Oral Presentation

Photochemical Escape and Corona Distribution of Oxygen on Early Mars

Jinjin Zhao and Feng Tian (Ministry of Education Key Laboratory for Earth System Modeling, Center for Earth System Science, Tsinghua University)

Photochemical escape is a crucial process for neutral oxygen atoms escape from present Mars. Energetic oxygen atoms are produced from O2+ dissociative recombination reactions (DR) in the thermosphere/ionosphere of Mars. Some of them have sufficient energy to escape Mars while others move along ballistic trajectories and return to the thermosphere. The later produces a corona around Mars which could lead to other escape processes such as solar wind pickup or sputtering. Recently the MAVEN mission observed the Martian corona. How oxygen would have escaped photochemically from early Mars and what role the early Martian corona played on the escape of early Martian remain an important unresolved question challenging our understanding of early Mars evolution.
A 1-D Monte-Carlo model simulating the photochemical escape of oxygen under 1, 3, 10, and 20 times present solar XUV fluxes was built recently (Zhao and Tian, Icarus 2015). The results show that oxygen escape rate increases from 1× to 3~10× present solar XUV conditions but decreases rapidly when increasing solar XUV flux further. This result suggests that photochemical escape be a less important escape mechanism than previously thought for the loss of water and/or CO2 from early Mars. In this work, a 3-D Monte-Carlo Model is built to check the validity of previous conclusions.
The 3-D model is also used to produce spatial distributions of the oxygen corona on early Mars under different solar XUV conditions. Such corona distribution will be used to calculate non-thermal escape rate of through ion pick-up by solar wind and sputtering in future studies. The relevance of our work with observed exoplanetary thermosphere and exosphere will also be discussed.