Penetrating Solar Wind Protons as a Source of Hydrogen to the Martian Atmosphere

Abstract

The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has observed a proton population in the upper Martian atmosphere with plasma velocities and temperatures consistent with the upstream solar wind. This population is believed to originate from solar wind protons that undergo charge exchange with neutral hydrogen in the exosphere, becoming energetic neutral atoms (ENAs) that then penetrate the atmosphere, unaffected by magnetic fields. As these ENAs precipitate, their charge state evolves through subsequent interactions, ultimately depositing as neutral hydrogen. We investigate this deposition process and its variability by combining model predictions with observations of the charged population measured by the Solar Wind Ion Analyzer onboard MAVEN. A Monte Carlo transport model was employed to simulate the competing processes of deposition and backscattering under a range of exospheric and solar wind conditions. Our results show that solar wind hydrogen absorption can exceed ${10}^{25}$ $s^{-1}$ at the current epoch, representing 10%–20% of the total hydrogen escape from Mars. This absorption mechanism likely played a significant role in shaping the early Martian atmosphere and climate, when solar wind proton densities were substantially higher.