these observed changes are most likely due to seasonal variations. Detailed examination of the interior cap showed that the albedo varies spatially but is generally repeatable interannually. Hyperspectral data from OMEGA acquired between LS ~93° and ~127° in 2004/2005 revealed that the albedo decrease on the polar cap is due to the sublimation of fine-grained frost which ex- poses older large-grained ice. In outlying regions, dominated by large-grained ice, the albedo increases with time. We tracked the seasonal and possible interannual albedo changes of the NRC using the Lambert albedo derived from High Resolution Stereo Camera (HRSC) image data of the summer seasons 2004/2005 (LS ~120°-~160°) and 2006 (LS ~90°- ~150).