Directly Determining the Source of Mercury’s Exosphere From MESSENGER Data

The UltraViolet and Visible Spectrometer (UVVS), one of two detectors on the MESSENGER Mercury Atmospheric and Surface Spectrometer (MASCS), observed sodium, calcium, and magnesium in Mercury’s surface-bounded exosphere on a near daily cadence for the entire four year MESSENGER mission. UVVS was a scanning grating monochromator and could observe only one emission 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. A full understanding of the neutral distribution in Mercury’s exosphere requires a model that can simulate the UVVS lines of sight for direct comparison of simulated emission with observed radiance.

The previously developed Monte Carlo model used to understand the calcium and sodium components of the exosphere (e.g., Burger et al. 2014; Cassidy et al. 2021) has been rewritten entirely in Python making it faster and more flexible that the previous IDL version. We have also added the capability to do back-modeling of the exosphere to determine what regions on the surface contributed neutrals to each UVVS line of sight. This information can then be used to estimate the flux of neutrals from each point on the surface that UVVS was sensitive to. We present initial results using this method to determine the source flux of calcium finding qualitative agreement with previous studies (Burger et al. 2014) showing a calcium source predominantly located near Mercury dawn (orbital ram direction) consistent with impact vaporization as the primary source mechanism.