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 
in diameter. The season was near summer solstice
( 
), and the subearth latitude was +22.6 
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 for HDO
and 4 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 
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.