Variations of the HDO/H2O ratio in the martian atmosphere and loss of water from Mars

Abstract

Ground-based spatially-resolved high-resolution spectroscopy is currently the only means to observe variations of the HDO/H2O ratio in the martian atmosphere. These observations are difficult because telluric water exceeds the martian water by two orders of magnitude even at the excellent conditions of NASA IRTF. Our observations of HDO and H2O at the close wavenumbers of 2722 and 2994cm−1, respectively, cover six martian seasons in the period from 2007 to 2014. Infrared properties of water ice and dust are rather similar at these wavenumbers, and the HDO and H2O line equivalent widths are comparable; therefore effects of aerosol absorption and scattering significantly cancel out in the HDO/H2O ratios. These ratios are rather constant in wide latitude ranges at four observing sessions, in accord with the GCM model by Montmessin et al. (Montmessin, F., Fouchet, T., Forget, F. [2005]. J. Geophys. Res. 110, E03006). Results of two other sessions demonstrate significant deviations from the model predictions and strong correlation between HDO/H2O and temperature at ∼7km above the surface with correlation coefficients of 0.9. The observed global-mean HDO/H2O ratio is 4.6±0.7 times the terrestrial ratio, the ratio in vapor released by the north polar cap is 6.2±1.4, and the ratio in the north polar cap ice is 7.1±1.6. Updating the model of isotope fractionation in hydrogen escape by Krasnopolsky and Feldman (Krasnopolsky, V.A., Feldman, P.D. [2001]. Science 294, 1914–1917), 60m of the global water layer was lost in the last 4Byr and more than 1200m could be lost by hydrodynamic escape of H2 released in the reaction between water and iron. Variations of telluric D/H above Mauna Kea (Hawaii, elevation 4.2km) are by-products of our observations; D/H varies from 0.4 to 0.9 in nine observations with a mean D/H=0.67.