Mars Orbiter Camera observations of the Martian south polar cap in 1999-2000

The spring-summer recession of the south polar cap of Mars in 1999-2000 has been investigated using the wide-angle cameras of the Mars Orbiter Camera experiment on Mars Global Surveyor. The 1999-2000 regression closely resemb les the recession observed by Viking orbiters in 1977. Although the rates o f recession are quite similar, the current recession was slightly ahead of that in 1977 throughout spring. The rapid development of dark terrain, dubb ed "cryptic terrain" by Hugh Kieffer, in one sector of the south cap betwee n L-S = 198 degrees and 223 degrees is documented. The emergence and separa tion of the Mountains of Mitchel has also been recorded in detail, culminat ing with its complete disappearance at solstice. The 2000 residual cap is a lmost identical to that in 1977; this observation does not resolve the myst ery of the very different appearance of the residual cap in 1972. The albed os in the residual cap region and in bright, seasonal frost increase rapidl y to high values at around solstice and then level out until the seasonal f rost disappears. Observations of more Martian years will be needed to const rain the amount of interannual variability and its relation to other phenom ena such as dust storms.