Constraining the global composition of D/H and 18O/16O in Martian water from SOFIA/EXES
Monthly Notices of the Royal Astronomical Society Oxford University Press 530:3 (2024) 2919-2932
Authors:
Juan Alday, S Aoki, C DeWitt, Franck Montmessin, J Holmes, M Patel, J Mason, Therese Encrenaz, M Richter, Patrick Irwin, F Daerden, N Terada, H Nakagawa
Abstract:
Isotopic ratios in water vapour carry important information about the water reservoir on Mars. Localised variations in these ratios
can inform us about the water cycle and surface-atmosphere exchanges. On the other hand, the global isotopic composition of
the atmosphere carries the imprints of the long-term fractionation, providing crucial information about the early water reservoir
and its evolution throughout history. Here, we report the analysis of measurements of the D/H and 18O/16O isotopic ratios
in water vapour in different seasons (𝐿S = 15◦
, 127◦
, 272◦
, 305◦
) made with SOFIA/EXES. These measurements, free of
telluric absorption, provide a unique tool for constraining the global isotopic composition of Martian water vapour. We find the
maximum planetary D/H ratio in our observations during the northern summer (D/H = 5.2 ± 0.2 with respect to the Vienna
Standard Mean Ocean Water, VSMOW) and to exhibit relatively small variations throughout the year (D/H = 5.0 ± 0.2 and
4.3 ± 0.4 VSMOW during the northern winter and spring, respectively), which are to first order consistent though noticeably
larger than the expectations from condensation-induced fractionation. Our measurements reveal the annually-averaged isotopic
composition of water vapour to be consistent with D/H = 5.0 ± 0.2 and 18O/16O = 1.09 ± 0.08 VSMOW. In addition, based on
a comparison between the SOFIA/EXES measurements and the predictions from a Global Climate Model, we estimate the D/H
in the northern polar ice cap to be ∼5% larger than that in the atmospheric reservoir (D/Hice = 5.3 ± 0.3 VSMOW).
