Carbon photochemistry at Mars: Updates with recent data

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. •Atomic carbon in Mars atmosphere is significantly more than previously reported.•New production channels drive the higher carbon densities.•Carbon column density is lower at perihelion season than aphelion season.•Carbon densities are insensitive to temperature-dependence of cross-sections.