Evidence of non‐thermal hydrogen in the exosphere of Mars resulting in enhanced water loss

Atomic H is a direct tracer of water loss at Mars. The recent discovery of annual enhancements in its escape rate near perihelion, in excess of the previously established theory, indicates that Mars has lost substantial amounts of water to space. However, these loss rates are often estimated assuming thermal properties for the exospheric H atoms and are therefore a lower limit. Past analyses of spacecraft observations delivered indirect evidence for the existence of an energetic non-thermal H population without a clear detection. Here, we present the unambiguous observational signature of non-thermal H at Mars, consistent with solar wind charge exchange as the primary driver for its production. The calculated non-thermal escape rates are as high as ∼26% of the thermal rate near aphelion and solar minima. An active Sun would increase the present-day escape rate and a younger energetic Sun likely contributed toward the significant historic loss of water from Mars.