Hydrogen escape from Mars enhanced by deep convection in dust storms

Present-day water loss from Mars provides insight into Mars's past habitability(1-3). Its main mechanism is thought to be Jeans escape of a steady hydrogen reservoir sourced from odd-oxygen reactions with near-surface water vapour(2,4,5). The observed escape rate, however, is strongly variable and correlates poorly with solar extreme-ultraviolet radiation flux(6-8), which was predicted to modulate escape(9). This variability has recently been attributed to hydrogen sourced from photolysed middle atmospheric water vapour(10), whose vertical and seasonal distribution is only partly characterized and understood(11-13). Here, we report multi-annual observational estimates of water content and dust and water transport to the middle atmosphere from Mars Climate Sounder data. We provide strong evidence that the transport of water vapour and ice to the middle atmosphere by deep convection in Martian dust storms can enhance hydrogen escape. Planet-encircling dust storms can raise the effective hygropause (where water content rapidly decreases to effectively zero) from 50 to 80 km above the areoid (the reference equipotential surface). Smaller dust storms contribute to an annual mode in water content at 40-50 km that may explain seasonal variability in escape. Our results imply that Martian atmospheric chemistry and evolution can be strongly affected by the meteorology of the lower and middle atmosphere of Mars.