Scattering cross sections for O(3P), C(3P), and H colliding with a CO2 molecule for planetary aeronomy

We present scattering cross sections for O(³P), C(³P), and H colliding with a CO₂ molecule at collision energies between 0.1 and 5 eV. The kinetics, transport, and energy relaxation associated with collisions between fast atoms and thermal background atomic and molecular species are of fundamental interest for escape processes in the Martian atmosphere. Two of three primary objectives of the Emirates Mars Mission are related to hydrogen and oxygen escape processes and the collision cross sections are used in the models needed to interpret the observations by the EMUS and EMIRS instruments.

In this work, the collision cross sections have been computed using first principles electronic potential energy surfaces constructed in reduced dimensionality for the lowest-energy asymptotes corresponding to the ground states of the interacting pairs. For the three systems, namely C(³P)-CO₂, O(³P)-CO₂, and H-CO₂, velocity-dependent elastic, rotationally inelastic, and corresponding differential cross sections and derived quantities, including the momentum-transfer cross sections, were constructed. In all cases, the CO₂ molecule was modeled using the rigid-rotor approximation, and the collisions were treated as non-reactive. The cross sections were calculated from the first principles (no external parameters) by solving quantum-mechanical coupled channel equations following the approach of Arthurs-Dalgarno, with the coupled-state approximation^1,2.

We estimate the impact of the collision cross sections impact on the O, C, and H escape rates at Mars using simple 1D transport models and find significant differences compared to the values in the literature. In the transport model, we used the altitude density profiles taken from NASA’s MAVEN mission. In case of all three energetic atoms, the inelastic cross sections are found to be a significant part of the total cross sections. In case of O escape, we obtain a larger escape flux closer to the estimates based on the MAVEN measurements. We find that that O+CO flux affects the O escape more significantly than expected³, due to its effects on the available energetic O flux. We expect a similar effect to be present not only at Mars but at all CO₂-rich planets due to the O-CO-CO₂ photochemistry. The impact on C escape is inconclusive, suggesting that the accounted mechanisms of photochemical escape of C are not sufficient to explain the missing carbon at Mars^4,5. Please insert your abstract HTML here.