Photochemical escape of atomic C, N, and O during the 2018 global dust storm on Mars

Dust storm is an important meteorological phenomenon on Mars. By modifying the structure of the Martian atmosphere and ionosphere, it plays an indispensable role in the Martian photochemistry and atmospheric loss. This study is devoted to e v aluating the effects of the 2018 global dust storm (GDS) on the photochemical escape of atomic C, N, and O on Mars based on multi-instrument measurements made by the Mars Atmosphere and Volatile EvolutioN spacecraft. The data set is divided into the non-dusty and dusty stages, for which the hot atom production rates from a variety of channels are calculated. A one-dimensional Monte Carlo model is then constructed to obtain the escape probability profile for each channel. By combining the above results, we derive the photochemical escape rates, both prior to and during the GDS. Our calculations suggest that the GDS-induced C, N, and O escape is generally reduced by similar to 30-40% relative to the quiet, pre-GDS state, in direct contrast to the well-known result of GDS-induced strong enhancement of atomic H escape. We further propose that the GDS-induced variation of photochemical escape essentially reflects the competition between two effects: the modification of hot atom production (enhancement for photodissociation or reduction for dissociative recombination) driven by the variation of the background atmosphere and the reduction of escape probability due to atmospheric expansion. During the GDS, the latter is usually more effective and responsible for the overall reduction of photochemical escape on Mars.