Mapping Mars water vapor with the very large array

Abstract

We observed Mars with the Very Large Array (VLA) in the spectral line mode on December 3/4 and 6/7 of 1990. Operating at a wavelength of 1.35 cm, we were able to map the 22-GHz water emission line around the atmospheric limb of Mars. The variation of the emission around the limb and the measured lineshapes yield information on the diurnal, latitudinal, and vertical distributions of water vapor in the Mars atmosphere. The VLA limb spectra yield a global average water column of 3.0 ± 0.8 pr μm, which is significantly lower than that returned by the 1988 IR reflectance observations of Rizk et al. (1991) for the same Mars season ( L S = 340−350°, early northern spring). The Rizk et al. measurement of 10 pr μm is roughly twice that returned by Viking MAWD observations for this driest of Mars seasons (Farmer et al. 1977), whereas our 1.35-cm emission determination is roughly one half that of the Viking MAWD determination. This implies interannual variations in global water vapor of the same order as the seasonal variations observed by Viking. The latitudinal variation measured by this VLA experiment indicates a maximum water vapor abundance of 4.5 ± 1.0 pr μm (75 ppmv for an altitude-independent mixing ratio) at equatorial latitudes, decreasing to 2.9 ± 0.7 pr μm (50 ppmv) at midlatitudes. The polar (60–90°NS) water mixing ratio fell below 20 ppmv at altitudes of 15–50 km. We do not find significant diurnal (5 AM vs 5 PM) variation of water vapor at low latitudes. However, it does appear that the water column at mid latitudes (40–60°NS) is ∼30% smaller for the morning versus the evening limb. An analysis of the pressure-broadened 22-GHz lineshape indicates that water vapor was well mixed to an altitude of ≥45 km, although a vertical mixing gradient as large as 50% over the 0 to 50-km altitude range would fall within the measurement uncertainties.