Hydrogen escape on Mars dominated by water vapor photolysis above the hygropause

Loss of water on Mars to space largely occurs through the decomposition of water vapor after transport to high altitudes with subsequent escape of atomic hydrogen. The relative importance of water photolysis and ion chemistry as decomposition mechanisms as well as seasonally recurring vs. impulsive transport driven by dust storms are open questions. Using photochemical modeling based on a synergistic dataset from three Mars orbiters, we show that water photolysis above the main region of cloud formation (the hygropause) is the dominant source of hydrogen available for escape, significantly exceeding hydrogen production through ion chemistry. We also show that seasonally recurring transport dominates hydrogen escape over impulsive transport, suggesting that dust storms play a smaller role in atmospheric water loss than previously thought.