Interferometric millimeter observations of water vapour on Mars and comparison with Mars Express measurements
Résumé
We present interferometric mapping of the 225.9-GHz HDO and 203.4-GHz H218O lines on Mars obtained with the IRAM Plateau de Bure facility (PdBI). The observations were performed during martian year 28 (MY28), at Ls=320.3deg for the HDO line, and at Ls=324.3deg for the H218Oline. The HDO line is detected at the eastern (morning) and western (evening) limbs of the northern hemisphere, corresponding to a water column density in the range 3-6 pr.-μm. The H218O line is not detected, which is compatible with the column densities derived from the HDO line. Quasi-simultaneous far infrared measurements obtained by the Planetary Fourier Spectrometer (PFS) onboard the Mars Express spacecraft confirm our PdBI results, yielding a 5±1 pr.-μm meridionally constant water column abundance. Such a low water abundance during the southern mid-autumn of MY28 does not correspond to the standard martian climatology as observed during the previous years. It was however already retrieved from near-infrared observations performed by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter spacecraft [Smith, M. D., Wolff, M. J., Clancy, R. T., Murchie, S. L. 2009. CRISM observations of water vapor and carbon monoxide. J. Geophys. Res.114, doi:10.1029/2008JE003288]. Our observations thus confirm that the planet-encircling dust storm that occured during MY28 significantly affected the martian water cycle. Our observations also demonstrate the usefulness of interferometric submillimeter observations to survey the martian water cycle from ground-based facilities.