[P-PS04] Mercury Science and Exploration

[PPS04-11] The link between exosphere and surface of Mercury's Mg revealed by the BepiColombo and MESSENGER spacecraft

*Yudai Suzuki¹, Eric Quémerais², Jean-Yves Chaufray², Rozenn Robidel³, Go Murakami¹, François Leblanc², Kazuo Yoshioka⁴, Ichiro Yoshikawa⁴, Oleg Korablev⁵ (1.Japan Aerospace Exploration Agency (JAXA), 2.The Laboratory for Atmospheres, Observations, and Space (LATMOS), 3.Eeropean Space Agency, 4.The University of Tokyo, 5.Russian Academy of Sciences (RAS))

Keywords:Mercury, Exosphere, BepiColombo, MESSENGER, Surface Composition

Mercury’s exosphere is an important target for understanding the dynamics of coupled systems in space environments, tenuous planetary atmospheres, and planetary surfaces. Magnesium (Mg) is especially crucial for establishing methods for estimating the surface chemical composition distribution through observations of the exosphere because its distribution in the exosphere and on the surface are strongly correlated. However, owing to its low radiance, the Hermean Mg exosphere has only been detected by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft. Thus, we have few observation data for areas other than low latitude regions in addition to few detection cases of short-term or sporadic fluctuations, resulting in a poor understanding of ejection and transportation mechanisms of the Mg exosphere.
In this study, we analyzed the distribution of the Hermean Mg exosphere by the Probing of Hermean Exosphere by Ultraviolet Spectroscopy (PHEBUS) onboard the Mercury Planetary Orbiter (MPO) of the BepiColombo mission during its second to fourth Mercury swing-bys (MSBs).
First, we constructed a calibration method including background subtraction and calibration using stellar observations. Mg light curves at three true anomaly angles were obtained, which were in agreement with Chamberlain model and a three-dimensional numerical calculation for MSBs 2 and 3 cases. Comparing the Mg and calcium (Ca) radiances obtained by PHEBUS during MSBs, the exospheric Mg atoms have a lower energy than the exospheric Ca atoms. This is consistent with the lower energy necessary for producing the Mg atoms produced by molecular photodissociation than for Ca atoms. For MSB4 cases, Mg radiance much lower than model calculation was obtained, the cause of which is under discussion.
In this presentation, we summarize the latest results related to Mercury’s Mg exosphere obtained by BepiColombo MPO/PHEBUS during its MSBs and by MESSENGER/MASCS.