Mars Upper Atmospheric Temperature and Atomic Oxygen Density Derived from the O i 130.4 nm Emission Observed by NASA’s MAVEN Mission

Abstract

The abundance of atomic oxygen is a major unknown for the Mars upper atmosphere, and systematic measurements of this key species is a primary objective of NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission. Moreover, the Martian dayside temperature structure and its variability has been a subject of considerable debate in the past. In this study, an inversion analysis of the periapsis limb scans of the O i 130.4 nm emission observed by MAVEN is performed to quantify the neutral temperature and atomic oxygen abundance in the Mars upper atmosphere. Our analysis yields unprecedented resolution of the spatial variation of Mars atomic oxygen, showing that its exobase density decreases with the solar zenith angle (SZA) from ∼0° to 60° by about an order of magnitude, from ∼10 8 to ∼10 7 cm −3 near perihelion (2015 April) and from ∼10 7 to ∼10 6 cm −3 near aphelion (2017 November). For a given SZA the exobase densities near perihelion are about an order of magnitude higher than those near aphelion. The [O]/[CO 2 ] mixing ratio at 130 km altitude can vary from ∼0.1% to 4.6%, which also decreases with increasing SZA. Moreover, it is shown that the Mars exobase temperature does not vary significantly with SZA from ∼0° to ∼60°, which is estimated to be ∼239.2 ± 27.6 K near perihelion and ∼162.4 ± 19.9 K near aphelion, with a difference of 76.8 K between these two seasonal extremes.