Mars’ water vapor mapping by the SPICAM IR spectrometer: Five Martian years of observations
Résumé
The SPICAM IR instrument on the Mars Express mission continuously observes the water vapor in the Martian atmosphere starting from 2004 in the 1.38-μm spectral band. The water vapor column abundance is retrieved from nadir observations to characterize its spatial, seasonal and interannual variations. A reference set of SPICAM water vapor column abundances (zonally averaged) covering the time period from 2004 to 2013 (Martian Years 27-31) is available for a grid of 2° Ls x 2° latitude, along with an average reference map of water vapor abundance combining all the Martian years of Mars Express observations. Compared to the previous data retrieval by Fedorova et al. [2006] the new processing algorithm includes many improvements concerning the calibration and assumed parameters. A major improvement is the account for aerosol scattering based on dust and water ice cloud optical depths measured by THEMIS/Mars Odyssey [Smith et al., 2009a]. The account for multiple scattering by aerosol particles increases the retrieved water vapor amount by ∼10% in polar areas during summer, and up to 60-70% for large solar zenith angles. The sensitivity of the results to aerosol properties, surface albedo, solar spectrum, and water vapor vertical distribution has also been studied. The retrieved water vapor reveals nominal annual cycle with maximum abundance of about 60-70 pr. μm for the Northern summer and ∼20 pr. μm for the Southern summer. The annual average amount of water has been estimated to be of 10 to 20 pr. μm, in agreement with other measurements. From year to year the seasonal cycle of water vapor abundance is very stable. An observed decrease during the MY 28 global dust storm cannot be fully attributed to the masking effect of dust, and indicates a real decrease of water amount near or above the surface. No evidence of diurnal variation of column water vapor amount was found, even though the 1.38-μm measurements are sensitive to the few lowermost kilometers above the surface.