Carbon Photochemistry and Densities in the Martian Atmosphere under MAVEN Deep Dip 2 Conditions

Atomic C in the Martian atmosphere is the final reservoir of carbon in the Martian system before its loss into space, and a good understanding of the size of this reservoir and the processes leading into and out of it today is a necessary step in understanding present loss rates of CO₂ from Mars, and thus the integrated loss over its history. Using a 1-D photochemical model, we provide an updated picture of atomic C densities and its various production and loss channels in the Martian atmosphere. Our model makes use of an extensive reaction list to search exhaustively for previously neglected reaction pathways, updated high-resolution photodissociation cross-sections, and new atmospheric data from the MAVEN (Mars Atmosphere and Volatile EvolutioN) mission. We found C densities to peak at 10⁵ cm^-3 at an altitude of 155 km, decreasing drastically at lower altitudes from the increasing ultraviolet optical depth, and decreasing exponentially towards the higher altitudes with a scale height consistent with molecular diffusion. CO₂ photodissociation, described by new cross-sections from Lu et al. [2014], is the main production channel for C, with secondary production from CO photodissociation towards the higher altitudes. We also discovered a new secondary C production pathway involving HCO⁺ dissociative recombination that is significant at the lower altitudes. Loss is through reaction with O₂. While C densities are not sensitive to the annual variations in atmospheric H₂O densities, they are sensitive to CO densities through the two secondary production processes of CO photodissociation and HCO⁺ dissociative recombination. This implies that the accuracy of CO densities, which are traditionally poorly modeled by 1-D Mars photochemical models, is critical for determining C densities at Mars. We believe that this difficulty arises from the use of H₂O density profiles that are too high in the troposphere. In our model, we made use of H₂O density profiles obtained from the Laboratoire de Météorologie Dynamique (LMD) GCM, which were calculated with a comprehensive water cycle module [Navarro et al., 2014]. We were able to produce CO densities consistent with observational constraints.