Carbon photochemistry at Mars: Updates with recent data

Abstract

We provide a comprehensive characterization of the photochemistry behind atomic carbon in the Mars atmosphere. Using a one-dimensional photochemical model, with an extensive reaction list incorporating new high-resolution photodissociation cross-sections (Heays et al., 2017) and the recently experimentally confirmed CO₂ + hν →C +O₂ (Lu et al., 2014), we investigate the dominant channels for the production and loss of atomic carbon, against a subsolar background atmosphere based on MAVEN Deep Dip 2 observations. We confirm the results from previous studies that CO photodissociation and CO+ dissociative recombination are important contributors to atomic C production, and that reaction with O₂ to form CO is the main loss channel. However, we also find significant contributions from CO₂ + hν →C +O₂ , HCO⁺ + e →C +OH and charge exchange of C+ with CO₂. These additional production channels give rise to significantly higher C densities than have been previously reported, with a peak at 4 × 10⁵ cm^-3 at a CO₂ density of 1 . 7 × 10¹⁰ cm^-3 (∼ 146 km altitude). We find the C densities to vary with H₂O densities over a Martian year, with the wetter perihelion season having 13% lower C column densities. Contrary to Anbar et al. (1993), we find C densities to be relatively insensitive to the temperature-dependence of cross-sections for CO₂ and CO photodissociation. A good understanding of carbon photochemistry in the present-day Martian atmosphere provides the essential foundational framework for determining the fate of atmospheric carbon in the study of Mars' climate evolution.