Emirates Mars Ultraviolet Spectrometer's (EMUS) Observation of Argon in the Martian Thermosphere

The Emirates Mars Ultraviolet Spectrometer (EMUS) is a far ultraviolet spectrometer on-board the Emirates Mars Mission's (EMM) which arrived at Mars on 9 February 2021. EMUS is designed to observe key neutral species in the Martian thermosphere (100-200 km) and exosphere (>= 200 km). EMUS has observed two neutral argon (Ar) emission lines, Ar I 104.8 nm and Ar I 106.6 nm, in the thermosphere. Our interest in these emissions stem from argon's non-reactiveness, making it a tracer for transport between the upper and lower atmosphere. We report average argon disk brightness measured by EMUS and compare them to measurements from Earth orbiting observatories. For the first time, this work investigates the variability of Ar I 106.6 nm brightness due to emission angles, solar zenith angles, solar longitudes, local times and latitudes. To contrast the behavior of inert argon we compare these results with the photochemically reactive oxygen 135.6 nm.