Atomic Oxygen Escape on Mars Driven by Electron Impact Excitation and Ionization

Abstract

Neutral escape on Mars is well known to be dominated by atomic oxygen (O) escape via the dissociative recombination (DR) of , the most abundant ion species in the Martian upper atmosphere. In this study, we suggest an alternative mechanism due to the impact excitation and ionization of CO 2 by photoelectrons on the dayside or precipitating solar wind electrons on the nightside. These electron impact (EI) processes are capable of producing hot O atoms via the predissociation of CO 2 and in electronically excited states. An evaluation with the aid of the combined data set accumulated by several instruments on board the Mars Atmosphere and Volatile Evolution spacecraft reveals that CO 2 EI makes a non-negligible contribution to total atomic O escape on Mars, with a median EI-to-DR escape flux ratio of 10% on the dayside and 20% on the nightside. CO 2 EI is found to be able to drive stronger O escape than atmospheric sputtering, an alternative escape mechanism well studied on Mars. Our results highlight the importance of a neutral escape mechanism largely ignored in previous studies.