The exobase is defined as the interface between the strongly collisional and the collisionless parts of an atmosphere. Although in reality the exobase is a transition region of finite depth, it is conventionally defined as the boundary above which an upwardly ejected neutral particle makes one collision at higher altitudes. Such an idealized definition is of practical use and serves as a good tracer of the overall size of an atmosphere as it expands and contracts under the influences of both external and internal sources. Knowledge of the atmospheric properties near the exobase is crucial to first-order estimates of atmospheric escape rates on terrestrial planets. Since its arrival at Mars on 21 September 2014, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has provided comprehensive maps of the Martian upper atmosphere under a variety of conditions. This allows, for the first time, a thorough investigation of the variations of the exobase altitude on this red planet. In this study, we use the N2 density measurements accumulated by MAVEN’s Neutral Gas and Ion Mass Spectrometer from October 2014 to November 2018 to determine the exobase altitudes for a large number of MAVEN orbits. Our analysis reveals clearly the variations of exobase altitude with local time and solar extreme ultraviolet (EUV) flux, as well as tentative evidence for the impact of global dust storms. These observations are indicative of thermal expansion of the Martian upper atmosphere, driven either externally by solar EUV energy deposition or internally by global dust storms.
In recent studies of the Martian atmosphere, strong diurnal variation in the dust was discovered in the southern hemisphere during major dust storms, which provides strong evidence that the commonly recognized meridional transport process is driven by thermal tides. This process, when coupled with deep convection, could be an important part of the short-term atmospheric dynamics of water escape. However, the potential of this process to alter the horizontal distribution of moist air has not been systematically investigated. In this work, we conducted pre-research on the horizontal transport of water vapor associated with the migrating diurnal tide (DW1) at 50 Pa in the upper troposphere during major dust storms based on the Mars Climate Database (MCD) 5.3, a state-of-the-art database for Martian atmospheric research that has been validated as simulating the relevant short-period atmospheric dynamics well. We found westward-propagating diurnal patterns in the global water vapor front during nearly all the major dust storms from Martian years (MYs) 24 to 32. Statistical and correlation analyses showed that the diurnal transport of water vapor during global and A-season regional dust storms is dominated by the DW1. The effect of the tidal transport of water vapor varies with the types of dust storms in different seasons. During regional dust storms, the tidal transport induces only limited diurnal motion of the water vapor. However, the horizontal tidal wind tends to increase the abundance of daytime water vapor at mid- to low latitudes during the MY 28 southern summer global dust storm while decreasing it during the MY 25 southern spring global dust storm. The tidal transport process during these two global dust storms can induce opposite effects on water escape.