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

Abstract

AbstractUsing 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.