The HDO Cycle on Mars: Comparison of ACS Observations With GCM Simulations.

The D/H ratio and its implications on the atmospheric escape, make it an essential observable to study the current and past inventory of water on Mars. With the arrival of the Trace Gas Orbiter around Mars, new measurements of the D/H ratio are now available and require tools to interpret the observations and understand the HDO cycle. We here present simulations of an updated version of the Laboratoire de Météorologie Dynamique Mars Global Climate Model which includes HDO and in particular the fractionation processes it undergoes. We compare our model simulations with the HDO observations in solar occultation from the Atmospheric Chemistry Suite mid‐infrared channel on board the Trace Gas Orbiter (Alday et al., 2021; https://doi.org/10.5281/ZENODO.5100448). The model successfully reproduces the general trends of the D/H ratio, indicating that the main physical processes are captured by theory. A consistent simulation of condensation processes is found to be key in the representation of the D/H ratio. Improvements in the representation of clouds and on the water cycle will help improving the representation of the HDO cycle and better help extrapolate back in times the conditions of water escape on Mars. Plain Language Summary: Understanding how the Martian climate affects the isotopic ratio of water is key to understand the history of water on the planet. We use a general circulation model to simulate the cycle of HDO, an isotope of water, in the atmosphere of Mars. We compare our model results with spacecraft observations from the Atmospheric Chemistry Suite spectrometer on board the Trace Gas Orbiter, currently in orbit around Mars. Our model provides a good qualitative agreement with the observations. We find that the condensation of water vapor into ice is a critical process for determining the isotopic ratio of the vapor phase. Key Points: We simulated the HDO cycle on Mars using a 3D Global Climate ModelGCM simulations were compared with solar occultations from Atmospheric Chemistry Suite on board the Trace Gas OrbiterA good representation of the condensation processes is key to reproduce the observed D/H ratio [ABSTRACT FROM AUTHOR]