Photochemical modeling of seasonal and dust storm driven hydrogen escape on Mars

The loss of water on Mars through atmospheric escape of hydrogen is an area of great interest and activity in current Mars research. Observations of upper atmospheric hydrogen in recent years show rapid changes in atomic hydrogen amounts on time scales of months. Significant increases in hydrogen amounts and subsequent escape have been reported for the perihelion season of Mars, with a maximum of hydrogen escape coinciding with southern summer solstice. In addition to this seasonal variation, more short-term increases have been reported in conjunction with global or large-scale regional dust storms. While it was shown that a regional storm could lead to a significant increase in hydrogen escape against the seasonal trend, the relative importance of seasonal vs. dust storm driven impulsive increases in hydrogen escape remains an open question.

Photochemical modeling is a powerful tool to address this question. We use the established photochemical model KINETICS in its one-dimensional formulation, which includes vertical transport by both molecular and eddy diffusion. The advantage of this approach over more sophisticated approaches, such as General Circulation Models, is that atmospheric conditions, such as temperature and water vapor distribution, can be directly constrained by observations and used as input to the model. We use a synergistic approach to constrain temperature, density, and water vapor based on measurements by the Mars Climate Sounder on the Mars Reconnaissance Orbiter, the ACS and NOMAD instruments on the ExoMars Trace Gas Orbiter, and the Neutral Gas and Ion Mass Spectrometer on MAVEN. We focus on the dusty season of Mars Year 34, which exhibits a global dust storm early in the season as well as a large regional dust storm late in the season that overlay the recurring seasonal variation. We show that the expansion of the atmosphere during a global dust storm significantly increases shielding of solar UV radiation by CO2 absorption, thus reducing the effectiveness of photolysis of middle atmospheric water vapor. Our results suggest that the annually recurring seasonal variation in middle and upper atmospheric water vapor have a larger effect on atmospheric hydrogen escape than impulsive effects such as regional or global dust storms.