The O( 1 S) 297.2-nm Dayglow Emission: A Tracer of CO 2 Density Variations in the Martian Lower Thermosphere

Abstract

The O( 1 S) metastable atoms can radiatively relax by emitting airglow at 557.7 and 297.2 nm. The latter one has been observed with the Imaging Ultraviolet Spectrograph onboard the Mars Atmosphere and Volatile Evolution Mars orbiter since 2014. Limb profiles of the 297.2-nm dayglow have been collected near periapsis with a spatial resolution of 5 km or less. They show a double-peak structure that was previously predicted but never observed during earlier Mars missions. The production of both 297.2-nm layers is dominated by photodissociation of CO 2 . Their altitude and brightness is variable with season and latitude, reflecting changes in the total column of CO 2 present in the lower thermosphere. Since the lower emission peak near 85 km is solely produced by photodissociation, its peak is an indicator of the unit optical depth pressure level and the overlying CO 2 column density. Its intensity is directly controlled by the Lyman-α solar flux reaching the Martian upper atmosphere. We take advantage of the Lyman-α flux measurements of the solar Extreme Ultraviolet Monitor instrument onboard Mars Atmosphere and Volatile Evolution to model the observed OI 297.2-nm limb profiles. For this, we combine photodissociation sources with chemical processes and photoelectron impact excitation. To determine the relative importance of the excitation processes, we apply the model to the atmospheric structure measured by the Viking 1 lander before applying it to a model atmosphere. We find very good agreement with the lower peak structure and intensity if the CO 2 density provided by the Mars Climate Database is scaled down by a factor between 0.50 and 0.66. We also determine that the previously uncertain quantum yield for production of O( 1 S) atoms by photodissociation of CO 2 at Lyman-α wavelength is about 8%.