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

Abstract

The loss of water on Mars to space largely occurs through the decomposition of water vapour after transport to high altitudes with the subsequent escape of atomic hydrogen. However, there are still open questions about the relative importance of water photolysis and ion chemistry as decomposition mechanisms. In addition, the relevance of seasonally recurring compared with impulsive vertical water transport driven by dust storms is not fully understood. Using photochemical modelling based on a synergistic dataset from three Mars orbiters, we show that water photolysis above the main region of cloud formation (above 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 only a minor role in atmospheric water loss. Modelled hydrogen escape rates show good agreement with available measurements, which demonstrates the importance of the mechanisms investigated here for improving quantitative estimates of long-term water loss on Mars.