Seasonal Recession of Martian South Polar Cap: 1992 HST Observations

The seasonal recession of the south polar cap of Mars has been studied using the Wide Field Planetary Camera images from Hubble Space Telescope obtained during the summer of 1992 when the angular size of Mars was between 5 and 6 arc sec. The observations cover the interesting period betweenLS= 259 and 284, when the recession is rapid and the asymmetry in the cap is large. These will be the only HST observations of this season during the 1990s because of the solar elongation constraint on telescope pointing. The well known “Mountains of Mitchel” outlier, which has been observed by Viking and by many ground based observers nearLS= 260, is clearly present in the first set of Hubble images. The latitude of the cap edge, the displacement of the center of the cap from the geographical pole, and the albedo of the polar deposits determined from these images are consistent with observations of these quantities by Viking in 1977; the data are not compatible with a recession as advanced as that deduced from the 1956 telescopic observations.