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The water vapor vertical distribution on mars from millimeter transitions of HDO and H218O

2001; Elsevier BV; Volume: 49; Issue: 7 Linguagem: Inglês

10.1016/s0032-0633(01)00009-5

1873-5088

Th. Encrenaz, E. Lellouch, G. Paubert, S. Gulkis,

Spaceflight effects on biology

Millimeter observations of Mars were performed in April 1997, near opposition, using heterodyne spectroscopy with the IRAM 30-m antenna at Pico Veleta (Spain). During the observations, the northern latitude of the sub-Earth point was about 24°. The planetocentric longitude of the Sun (Ls) was close to 100°, which corresponds to the beginning of Martian northern summer, when the water vapor abundance is expected to be maximum at high northern latitudes. A strong emission was detected in the 226GHz transition of HDO at the disk center (latitude=+24°) and around the north pole. The 203GHz transition of H218O was also unambiguously detected. The thermal profile was retrieved from the simultaneous observation of 12CO(2-1) at 230GHz, 13CO(2-1) at 220GHz and 12CO(1-0) at 115GHz. Using both HDO and H218O transitions, we derive a water vapor content of 52(+34,−23)prμm at the disk center and 85(+30,−33)prμm at the north pole. These high values are consistent with the seasonal water cycle found by the MAWD Viking data (Jakosky, Haberle, in: H.H. Kieffer, B.M. Jakosly, C.N. Synder, M.S. Mathews (Eds.), Mars, University of Arizona Press, 1992.) in the case of the northern region, but higher than the Viking estimates at low latitude. The water vapor profile was found to be saturated at a level of 9(+5/−4)km at the disk center and 5(+3,−2)km at the north pole. These saturation levels are consistent with MAWD Viking data (H.M. Hart, Seasonal changes in the abundance and vertical distribution of water vapor in the atmosphere of Mars, Ph.D. Thesis, University of Colorado, 1989) and with millimeter measurements performed by Clancy et al. (Icarus 122 (1996) 36); they confirm that water is confined to the lower troposphere near aphelion. The quality of our data did not allow us to derive a precise determination of the D/H Martian ratio, which is found to be, within a factor of 2, equal to 6 times the terrestrial ratio, in agreement with previous measurements.