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 resembles the recession observed by Viking orbiters in 1977. Although the rates of recession are quite similar, the current recession was slightly ahead of that in 1977 throughout spring. The rapid development of dark terrain, dubbed "cryptic terrain" by Hugh Kieffer, in one sector of the south cap between L-S = 198 degrees and 223 degrees is documented. The emergence and separation of the Mountains of Mitchel has also been recorded in detail, culminating with its complete disappearance at solstice. The 2000 residual cap is almost identical to that in 1977; this observation does not resolve the mystery of the very different appearance of the residual cap in 1972. The albedos in the residual cap region and in bright, seasonal frost increase rapidly to high values at around solstice and then level out until the seasonal frost disappears. Observations of more Martian years will be needed to constrain the amount of interannual variability and its relation to other phenomena such as dust storms.