Solar Cycle, Seasonal, and Dust-storm-driven Variations of the Mars Upper Atmospheric State and H Escape Rate Derived from the Lyα Emission Observed by NASA’s MAVEN Mission

Abstract

An accurate and systematic quantification of the state variables in the Mars upper atmosphere is important for the study of Mars volatile evolution. In this study, we perform a comprehensive analysis of the Ly α limb scans observed by MAVEN during Martian years 32–35 to quantify the Mars upper atmospheric temperature, the CO 2 density, the H density, and the H thermal escape rate. A previously uncharacterized feature of the proton aurora at Mars is reported, which affects the overall shape of the limb scans without introducing peak intensity structures. Our inversion results excluding such scans demonstrate an unambiguous and systematic consistency of the Mars exobase temperature (∼150–280 K) derived from the dayside Ly α emission with those derived from the CO 2 + airglow emission and those from atmospheric drag measurements, resolving a long-standing discrepancy between those different estimates. The temperature is shown to be highly predictable over different solar cycles and seasons. However, the H density and escape rate can be episodically enhanced by a factor of ∼2–3, likely driven by dust storms that coincided with the enhancements. During the three Martian years, the averaged thermal escape rates between Mars solar longitude 0°–180° and 210°–330° are 0.6 × 10 7 cm −2 s −1 and 9.0 × 10 7 cm −2 s −1 , respectively, a factor of 15 seasonal difference, with a factor of ∼7.5 and ∼2 due to temperature and H density variations, respectively. These temperatures and thermal escape rates derived from the thermospheric Ly α emission are generally lower than those derived from the exospheric Ly α emission in the literature, indicating the presence of a hot H population in the Mars exosphere.