Atmospheric Hydrogen Deposition on Mars From Penetrating Solar Wind Protons

Upstream solar wind protons, being positively charged, are typically deflected by Lorentz forces around the Martian dayside bow shock, avoiding the atmosphere. However they can penetrate directly into the dayside Martian atmosphere when the protons charge exchange with neutral hydrogen atoms in the Martian exosphere becoming energetic neutral atoms (ENAs) that can precipitate directly into the thermosphere [Halekas et al., 2015]. Here the ENAs collide with neutral atoms and molecules, altering their charge, energy, and direction. The positively charged portion of this population can be observed by the Solar Wind Ion Analyzer (SWIA) onboard the MAVEN spacecraft. The primary tool in our investigation of this process is a Monte Carlo transport model called ASPEN [Jolitz et al., 2018] which simulates the transport of these ENAs as they undergo charge exchange and electron stripping reactions, losing energy, being deflected by electric and magnetic fields (when in proton form) and sometimes back scattering out of the atmosphere back into space. As a benchmarking exercise, we compared energy fluxes of these penetrating protons below the Martian exobase (~200 km altitude) measured by SWIA [Halekas et al., 2017] during MAVENs Deep Dip 2 campaign, with energy fluxes of protons (with respect to energy and angle) simulated by ASPEN. The model simulated from solar wind conditions observed by SWIA yielded an energy flux peak at a higher energy level (blue line) due to solar wind measurements being sampled from a disturbed (i.e. rapidly changing) period with a coronal mass ejection striking Mars. However, we found good agreement between an energy flux model simulated from a slower moving solar wind (green line) and the measured energy flux (red line). With this simulation, we will attempt to quantify the total flux of hydrogen thereby deposited in the Martian atmosphere and its variability at the present epoch and over the solar system history.