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ISSN  2096-3955

CN  10-1502/P

Citation: Fu, M. H., Cui, J., Wu, X. S., Wu, Z. P., and Li, J. (2020). The variations of the Martian exobase altitude. Earth Planet. Phys., 4(1), 4–10..

2020, 4(1): 4-10. doi: 10.26464/epp2020010


The variations of the Martian exobase altitude


Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China


School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, China


School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai Guangdong 519082, China


Center for Excellence in Comparative Planetology, Chinese Academy of Sciences, Hefei 230026, China

Corresponding author: Jun Cui,

Received Date: 2019-09-27
Web Publishing Date: 2019-12-11

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.

Key words: Mars, Exobase, MAVEN

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The variations of the Martian exobase altitude

MengHao Fu, Jun Cui, XiaoShu Wu, ZhaoPeng Wu, Jing Li