Abstract
The global D/H ratio on Mars is an important measurement for understanding the past history of water on Mars; locally, through condensation and sublimation processes, it is a possible tracer of the sources and sinks of water vapor on Mars. Measuring D/H as a function of longitude, latitude and season is necessary for determining the present averaged value of D/H on Mars. Following an earlier measurement in April 2014, we used the Echelon Cross Echelle Spectrograph (EXES) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) facility to map D/H on Mars on two occasions, on March 24, 2016 (Ls = 127°), and January 24, 2017 (Ls = 304°), by measuring simultaneously the abundances of H2O and HDO in the 1383–1391 cm−1 range (7.2 μm). The D/H disk-integrated values are 4.0 (+0.8, −0.6) × Vienna Standard Mean Ocean Water (VSMOW) and 4.5 (+0.7, −0.6) × VSMOW, respectively, in agreement with our earlier result. The main result of this study is that there is no evidence of strong local variations in the D/H ratio nor for seasonal variations in the global D/H ratio between northern summer and southern summer.
Highlights
Measuring the D/H ratio in the Martian atmosphere is important for estimating the abundance of the early water reservoir on Mars
In the case of the March 2016 data, we used a surface temperature of 255 K, a surface pressure of 6.0 mbar, and temperatures of 230, 200, 170, and 140 K at altitude levels of 0, 10, 20, and 40 km respectively, with an isothermal profile above 40 km. This profile, close to that predicted by the European Mars Climate Database (EMCD; Forget et al 1999), is the same as the one we used for our data of April 2014, which corresponded to a similar season and the same longitude range
In our previous analyses of the Texas Echelon Cross Echelle Spectrograph (TEXES) spectra of Mars (Encrenaz et al 2015), we showed that the mixing ratio of two atmospheric species can be directly estimated from the ratio of their line depths provided that the lines are weak, with line depths less than about 10%
Summary
Measuring the D/H ratio in the Martian atmosphere is important for estimating the abundance of the early water reservoir on Mars. For a precise measurement of the present D/H ratio on Mars and in order to avoid any possible seasonal variation, D/H should be, as much as possible, integrated over an entire Martian year. In addition to its cosmogonical interest, the measurement of D/H in water on Mars as a function of altitude, location, and season can bring important constraints on the mechanisms responsible for the sources and sinks of water vapor on the modern day Martian surface and atmosphere. We continued our observing campaign with EXES on board SOFIA in order to map D/H on Mars for different longitude ranges and different seasons.
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