Detection of O II 247 nm Airglow at Mars by MAVEN/IUVS

Emission of O II (²P°→⁴S°) 247 nm photons by O⁺ ions is produced following ionization of atomic oxygen in planetary atmospheres. This ultraviolet airglow is a forbidden radiative transition, and only emitted at the highest reaches of the upper atmosphere where collisional quenching is infrequent (> 5 seconds). It has previously been studied at Earth for its important role in photochemistry, as a source of the non-thermal oxygen geocorona, and for making retrievals of neutral atomic oxygen density. While this emission is also expected at Mars, detection has been elusive because it is relatively dim and the data reduction requires careful attention to the spectral blending with the bright CO Cameron band system. Here we report on the detection of O II 247 nm in the airglow of Mars, as measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) Imaging Ultraviolet Spectrograph (IUVS) instrument. This represents the first observation of this emission feature at a planet other than Earth. Implications for our understanding of Martian thermospheric photochemistry and non-thermal oxygen corona are discussed.