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Th. Encrenaz, E. Lellouch (Obs.Paris), G. Paubert (IRAM,Granada), S. Gulkis (JPL)
Observations of water vapor in the Martian atmosphere have been obtained in 1997 and 1999 using the IRAM 30-m antenna at Pico Veleta. In April 1997 (Ls = 100-102 deg.), the 226 GHz HDO and the 203 GHz H218O transitions were both detected in the northern hemisphere, leading to a H2O column abundance of 55 +/- 20 pr-\mum at a latitude of +24 deg. The water vapor was found to be confined in the lower atmosphere, in agreement with the results of Clancy et al. (Icarus 122, 36, 1996), with a saturation level at 9 +/- 2 km. In April 1999, (Ls = 120 deg.) an upper limit of about 15 pr-\mum was derived. In November 1999 (Ls = 238 deg.), the HDO 226 GHz transition was again detected, corresponding to a mean H2O column density of 7 pr-\mum. The April 1997 and November 1999 results are consistent with the abundances expected from the seasonal variations inferred by Viking, but the April 1999 result is significantly lower, which seems to show evidence for interannual variations.