Spatial structure and relative variability of CO abundance in the Martian thermosphere observed by the Emirates Mars Mission

From its vantage point in orbit, the Emirates Ultraviolet Spectrometer (EMUS) onboard the Emirates Mars Mission (EMM) Hope probe images Mars at extreme- and far-ultraviolet wavelengths extending from approximately 100 to 170 nm. The EMUS CO/CO2 algorithm (where indicates a slant path integral) provides a measure of relative composition variability within the Martian thermosphere by deriving the column abundance of CO above a fixed reference column density (or pressure level) of CO2 from EMUS images. The CO/CO2 algorithm traces its origins to the O/N2 algorithm used at Earth for studying relative variability in O abundance from remote sensing observations (Strickland et al., 1995; Evans et al., 1995). The algorithm uses a lookup table approach by first generating a series of model atmospheres that span the expected range of physically realistic atmospheres. The AURIC model (Strickland et al., 1999; Evans et al., 2015) is then used to calculate column emission rates and spectral radiances as functions of CO/CO2 column density ratio, solar zenith angle, and emission angle for the range of input model atmospheres. When used with EMUS observations, the lookup table provides a unique mapping from intensity ratio to CO/CO2 column density ratio for a given solar zenith angle and emission angle. Intensities from carefully selected bands of the optically allowed CO Fourth Positive Group (4PG) band system (A1 X1+) are used as signatures of CO and CO2. In this presentation we report on the EMUS Level 3 data product CO/CO2, describe the algorithm from which the data are derived, and discuss simulations that were used to test the algorithm. Using EMUS observations for a range of solar and geophysical conditions, we demonstrate that the algorithm produces CO/CO2 values with sufficient fidelity to characterize the spatial structure and relative variability of CO abundance in the Martian thermosphere.