Vol. 630
In section 10. Planets and planetary systems

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

Isotopic ratios of several elements (H, N, O, and noble gases) in the martian atmosphere can depart from their terrestrial counterparts, providing diagnostics of evolution processes such as atmospheric loss or interaction with the surface, as well as possible fractionation effects within the present-day martian atmosphere. While the martian D/H ratio, enhanced by a typical factor of approximately six over the Earth value, is well-documented, much less is known about the oxygen isotope ratios (18O/16O and 17O/16O) in water vapor. Furthermore, published measurements are thus far limited to a value of 18O/16O in the near-surface atmosphere obtained by the Curiosity Rover. Using early solar occultation infrared measurements of H216O, H218O, and H217O with the ACS (Atmospheric Chemistry Suite) instrument onboard the Trace Gas Orbiter mission, Alday et al. report vertically resolved profiles of 18O/16O and 17O/16O in H2O for the first time. They find these two ratios to be only mildly enhanced, by 200+/-80 per mil for 18O/16O and 230+/-110 per mil for 17O/16O, over the telluric standard values. This result likely stems from the combination of a low oxygen escape and a large oxygen reservoir. Within measurement errors, no vertical variability of the isotopic ratios can be distinguished from these early data. Accumulating measurements over varying seasonal and spatial conditions will allow for the search of fractionation effects, such as at water condensation, within the atmosphere.