MAVEN Observations of a State-Transition in Ion Escape from Mars

Fundamentally, what limits the rate of atmospheric ion escape from non-magnetized planets? Previous orbit-based in situ measurements of escaping heavy ions (O⁺, O₂⁺ and heavier species) at Mars have yielded conflicting estimates of the dependencies on upstream solar wind and solar extreme ultraviolet (EUV) conditions. We compile 7 years (2014-2021) of measured 0.1 eV – 30 keV ion distributions from the STATIC instrument on the MAVEN orbiter to globally map the phase-space ion flux distribution, from which we derive globally integrated outflow, inflow, and net ion fluxes. Through binning the measurements by upstream solar wind (measured simultaneously by the Mars Express orbiter) and EUV conditions, we separately quantify the O⁺ and O₂⁺ escape dependencies on these external drivers. The found trends indicate that ion escape from Mars is a 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 can transition to a Venus-like energy-limited state. The findings show that the state of the ion escape process does not only differ between planets, but the state and thus the drivers of ion escape can also differ under varying external conditions. We discuss the implications for ion observations at Mars during the upcoming solar activity maximum, 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.