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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Discussion papers
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Regular paper 23 Oct 2018

Regular paper | 23 Oct 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Annales Geophysicae (ANGEO).

Mercury’s Sodium Exosphere: An ab initio Calculation to Interpret MASCS/UVVS Observations from MESSENGER

Diana Gamborino, Audrey Vorburger, and Peter Wurz Diana Gamborino et al.
  • Space Research and Planetary Sciences, Physics Institute, University of Bern, 3012 Bern, Switzerland

Abstract. The optical spectroscopy measurements of sodium in Mercury’s exosphere by MESSENGER MASCS/UVVS have been interpreted before with a model employing two exospheric components of different temperatures. Here we use an updated version of the Monte Carlo (MC) exosphere model developed by Wurz and Lammer (2003) to calculate the Na content of the exosphere for the observation conditions. In addition, we compare our results to the ones according to Chamberlain theory. Studying several release mechanisms, we find that close to the surface thermal desorption dominates driven by a surface temperature of 594 K, whereas at higher altitudes micro-meteorite impact vaporization prevails with a characteristic energy of 0.34 eV. From the surface up to 500 km the MC model results agree with the Chamberlain model, and both agree well with the observations. At higher altitudes, the MC model using micro-meteorite impact vaporization explains the observation well. We find that the combination of thermal desorption and micro-meteorite impact vaporization reproduces the observation of the selected day quantitatively over the entire observed altitude range, with the calculations performed based on the prevailing environment and orbit parameters. These findings may help to improve our understanding of the physical conditions at Mercury’s exosphere, as well as to better interpret mass-spectrometry data obtained to date and in future missions, such as BepiColombo.

Diana Gamborino et al.
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Status: final response (author comments only)
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Diana Gamborino et al.
Diana Gamborino et al.
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Short summary
We propose that the temperature of the Na exosphere of Mercury near the sub-solar point is not at high as proposed in previous works. Using a numerical model and the appropriate energy distributions for each release mechanism we can explain observations made by MESSENGER in April 2012. Our results show that close to the surface the dominant release mechanism for Na is evaporation due to the solar irradiation, and at high altitudes the best candidate is the release by micro-meteoroid impacts.
We propose that the temperature of the Na exosphere of Mercury near the sub-solar point is not...