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Constraining the Spatial Distribution of Mercury’s Exospheric Calcium Source

Presentation #116.07 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
Constraining the Spatial Distribution of Mercury’s Exospheric Calcium Source

Sodium, calcium, and magnesium in Mercury’s surface-bounded exosphere were observed on a near daily cadence for the entire four-year MESSENGER mission by the UltraViolet and Visible Spectrometer (UVVS), one of two detectors on the MESSENGER Mercury Atmospheric and Surface Composition Spectrometer (MASCS). UVVS was a scanning grating monochromator and could observe only one emission line at a time along a single line of sight at a time. Spatial information was derived by rotating the spacecraft along the axis perpendicular to the sunshade. Therefore, the observing geometry was very complicated and was unique for each orbit. We have developed a new technique for estimating the emitted flux of neutral atoms from Mercury’s surface by determining which regions of Mercury’s surface UVVS contributed neutrals to each UVVS line of sight and calculating the flux from these regions that produce the radiance measured by UVVS. For calcium, we find that the flux distribution is largely consistent with previous studies such as Burger et al. (2012, 2014) that showed a source centered at the dawn point with a total flux that varies over the course of Mercury’s year. Our new analysis suggests that the location on the surface of the peak also varies with Mercury true anomaly angle, largely lining up with the ram direction and consistent with previous studies suggesting micrometeoroid impact vaporization is the primary source of calcium in Mercury’s exosphere.

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