A map of D/H on Mars in the thermal infrared using EXES aboard SOFIA

On a planetary scale, the D/H ratio on Mars is a key diagnostic for understanding the past history of water on the planet; locally, it can help to constrain the sources and sinks of water vapor through the monitoring of condensation and sublimation processes. To obtain simultaneous measurements of H₂O and HDO lines, we have used the Echelle Cross Echelle Spectrograph (EXES) instrument aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) facility to map the abundances of these two species over the Martian disk. High-resolution spectra (R = 6 × 10⁴) were recorded in the 1383-1390 cm^-1 range (7.2 μm) on April 08, 2014. Mars was very close to opposition and near northern summer solstice (Ls = 113°). Maps of the H₂O and HDO mixing ratios were retrieved from the line depth ratios of weak H₂O and HDO transitions divided by a weak CO₂ line. As expected for this season, the H₂O and HDO maps show a distinct enhancement toward polar regions, and their mixing ratios are consistent with previous measurements and with predictions by the global climate models, except at the north pole where the EXES values are weaker. We derive a disk-integrated D/H ratio of 6.8 (+1.6, -1.0) × 10^-4. It is higher than the value in Earth's oceans by a factor 4.4 (+1.0, -0.6). The D/H map also shows an enhancement from southern to northern latitudes, with values ranging from about 3.5 times to 6.0 times the VSMOW (Vienna standard mean ocean water) value. The D/H distribution shows a depletion over the Tharsis mountains and is consistent with observed latitudinal variations. The variations in D/H with latitude and altitude agree with the models and with the isotope fractionation expected from condensation and sublimation processes.