MAVEN/IUVS observations of C I 156.1 nm and 165.7 nm dayglow: Direct detection of carbon and implications on photochemical escape

We provide the first detailed characterization and analysis of the C I 156.1 nm (³D^o→³ P) and 165.7 nm (³P^o→³ P) dayglow in the Martian atmosphere. Using MAVEN/IUVS periapse limb observations from two time periods corresponding to the aphelion and perihelion seasons, we determine through photochemical and radiative transfer modeling that both emissions are driven by electron impact dissociation (EID) of CO₂ and resonant scattering of solar photons by atomic C. EID of CO₂ is dominant at the lower latitudes, while resonant scattering by C is dominant at the higher altitudes, with the cross-over point occurring at ∼155 km. The high-altitude C I 156.1 nm dayglow allows us to make the first direct measurement of C densities in the upper Martian atmosphere: at aphelion, (4 . 0 ± 0 . 3) × 10⁵ cm^-3 at ∼165 km altitude, and at perihelion, (4 . 4 ± 0 . 5) × 10⁵ cm^-3 at ∼175 km altitude. These high C densities, while providing support for the addition of CO₂ photodissociation as a major C production channel, suggest that C production rates in photochemical models still have to be higher. Constraints from the dayglow observations also indicate that actual CO₂ + e rates are as much as a factor of 2 compared to photochemical modeling, with implications on slightly higher overall C photochemical escape rates. Contrary to model predictions, we found the altitude of the emission peak at perihelion to be higher than it is at aphelion, and higher for the C I 156.1 nm emission than the C I 165.7 nm emission at the same season. The former seasonal variation can be attributed to low modeled atmospheric temperatures at perihelion.