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Mars has lost most of its initial water to space as atomic hydrogen and oxygen. Spacecraft measurements have determined that present-day hydrogen escape undergoes large variations with season that are inconsistent with long-standing explanations. The cause is incompletely understood, with likely contributions from seasonal changes in atmospheric circulation, dust activity and solar extreme ultraviolet input. Although some modelling and indirect observational evidence suggest that dust activity can explain the seasonal trend, no previous study has been able to unambiguously distinguish seasonal from dust-driven forcing. Here we present synoptic measurements of dust, temperature, ice, water and hydrogen on Mars during a regional dust event, demonstrating that individual dust events can boost planetary H loss by a factor of five to ten. This regional storm occurred in the declining phase of the known seasonal trend, establishing that dust forcing can override this trend to drive enhanced escape. Because similar regional storms occur in most Mars years, these storms may be responsible for a large fraction of Martian water loss and represent an important driver of Mars atmospheric evolution.

Water escape on Mars has recently undergone a paradigm shift with the discovery of unexpected seasonal variations in the population of hydrogen atoms in the exosphere where thermal escape occurs and results in water lost to space. This discovery led to the hypothesis that, contradicting the accepted pathway, atomic hydrogen in the exosphere was not only produced by molecular hydrogen but mostly by high altitude water vapor. Enhanced presence of water at high altitude during southern spring and summer, due to atmospheric warming and intensified transport, favors production of H through photon-induced ion chemistry of water molecules and thus appears to be the main cause of the observed seasonal variability in escaping hydrogen. This hypothesis is supported by the observation of large concentrations of water vapor between 50 km and 150 km during the southern summer solstice and global dust events. Using a simplified yet representative air parcel transport model, we show that in addition to the formation of atomic hydrogen from water photolysis above 80 km, a major fraction of the exospheric hydrogen is formed at altitudes as low as 60 km and is then directly advected to the upper atmosphere. Comparing the injection modes of a variety of events (global dust storm, perihelion periods, regional storm), we conclude that southern spring/summer controls H production and further ascent into the upper atmosphere on the long term with direct implication for water escape.

Mars has lost most of its initial water to space as atomic hydrogen and oxygen. Spacecraft measurements have determined that present-day hydrogen escape undergoes large variations with season that are inconsistent with long-standing explanations. The cause is incompletely understood, with likely contributions from seasonal changes in atmospheric circulation, dust activity and solar extreme ultraviolet input. Although some modelling and indirect observational evidence suggest that dust activity can explain the seasonal trend, no previous study has been able to unambiguously distinguish seasonal from dust-driven forcing. Here we present synoptic measurements of dust, temperature, ice, water and hydrogen on Mars during a regional dust event, demonstrating that individual dust events can boost planetary H loss by a factor of five to ten. This regional storm occurred in the declining phase of the known seasonal trend, establishing that dust forcing can override this trend to drive enhanced escape. Because similar regional storms occur in most Mars years, these storms may be responsible for a large fraction of Martian water loss and represent an important driver of Mars atmospheric evolution.

Water escape on Mars has recently undergone a paradigm shift with the discovery of unexpected seasonal variations in the population of hydrogen atoms in the exosphere where thermal escape occurs and results in water lost to space. This discovery led to the hypothesis that, contradicting the accepted pathway, atomic hydrogen in the exosphere was not only produced by molecular hydrogen but mostly by high altitude water vapor. Enhanced presence of water at high altitude during southern spring and summer, due to atmospheric warming and intensified transport, favors production of H through photon-induced ion chemistry of water molecules and thus appears to be the main cause of the observed seasonal variability in escaping hydrogen. This hypothesis is supported by the observation of large concentrations of water vapor between 50 km and 150 km during the southern summer solstice and global dust events. Using a simplified yet representative air parcel transport model, we show that in addition to the formation of atomic hydrogen from water photolysis above 80 km, a major fraction of the exospheric hydrogen is formed at altitudes as low as 60 km and is then directly advected to the upper atmosphere. Comparing the injection modes of a variety of events (global dust storm, perihelion periods, regional storm), we conclude that southern spring/summer controls H production and further ascent into the upper atmosphere on the long term with direct implication for water escape.