MAVEN-based investigation of Martian exobase temperatures: Diurnal and solar cycle variations

Abstract

The exobase is a key concept for understanding atmospheric escape and evolution on Mars. With the aid of neutral density data measured via the Neutral Gas and Ion Mass Spectrometer onboard the Mars Atmosphere and Volatile Evolution spacecraft, we calculated the exobase temperatures of four relatively abundant species on Mars: CO2, O, N2, and CO. Our calculations revealed that the exobase temperatures are highly variable, with median temperatures of 174, 152, 195, and 193 K for CO2, O, N2, and CO, respectively. Moreover, the calculated exobase temperatures for the four species increased systematically with an increase in the exobase altitude or a decrease in the exobase density. Further investigations indicated strong diurnal and solar cycle variations in the exobase temperature: (1) the exobase temperature on the day side was higher than that on the night side, revealing a maximum temperature near the local time of 14 h and a minimum temperature near the local time of 2 h; and (2) the exobase temperatures of the four species increased with the solar extreme ultraviolet (EUV) flux, a feature that was restricted to the day side and absent on the night side, where non-solar energy inputs are likely important. These variations originate from the varying degrees of upper atmospheric heating on Mars in response to the different solar EUV energy inputs.

Keywords

• mars
• atmospheric escape
• exobase