Martian planetary heavy ion sputtering of Phobos

Abstract

The Martian moons, Phobos and Deimos, have long been suspected to be the sources of tenuous neutral gas tori encircling Mars. While direct outgassing has been ruled out as a strong source, micrometeoroid impact vaporization and charged particle sputtering must operate based on observations at other airless bodies. Previous models have addressed solar wind sputtering of Phobos; however, Phobos and Deimos are also subject to a significant, yet temporally variable, flux of heavy planetary ions escaping from Mars. In this report, we use a combination MHD/test-particle model to calculate the planetary heavy ion flux to Phobos and the ensuing neutral sputtered flux. Depending on ambient solar wind conditions and the location of Phobos, heavy ion sputtering of Phobos generates neutral fluxes up to and exceeding that from solar wind sputtering. We model pickup ions from the Phobos torus itself with applications for observations by the upcoming Mars Atmospheric and Volatile Evolution mission. Key Points We assess neutral sputtering of Phobos by escaping Martian O+ ionsMartian O+ sputters at rates larger than solar wind depending on SW conditionsMAVEN may detect the Phobos torus via newly generated torus pickup ions