Regression of Martian North Polar Cap: 1990-1997 Hubble Space Telescope Observations

Hubble Space Telescope imaging of Mars from December 13, 1990, through April 17, 1997, has resulted in the synoptic monitoring of parts of four consecutive martian north polar cap regressional cycles spanning L-s = 335.65 degrees to L-s = 144.56 degrees. The 1990-1993 observations were obtained with the Wide Field Planetary Camera 1 (WFPC1) instrument while subsequent observations from 1994 to 1997 were obtained with the improved optics of the Wide Field Planetary Camera 2 (WFPC2). Mars observations were obtained from the ultraviolet (218 nm) to the near-infrared (1042 nm), which has allowed us to constrain the effects of clouds and dust particulates on cap edge measurements. HST observations of the polar cap regressions after L-s = 0 degrees are fairly consistent from year to year with some variations possible before L-s = 0 degrees. The 1991 observations at L-s = 35.25 degrees and L-s = 59.88 degrees in the longitude range 270 degrees-280 degrees possibly support a standstill in cap retreat at a latitude 70 degrees N. The standstill in cap retreat reported in previous years at a latitude 67 degrees N was not observed in 1992-1997. Comparison with previous ground-based and spacecraft observations suggests that interannual variations may be significant; they may be due to interannual atmospheric changes such as global dust storm activity. Lambert albedos of the north polar cap in red and violet bandpasses are similar to those reported before. (C) 1998 Academic Press.