Viking Lander I expired in November, 1982 following the orbiters a couple of years sooner. The United States' and Russia's attempts with the Mars Observer and the Phobos vehicles were spectacular failures although limited data were obtained from the latter mission. The Mars ``community'' has been limited to ingenious reanalysis of the old data from Viking and Mariner, leaving the field open for new discoveries to the Earth-bound remote sensors. Our knowledge of Mars is strongly colored by Mars at the Viking epoch. Todd Clancy has shown that the temperature structure of the atmosphere of Mars is strongly forced by the dust loading [ Clancy, Muhleman and Berge, 1990]. The size and shape of the microwave absorption lines of atmospheric CO are a very sensitive measure of the temperature profile, which has varied by more than to 25 K near the tropopause. A dust-free Martian atmosphere approaches the classical radiative equilibrium profile while the dusty atmosphere (typical of the Viking epoch) is much more isothermal.
New heterodyne techniques have been developed to study the atmospheric chemistry using IR and microwave spectroscopy. Ozone measurements were first done on Mariners 7 and 9 but more recently these measurements have been made from Earth at 9.6 microns [ Espernak, et al., 1991], making it possible to monitor the atmosphere through Martian seasons. New microwave techniques involve the use of the VLA in the 23 GHz spectral line of water vapor [ Clancy, Grossman and Muhleman, 1992]. The water vapor emission around the planet's limb can be mapped and its variations with latitude studied as a function of season. Although the work is only at the beginning stage, it seems clear that seasonal and spatially resolved measurements of ozone and water vapor will make our understanding of the Martian atmosphere and its dynamics nearly complete.
Perhaps the newest important discovery on Mars is ``Stealth'', revealed in
VLA radar imaging by Muhleman, et al. [1991] [see also Butler,
1994]. This surface structure lies in the middle of the Tharsis volcanic
region and stretches nearly 2000 km along the equator to the west of the
Tharsis volcanic ridge. Its southern boundary appears to be defined by the
North/South dichotomy line. The entire feature exhibits essentially zero
centimeter wavelength reflectivity (a black hole). The authors have
interpreted the phenomenon as arising from a deposit, perhaps an ash flow, of
basaltic material with density less than about 0.5 gm/cm
, with depths
greater than about 4 meters, but possibly much greater. No similar deposit
has been found on any other terrestrial planet or on the Moon. Is this
``continent'' a volcanic artifact or possibly a remnant of a near-polar deposit
remaining from epochs when the Martian obliquity was very large and the ice
deposits may have approached the equatorial zones?