Hydrogen Loss from Mars: Seasonal and Sourced from Upper Atmospheric Water

Mars has lost a significant fraction of its initial water inventory to space over its history. This loss proceeds via a chemical chain initiated in the lower and middle atmosphere below 100 km altitude, which results in H escape to space in the collisionless corona above about 200 km altitude. Hydrogen loss from Mars is tracked via brigtness measurements of this corona in Lyman alpha light at 121.6 nm, which is scattered by neutral H orbiting and escaping the planet. Here we present observations of H escape variability made by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission's Imaging Ultraviolet Spectrograph (IUVS) that demonstrate the seasonal dependence of H escape, adding to evidence initially gathered by Mars Express and the Hubble Space Telescope. We show that for two Mars years the atmosphere has exhibited enhanced H escape in Southern Summer near perihelion. We also present the result of photochemical model calculations which demonstrate that this variation can be explained as a result of high concentrations of water vapor in the upper atmosphere, consistent with Mars Express solar occultation measurements and several general circulation models. Our results demonstrate that the large variations in H escape at high altitudes can be driven by lower atmospheric dynamics, suggesting that Mars hydrogen escape may depend on climate, in addition to the long-term evolution of Martian climate depending on atmospheric escape.