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. Plain Language Summary: The total water lost by Mars is determined by calculating the number of H atoms permanently escaping its upper atmosphere into space at present and extrapolating it back in time. The present‐day escape rate of H does not account for energetic H atoms, which were postulated to exist but not previously observed to date. Such a population would enhance the H escape rate further, thereby increasing the total volume of the water lost by Mars. This work presents the first confirmed signature of energetic H atoms in the atmosphere of Mars and finds that interaction of the neutral H population in its uppermost atmospheric layer, that is, its exosphere, with the solar wind results in its creation. The amount of energetic H atoms produced depends on the Mars' atmospheric conditions such as temperature and density of neutral H atoms in the exosphere, solar activity, and Mars' position around the Sun. Since the younger Sun was much more active than today, it is likely that the interaction of an early Martian exosphere with the stronger solar wind resulted in the loss of a larger fraction of H atoms into space than has been estimated to date. Key Points: Observations of H brightness at high altitude in Mars' atmosphere during aphelion solar minimum show an abrupt slope change above 5,000 kmHot H atoms produced via solar wind charge exchange can reproduce the observed profiles, providing evidence for nonthermal hydrogenSince this escape scales with solar wind flux, elevated loss rates early in Mars' history could result in much larger overall water loss [ABSTRACT FROM AUTHOR]