The relation between the surface composition anomaly and the
distribution of the exosphere of Mercury
Abstract
In celestial bodies with collisionless atmospheres, such as Mercury, the
spatial distribution of the exosphere is expected to reflect the surface
composition. In this study, we discuss whether the distribution of Mg,
Ca, and Na, the primary exospheric components on Mercury, have
exosphere-surface correlation (ESC) by analyzing the observation data of
the Mercury Atmospheric and Surface Composition Spectrometer (MASCS)
onboard the MErcury Surface, Space ENvironment, GEochemistry, and
Ranging (MESSENGER) spacecraft. As a result, it was found that Mg has
strong ESC, Ca has weak ESC and Na has little ESC. The Monte Carlo
simulations of trajectory in the exosphere show that the weak ESC of Ca
is due to the relatively large solar radiation acceleration. Na has ESC
only in high-temperature regions around 0°E. This can be explained well
by considering that the weakly physisorbed Na layer on the surface is
depleted under high temperature and that the distribution of strongly
chemisorbed Na atoms is reflected in the exosphere. Based on these
results, the conditions for components with ESC in celestial bodies with
thin atmospheres include low volatility and little solar radiation
acceleration.