Mars Atmospheric Temperature and Water Abundance During Northern Summer Solstice

M. A. Gurwell (Harvard-Smithsonian CfA), D. O. Muhleman (Caltech)

We present preliminary results from our recent campaign to observe the distribution of water in the atmosphere of Mars. Observations of Mars were performed with the Owens Valley Radio Observatory Millimeter Array on March 1 and March 5, 1997, with Mars approximately 13.5 tex2html_wrap_inline14 in diameter. The season was near summer solstice ( tex2html_wrap_inline16 ), and the subearth latitude was +22.6 tex2html_wrap_inline18 providing a good view of the northern polar cap. Spectroscopic measurements of the HDO (3 1 2 - 2 2 1) transition near 225.9 GHz were made on both nights, and on the second night the CO (1-0) transition near 115.3 GHz was also simultaneously observed. The synthesized resolution of these interferometric observations is approximately 2 tex2html_wrap_inline14 for HDO and 4 tex2html_wrap_inline14 for CO.

At millimeter wavelengths spectroscopic transitions are isolated and easily resolved, allowing accurate determination of the lineshape. The lineshape in turn can be used to retrieve vertical profiles of temperature and absorber abundance. This process is made considerably easier because the source function is nearly linear with temperature (Rayleigh-Jeans limit) and scattering by suspended dust is unimportant, unlike in the infrared. Carbon monoxide has a well determined abundance and can be used effectively to measure the atmospheric temperature profile (cf. Clancy and Muhleman, JGR 95, 14545, 1990). With the temperature profile determined, the HDO observations can then be used to measure the vertical profile of water vapor (assuming HDO/H tex2html_wrap_inline24 O is known). Hence, these observations provide a measure of both the horizontal and vertical structure of the atmospheric temperature field and the distribution of water vapor. At the time of this writing, data reduction is not complete; however, early results suggest that the water vapor distribution is highly non-uniform, with significantly more vapor in the northern hemisphere at the time of the observations. A more complete analysis of the water vapor and atmospheric temperature results will be presented.