New C-CO2 scattering cross sections at eV energies and their impact on photochemical escape of Carbon from Mars

Abstract

<p>Recent years have seen a renewed interest in the problem of “missing carbon” at Mars and missing physical mechanisms that could explain it. At present, the carbon escape rates derived from in situ observations conducted by Mars orbiters<sup>1</sup> appear to be at least an order of magnitude lower than the observed oxygen escape rates. This would imply that majority of the oxygen escape from Mars that took place over the last epochs has been from water rather than from CO<sub>2</sub> – in contradiction with geological evidence and atmospheric models that would require significantly higher carbon escape at higher solar inputs in its past<sup>2</sup>.</p><p>It has been widely regarded that photochemistry is the major source of hot escaping carbon in the present-day Martian atmosphere. Recently, photodissociation of CO<sub>2</sub> by solar UV photons was identified as its leading contribution<sup>1,3</sup>. Theoretical models suggest that, regardless of the exact process, hot carbon atoms are predominantly produced at the altitudes near the exobase<sup>1</sup>, at about 200 km above the planet’s surface, and have to traverse through the exosphere without losing their kinetic energy in collisions in order to successfully escape. Thus, due to the abundance of atmospheric CO<sub>2</sub>, one of the key parameters that determine the C escape rate are the C-CO<sub>2</sub> scattering cross sections. These cross sections are not well known at the energy range of interest.</p><p>In this work, we constructed from first principles quantum-mechanical velocity-dependent elastic, inelastic, and corresponding differential and momentum-transfer cross sections for C(<sup>3</sup>P)-CO<sub>2</sub> scattering for collision energies up to 5 eV and estimated the impact on the C escape rate at Mars using simple column escape models.</p><p> </p><p><sup>1</sup>Lo et al., Carbon photochemical escape rates from the modern Mars atmosphere. Icarus 360, 114371 (2021)</p><p><sup>2</sup>Jakosky et al., Loss of the Martian atmosphere to space: Present-day loss rates determined from MAVEN observations and integrated loss through time. Icarus 315, 146 (2018)</p><p><sup>3</sup>Lu et al., Evidence for direct molecular oxygen production in CO2 photodissociation. Science 346, 61 (2014)</p>