The water vapor vertical distribution on mars from millimeter transitions of HDO and (H2O)-O-18

Millimeter observations of Mars were performed in April 1997, near oppositi on, using heterodyne spectroscopy with the IRAM 30-m antenna at Pico Veleta (Spain). During the observations, the northern latitude of the sub-Earth p oint was about 24 degrees. The planetocentric longitude of the Sun (L,) was close to 100 degrees, which corresponds to the beginning of Martian northe rn summer, when the water vapor abundance is expected to be maximum at high northern latitudes. A strong emission was detected in the 226 GHz transiti on of HDO at the disk center (latitude = +24 degrees) and around the north pole. The 203 GHz transition of (H2O)-O-18 was also unambiguously detected. The thermal profile was retrieved from the simultaneous observation of (CO )-C-12(2-1) at 230 GHz, (CO)-C-13(2-1) at 220 GHz and (CO)-C-12(1-0) at 115 GHz. Using both HDO and (H2O)-O-18 transitions, we derive a water vapor co ntent of 52 (+34, -23) pr mum at the disk center and 85 (+30, -33) pr mum a t the north pole. These high values are consistent with the seasonal water cycle found by the MAWD Viking data (Jakosky, Kaberle, in: H.H. Kieffer, B. M. Jakosly, C.N. Synder, M.S. Mathews (Eds.), Mars, University of Arizona P ress, 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 saturat ed at a level of 9 (+5/-4) km at the disk center and 5 (+3, -2) km at the n orth pole. These saturation levels are consistent with MAWD Viking data (H. M. Hart, Seasonal changes in the abundance and vertical distribution of wat er vapor in the atmosphere of Mars, Ph.D. Thesis, University of Colorado, 1 989) and with millimeter measurements performed by Clancy et al. (Icarus 12 2 (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 fac tor of 2, equal to 6 times the terrestrial ratio, in agreement with previou s measurements. (C) 2001 Elsevier Science Ltd. All rights reserved.