SIMBAD references

Isotopic ratios in water vapour carry important information about the water reservoir on Mars. Localized 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 (L_S = 15 , 127 , 272 , and 305 ) made with the Echelon-Cross-Echelle Spectrograph (EXES) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). 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 $\sim\!{5}~{{\ \rm per\ cent}}$ larger than that in the atmospheric reservoir (D/H_ice = 5.3 ± 0.3 VSMOW).