Neutral Composition and Horizontal Variations of the Martian Upper Atmosphere From MAVEN NGIMS

Using data from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS), we investigate the horizontal variations in composition of the Martian upper atmosphere. We focus on ratios of Ar, N₂, O, He, and H₂ to CO₂ and their variation with local time, latitude, season, and temperature. These are the first direct measurements of H₂ in the upper atmosphere, a species important for the delivery of H to this region because H and H₂ escape to space. The NGIMS data nominally cover an altitude range of ∼150-300 km, from near the mesopause to above the exobase. The ratios to CO₂ of He and H₂ are larger toward the poles and on the nightside of the planet, where the atmosphere is colder and downwelling leads to the enrichment of these lighter species. For example, the H₂ to CO₂ ratio, at a CO₂ density of 1 × 10⁸ cm⁻³, is observed to increase by a factor of 2.8 from 52.5°S to 72.5°S and by a factor of 5.5 from 3 p.m. to 3 a.m. around the equator. We observe seasonal variation of the Ar to CO₂ ratio due to deposition and sublimation of CO₂ at the polar ice caps. Finally, we find good agreement between the Ar and N₂ to CO₂ ratios obtained by the Viking Lander Upper Atmospheric Mass Spectrometers and those obtained by NGIMS, and significant disagreement between the N₂ to CO₂ ratios obtained by the Imaging Ultraviolet Spectrograph on MAVEN and those obtained by NGIMS.