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

Abstract

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 H 2 O 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 × 10 4 ) 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 H 2 O and HDO mixing ratios were retrieved from the line depth ratios of weak H 2 O and HDO transitions divided by a weak CO 2 line. As expected for this season, the H 2 O 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 diskintegrated 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.

New measurements of D/H on Mars using EXES aboard SOFIA

“…As in our previous analysis, we note that this profile does not perfectly fit the EXES lines, which show broader wings than the Gaussian profile, while the central core tends to be narrower. As shown in our earlier analysis (Encrenaz et al 2016), the sinc 2 function is very close to the Gaussian curve and does not improve the fit with our data. We adopted these Gaussian profiles and these resolving powers as a compromise to minimize the discrepancies between the data and the model.…”

“…obtained in May 2014 (Encrenaz et al 2016). However, we must be aware of the limitations of this method.…”

“…To model the Martian synthetic spectrum, we used the radiative transfer code developed for our analysis of the TEXES data of Mars (Encrenaz et al 2004(Encrenaz et al , 2015, and also used for the analysis of our EXES data of April 2014 (Encrenaz et al 2016). Spectroscopic data were extracted from the GEISA molecular database (Jacquinet-Husson et al 2008).…”

“…Many observations have been performed since 1988 (Bjoraker et al 1989;Krasnopolsky et al 1997;Encrenaz et al 2001), all consistent with the first detection. More recently, attempts to map D/H on Mars as a function of latitude and season have been performed by Novak et al (2011), Villanueva et al (2015), Krasnopolsky (2015), Aoki et al (2015) and Encrenaz et al (2016).…”

The Main Belt Comets and ice in the Solar System

“…There is strong geologic and mineralogic evidence that liquid water was more abundant on Mars in the past, when the atmosphere was thicker (Sagan et al 1973;Lasue et al 2013). Like on Venus, an elevated D/H ratio indicates that a significant amount of water vapour escaped to space from the atmosphere, affecting H 2 O more than D 2 O (Encrenaz et al 2016). The original water content of Mars may have been sufficient to cover the planet with a layer of up to 1 km depth (Lasue et al 2013).…”