Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board the ExoMars Trace Gas Orbiter

Abstract : Oxygen isotope ratios provide important constraints on the history of the Martian volatile system, revealing the impact of several processes that might fractionate them, such as atmospheric loss into space or interaction with the surface. We report infrared measurements of the Martian atmosphere obtained with the mid-infrared channel (MIR) of the Atmospheric Chemistry Suite (ACS), onboard the ExoMars Trace Gas Orbiter. Absorption lines of the three main oxygen isotopologues of water vapour (H₂¹⁶O, H₂¹⁸O, and H₂¹⁷O) observed in the transmission spectra allow, for the first time, the measurement of vertical profiles of the ¹⁸O/¹⁶O and ¹⁷O/¹⁶O ratios in atmospheric water vapour. The observed ratios are enriched with respect to Earth-like values (δ¹⁸O = 200 ± 80‰ and δ¹⁷O = 230 ± 110‰ corresponding to the Vienna Standard Mean Ocean Water). The vertical structure of these ratios does not appear to show significant evidence of altitudinal variations.