The 1997 spring regression of the Martian south polar cap: Mars Orbiter camera observations

The Mars Orbiter cameras (MOC) on Mars Global. Surveyor observed the south polar cap of Mars during its spring recession in 1997. The images acquired by the wide angle cameras reveal a pattern of recession that is qualitative ly similar to that observed by Viking in 1977 but that does differ in at le ast two respects. The 1977 recession in the 0 degrees to 120 degrees longit ude sector was accelerated relative to the 1997 observations after L-S = 24 0 degrees; the Mountains of Mitchel also detached from the main cap earlier in 1997. Comparison of the MOC images with Mars Orbiter Laser Altimeter da ta shows that the Mountains of Mitchel feature is controlled by local topog raphy. Relatively dark, low albedo regions well within the boundaries of th e seasonal cap were observed to have red-to-violet ratios that characterize them as frost units rather than unfrosted or partially frosted ground; thi s suggests the possibility of regions covered by CO2 frost having different grain sizes. (C) 2000 Academic Press.