Oxygen Exosphere of Mars: Evidence from Pickup Ions Measured by MAVEN

Abstract

Mars possesses a hot oxygen exosphere that extends out to several Martian radii. The main source for populating this extended exosphere is the dissociative recombination of molecular oxygen ions with electrons in the Mars ionosphere. The dissociative recombination reaction creates two hot oxygen atoms that can gain energies above the escape energy at Mars and escape from the planet. Oxygen loss through this photochemical reaction is thought to be one of the main mechanisms of atmosphere escape at Mars, leading to the disappearance of water on the surface. In this work the hot oxygen exosphere of Mars is modeled using a two-stream/Liouville approach as well as a Monte-Carlo simulation. The modeled exosphere is used in a pickup ion simulation to predict the flux of energetic oxygen pickup ions at Mars. The pickup ions are created via ionization of neutral exospheric oxygen atoms through photo-ionization, charge exchange with solar wind protons, and electron impact ionization. Once ionized, the pickup ions are accelerated by the solar wind motional electric field to high energies, thus detectable by spacecraft instruments. The MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft arrived at Mars in September of 2014 and has been taking measurements in the upper atmosphere of Mars, with the goal of determining the drivers and rates of atmospheric escape. In this work comparisons are made between the pickup ion model results and the MAVEN data from the SEP (Solar Energetic Particle) and SWIA (Solar Wind Ion Analyzer) instruments. It is shown that these model-data comparisons can be used to constrain the hot oxygen exospheric densities and the associated escape rates of oxygen from Mars.