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

B. A. Cantor, M. J. Wolff, P. B. James (The University of Toledo), E. Higgs (Xavier University)

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 from tex2html_wrap_inline21 =335.65 tex2html_wrap_inline23 to tex2html_wrap_inline21 =144.56 tex2html_wrap_inline23 . The 1990-1993 observations were obtained with the Wide Field Planetary Camera 1 (WFPC1) instrument while subsequent observations from 1994-1997 were obtained with the improved optics of the Wide Field Planetary Camera 2 (WFPC2). Mars observations were obtained in a wide multispectral range from the ultraviolet (218nm) to the near-infrared (1042nm) 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 tex2html_wrap_inline21 =0 tex2html_wrap_inline23 are fairly consistent from year to year with some variations possible before tex2html_wrap_inline21 =0 tex2html_wrap_inline23 . The 1991 observations at tex2html_wrap_inline21 =35.25 tex2html_wrap_inline23 and tex2html_wrap_inline21 =59.88 tex2html_wrap_inline23 in the longitude range from 270 tex2html_wrap_inline23 -280 tex2html_wrap_inline23 , possibly support a standstill in cap retreat at a latitude tex2html_wrap_inline49 70 tex2html_wrap_inline23 N. The standstill in cap retreat reported in previous years at a latitude tex2html_wrap_inline49 66.6 tex2html_wrap_inline23 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.