Constrained estimates of water loss from the martian atmosphere in a coupled TGO/MRO assimilation

Abstract

The evolution of water on Mars is a major scientific topic that can in part be advanced through the provision of bounded constraints on the rates of water loss. The Mars Atmosphere and Volatile EvolutioN Imaging Ultraviolet Spectrograph instrument continues to monitor the Lyman alpha brightness from Mars that is intricately linked to the hydrogen escape flux, but converting the emission data to hydrogen escape fluxes is difficult. We provide a complementary approach by combining retrievals of water vapour, temperature and dust from multiple spacecraft in orbit around Mars through a process called data assimilation. Recent instrument data from the Nadir and Occultation for MArs Discovery (NOMAD) and Atmospheric Chemistry Suite (ACS) instruments on the ExoMars Trace Gas Orbiter (TGO) provide a relative wealth of water vapor vertical profiles that are critical in constraining the vertical distribution and transport of water, the main chemical species that influences hydrogen escape rates. This lower atmosphere assimilation, that forms part of the OpenMARS (Open access to Mars Assimilated Remote Soundings) dataset, is coupled to a state of the art upper atmosphere model. Number densities of key chemical species in the assimilation are passed as a lower boundary to the upper atmosphere model to drive calculations of hydrogen escape at 200 km altitude.We investigate the escape of hydrogen during the time period that covers the primary science phase of the ExoMars TGO mission until the end of Mars Year (MY) 35 (around 1.5 Mars years). Hydrogen escape during the perihelion season is found to be a factor of 10 increased compared to aphelion, as a result of the seasonal pump of water vapor from the southern polar cap twinned with increased levels of atmospheric dust. The general trend over the perihelion season is in line with the first-order fit of Hubble Space Telescope hydrogen escape fluxes, although larger temporal variations are evident in the model simulations as these can compute the hydrogen escape flux at much finer temporal resolution. Variations in the peak intensity of hydrogen escape flux in both MY are evident, with the global dust storm in MY34 creating an early dust season peak which seems to diminish the peak at perihelion found a year later. The southern summer regional-scale dust storms that occur each year create a late season peak in hydrogen escape flux, with year-to-year variability in the exact timing of this particular dust storm as leading to a shift in the timing of peak hydrogen escape.