Carbon Photochemical Escape Rates from the Modern Mars Atmosphere

Abstract:

We provide a comprehensive update of photochemical escape rates of atomic carbon from the present-day Martian atmosphere using the one-dimensional photochemical model from Lo et al. [2020] and the Monte Carlo escape model from Lillis et al. [2017]. The photochemical model incorporates new results relevant to carbon photochemistry at Mars, including new cross sections for photodissociation of CO₂ into C and O₂ [Lu et al., 2014] and electron impact dissociation of CO [Ajello et al, 2019]. We find the newly included channel of CO₂ photodissociation to be the largest contributor to C escape. CO photodissociation and CO⁺ dissociative recombination, which have been discussed extensively in the literature, also show up as significant sources of hot C atoms. Electron impact dissociation of CO₂ and photoionization of CO also show up as important channels. Overall, modern escape rates vary over 4--15 x 10²³ s^-1, with an increase of 70% at perihelion compared to aphelion, and a much larger increase of 135% at solar maximum compared to solar minimum. When multiplied by 3.6 billion years, this modern escape rate gives an integrated loss of a mere 1.8 millibars of CO₂, significantly lower than the hundreds of millibars of loss suggested by the geological record. This suggests that photochemical escape rates of C in the past could have been as much as 2 orders of magnitude higher than that of today, due to the significantly different atmospheric and solar conditions. Our model provides the theoretical foundation for future investigations into how exactly these different conditions could result in the required higher loss rates.