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A.L. Sprague, D.M. Hunten, R.E. Hill, L.R. Doose, B. Rizk (Lunar and Planetary Laboratory, University of Arizona)
Martian atmospheric water vapor abundances measured during Ls 18 - 242 (two mars apparitions in 1996 - 1999) show highly variable water vapor abundances from no measurable amount to more than 90 ppt. \mum. We observed the same seasonal variations of water vapor as observed by MAWD on the Viking orbiters and previous ground-based observations. In addition we observed highly variable water vapor abundances with hourly and daily variations indicating active weather. Observation and analysis of dust opacity at the locations of our water vapor measurements have permitted us to adjust our high airmass measurements for the "Davies effect." By doing so we have been able to show that apparent morning and evening lows in water vapor are real and not just an artifact of dust scattering and absorption. Another important result of our work is to note that ground-based polar measurements under estimate the amount of water vapor present by up to as much as factors of 2 and 3 if dust opacity is not taken into account. Our measurements augment those of the Thermal Emission Spectrometer on Mars Global Surveyor that were obtained during 1998 - 1999 for the 2p.m. / 2a.m. orbital path. We have reduced more than 1000 measurements at different locations and local times on the planet. These are being studied and searched for persistently wet and dry regions. This work was funded by the NASA Planetary Astronomy Program.