Seasonal Variability of Mercury's Sodium Exosphere Deduced From MESSENGER Data and Numerical Simulation. Issue 9 (11th September 2020)

• Record Type: Journal Article
• Authors: Suzuki, Y.; Yoshioka, K.; Murakami, G.; Yoshikawa, I.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 9(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-11
• Subjects: Mercury -- sodium exosphere -- MESSENGER -- seasonal variability -- comet dust streams; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2020JE006472
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: Mercury is valuable to us because we can see the interactions between the planet and its space environment. This research aims to clarify how Mercury's neutral Na exosphere was produced. Data from MErcury Surface, Space ENvironment, GEochemistry, and Ranging/Mercury Atmospheric and Surface Composition Spectrometer (MESSENGER/MASCS) and model calculations that examine possible generation, transportation, and dissipation processes were compared. First, the seasonal variability in the amount of Na exosphere was analyzed for each local time (LT) using MASCS data. Previous research has shown that the amount of Na above LT12 reaches its maximum at the aphelion and that this maximum is recorded only at LT12. Following this result, we constructed a 3‐D Na exosphere model to understand the key seasonal variability processes occurring around LT12. The numerical calculation produced results that were consistent with the MASCS observations regarding the vertical profile and the seasonal variability at LT06 and LT18. However, the peak that occurs around the aphelion at LT12 could not be reproduced. However, the model produced results suggesting that less than 10 8 kg of comet stream dust particles per Mercury year could be the local and short‐term source of Na. Plain Language Summary: Mercury is valuable because we can see the interaction between the planet and its space environment. In this study, the seasonal variability of Mercury's Na exosphere was investigated using optical observations by MESSENGER spacecraft and a 3‐D Monte Carlo model. An analysis of the observations showed that the amount of Na exosphere increased at noon around the aphelion. We first attempted to reproduce this feature with a theoretical model considering four well‐known outgassing processes of the atmosphere. However, when we add comet dust streams to our model as a new outgassing process, we could resolve the inconsistencies of the first model. By considering Na supply to the surface by the impact of dust streams and tuning the parameters more precisely, we could reproduce observations at all local times. Key Points: The seasonal variability of Mercury's Na exosphere was investigated using observations by MESSENGER/MASCS and a 3‐D Monte Carlo model Model with thermal/photon‐stimulated desorption, solar wind sputtering, and micrometeoroid impact vaporization cannot reproduce observations The putative presence of comet dust streams could resolve this inconsistency