Carbon Photochemistry and Escape in the Present-Day Martian Atmosphere

Abstract

Loss of CO$_2$ from the Martian atmosphere has driven drastic changes in the Martian climate since the Noachian period 3.6 billion years ago. This loss can be into the surface or into space, and the loss into space is driven primarily by photochemical escape of atomic carbon. A reliable calculation of the CO$_2$ loss over the last 3.6 billion years requires a foundation grounded in a solid understanding of what is happening today. To this end, this dissertation provides an updated and comprehensive of carbon photochemistry and escape from the present-day Martian atmosphere, using the most updated characterization of the atmospheric conditions from both modeling and MAVEN observations, of the photochemical reactions that can occur in the atmosphere, and of the collisional cross sections for determining whether a freshly produced high-energy C atom can escape. Through our investigations, we not only confirm that the well-studied CO photodissociation to be important for the production of atomic C, but also find CO$_2$ photodissociation and HCO$^+$ dissociative recombination to be significant contributors. CO$_2$ photodissociation is also the largest contributor to C escape rates, with CO photodissociation and CO$_2$ electron impact dissociation being secondary contributors. Reactions such as $\text{CO} + h\nu \rightarrow \text C + \text O^+ + e$ and $\text{CO} + e \rightarrow \text C + \text O + e$ are found to be highly sensitive to the solar ultraviolet flux. Although they are not significant contributors today, the higher ultraviolet output from the younger Sun may make them relevant for photochemical escape from ancient Mars. We also present the first measurements of C densities in the Martian upper atmosphere from retrievals of C I emissions at 156.1~nm and 165.7~nm, providing crucial empirical constraints on our modeling results on C photochemistry and escape.