Atmospheric ion escape from Mars - MAVEN observations of a state-transition at high solar EUV

What fundamentally limits the rate of solar wind driven atmospheric ion escape from Mars? Previous orbit-based in situ measurements of escaping heavy ions (O⁺, O₂⁺ and heavier species) have yielded conflicting estimates of the dependencies on upstream solar wind and solar extreme ultraviolet (EUV) conditions. We compile 6 years (2014-2020) of measured 0.1 eV - 30 keV ion distributions from the SupraThermal and Thermal Ion Composition (STATIC) instrument on the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter to globally map the average phase-space ion flux distribution, from which we derive globally integrated outflow, inflow and net ion fluxes. Through binning the data by upstream solar wind (measured simultaneously by the Mars Express orbiter) and EUV conditions, we quantify the dependencies on these drivers. The found trends indicate that ion escape from Mars is an ion supply/source-limited process under low solar EUV conditions, however, the appearance and increase of gravitationally bound heavy ion return flows under moderate EUV conditions suggests that the escape process is close to transition ing to an energy-limited state. We discuss the implications for ion observations at Mars during the upcoming solar cycle, for the evolution of the Martian atmosphere, and for our understanding of atmospheric ion escape as a general process in the solar system and beyond.