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 Telesc ope obtained during the summer of 1992 when the angular size of Mars w as between 5 and 6 are sec. The observations cover the interesting per iod between L(s) = 259 and 284, when the recession is rapid and the as ymmetry in the cap is large. These will be the only HST observations o f this season during the 1990s because of the solar elongation constra int on telescope pointing. The well known ''Mountains of Mitchel'' out lier, which has been observed by Viking and by many ground based obser vers near L(s) = 260, is clearly present in the first set of Hubble im ages. The latitude of the cap edge, the displacement of the center of the cap from the geographical pole, and the albedo of the polar deposi ts determined from these images are consistent with observations of th ese quantities by Viking in 1977; the data are not compatible with a r ecession as advanced as that deduced from the 1956 telescopic observat ions. (C) 1996 Academic Press, Inc.