Numerical Prediction of Changes in Atmospheric Chemical Compositions During a Solar Energetic Particle Event on Mars

Precipitation of solar energetic particles (SEPs) into planetary atmospheres causes changes in atmospheric chemical composition through ionization, dissociation, and excitation of atmospheric molecules. In contrast to the terrestrial atmosphere, where depletion of ozone in the polar mesosphere has been studied by observations and models for decades, there have been no studies on the effects of SEPs on the neutral chemical composition of Mars' present-day atmosphere. This study provided the first estimate of the impacts of SEPs on neutral chemical composition in the present-day Martian atmosphere coupling a Monte Carlo model and a one-dimensional photochemical model. Our results showed that ozone density in the Martian atmosphere might decrease in the altitude range of 20-60 km with a factor 10 maximum enhancement occurring at 40 km during a Halloween-class SEP event due to an enhanced concentration of HOx. The depletion of ozone occurred in the altitude range of 20-60 km, corresponding to the penetration of protons with an energy range of 4.6-46 MeV. Such ozone depletion should be detected by Trace Gas Orbiter/Nadir and Occultation for MArs Discovery during intense SEP event. A 75% depletion of the ozone density at 40 km can be expected during SEP events occurring once in every 1 year. Therefore, ozone concentration in the Martian atmosphere might sufficiently decrease during a SEP event as on Earth, but through different chemical pathways driven by CO2 ionization and CO recombination catalytic cycle. The solar energetic particle (SEP) effects on the neutral chemical composition in the Martian atmosphere are investigated by numerical simulationsOzone depletion due to enhanced HOx during a large SEP event could be detected by Trace Gas Orbiter/Nadir and Occultation for MArs DiscoveryChemical pathways leading to ozone depletion on Mars differ from those on Earth