Mars ultra-violet dayglow from NOMAD/UVIS

Abstract

The UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer onboard the ExoMars Trace Gas Orbiter performs limb observations of the dayside of the Mars atmosphere in both the visible and the ultraviolet domains since April 2019. The recently discovered visible emissions of the oxygen green line (557.7 nm) and the oxygen red line doublet (630.0 and 636.4 nm) have already been investigated by Soret et al. (2022). The variation of their peak brightness and altitudes have been studied over seasons and compared to photochemical model simulations. In this work, we present the ultraviolet dayglow counterpart of this study. The [OI] 297.2 nm emission that, like the oxygen green line, originates from the O(1S) state is analyzed. The [OI] 297.2 nm has already been extensively studied with previous missions but it is the first time that both the 297.2 and 557.7 nm can be observed simultaneously by the same instrument. The correlation of those emissions is shown and the ratio I(557.7 nm)/I(297.2 nm) is established with NOMAD/UVIS concurrent measurements. The CO Cameron bands, that have not been studied from the UVIS observations so far, are also presented. Finally, the CO2+ Fox-Duffendack-Barker (FDB) system, that has never been observed on Mars since Mariner, is also investigated. We present averaged limb profiles of all these ultraviolet emissions, their seasonal variations and ratios of their brightness. NOMAD/UVIS spectrometer is the first instrument that allows studying these UV emissions simultaneously that can, thus, be directly compared