Photochemical Escape of Oxygen from Early Mars

TL;DR

This study models photochemical escape of oxygen from early Mars using a Monte-Carlo approach, revealing that increased solar XUV flux initially boosts escape rates but then decreases them at higher fluxes, suggesting a lesser role in water loss.

Contribution

Developed a 1-D Monte-Carlo model to quantify oxygen escape rates from early Mars under varying solar XUV conditions, highlighting a non-linear relationship.

Findings

Oxygen escape rate peaks at 10 times present solar XUV flux.

Higher XUV flux increases upper thermosphere atomic species more rapidly than O2+.

Photochemical escape is less significant for water loss than previously believed.

Abstract

Photochemical escape is an important process for oxygen escape from present Mars. In this work, a 1-D Monte-Carlo Model is developed to calculate escape rates of energetic oxygen atoms produced from O2+ dissociative recombination reactions (DR) under 1, 3, 10, and 20 times present solar XUV fluxes. We found that although the overall DR rates increase with solar XUV flux almost linearly, oxygen escape rate increases from 1 to 10 times present solar XUV conditions but decreases when increasing solar XUV flux further. Analysis shows that atomic species in the upper thermosphere of early Mars increases more rapidly than O2+ when increasing XUV fluxes. While the latter is the source of energetic O atoms, the former increases the collision probability and thus decreases the escape probability of energetic O. Our results suggest that photochemical escape be a less important escape mechanism than previously thought for the loss of water and/or CO2 from early Mars.