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
=335.65 to =144.56 . 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 =0 are fairly consistent from year to year with
some variations possible before =0 . The 1991
observations at =35.25 and =59.88 in the
longitude range from 270 -280 , possibly support a
standstill in cap retreat at a latitude 70 N. The
standstill in cap retreat reported in previous years at a
latitude 66.6 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.