Thermal Escape of Atomic Hydrogen on Mars: A Comprehensive View With a General Circulation Model

Abstract Mars loses water mainly through the photodissociation of water vapor, followed by the thermal escape of light hydrogen atoms. Observations have revealed large seasonal variations in hydrogen concentration in the upper thermosphere, peaking during southern summers and intensifying during dust storms. These variations were found to correlate with the presence of water vapor above approximately 70 km. This prompted a hypothesis that the water could be a source of hydrogen near the exobase. However, uncertainties remain about the mechanisms of hydrogen production at these heights, how seasonal and dust‐driven processes influence hydrogen loss, and the role of circulation in all of this. Using a Martian general circulation model covering altitudes from the surface to the exobase, we investigated the production, transport and escape of atomic hydrogen during Martian years (MY) 34 and 35, a period of low solar activity. Simulations show maximum photodissociation between 40 and 60 km across all seasons, with peaks following the Sun. The produced hydrogen and water vapor are carried upward by the meridional circulation. Production strengthens near perihelion due to higher water content and increased insolation. Intensified circulation, shortened pathways and faster transport to the thermosphere during this season enhance hydrogen escape. Enhancements during the MY34 global dust storm were 2.5 times less intense and even weaker during late‐year regional dust storms. However, their overall contribution to the annually integrated escape remains limited. The simulated seasonal maxima of global escape rates during southern summer solstice reach H atoms per second.