A TENTATIVE DETECTION OF THE 183-GHZ WATER-VAPOR LINE IN THE MARTIAN ATMOSPHERE - CONSTRAINTS UPON THE H2O ABUNDANCE AND VERTICAL-DISTRIBUTION

The 183-GHz water vapor line was tentatively detected on Mars in January 1991, with the IRAM 30-m millimeter antenna, under extremely dry atmospheric conditions. The measurement refers to the whole disk. The spectral line, although marginally detected, can be fit with a constant H2O mixing ratio of 1.0 x 10(-5), which corresponds to a water abundance of 1 pr-mu m; in any case, an upper limit of 3 pr-mu m is inferred. This value is comparable to the very small abundances measured by Clancy (1992, Icarus 100, 48-59) 5 five weeks before our observation and seems to imply both seasonal and long-term variations in the martian water cycle. (C) 1995 Academic Press, Inc.