Martian Middle Atmospheric Water Vapor and its Influence on Hydrogen Production by Photolysis during the Global Dust Storm of 2018

One of the biggest questions concerning martian habitability is how Mars lost most of its water over geologic timescales. Hydrogen escape from the upper atmosphere is believed to be the primary loss process. Recent measurements have detected a significant seasonal variability in upper atmospheric hydrogen, which suggests a connection to middle atmospheric water vapor. Water vapor in the middle atmosphere photolyzes rapidly, producing hydrogen that can then be transported to the upper atmosphere and eventually escape. Rapid water vapor transport to the middle atmosphere of Mars has been linked to dust storm activity.

The martian global dust storm of 2018 provides a unique opportunity to explore these processes. Temperature structure and the vertical distribution of dust and water ice was measured nearly continuously by the Mars Climate Sounder (MCS) on board Mars Reconnaissance Orbiter (MRO). We use the temperature and water ice cloud distribution measured by MCS as a proxy for water vapor in the middle atmosphere. The lateral and vertical distribution of middle atmospheric water vapor during the global dust storm are derived from water ice cloud occurrence assuming equilibrium conditions. We use a one-dimensional photochemical model of the martian atmosphere to model the influence of water vapor photolysis on hydrogen production. The model is constrained by the temperature and water vapor fields derived from the measurements at various latitudes. We calculate hydrogen production through water vapor photolysis and quantify the amount of hydrogen available for escape due to this process under the conditions of a global dust storm.