MESSENGER Observations of Sputtering in Mercury's Exosphere

Abstract:

MESSENGER provided new insights into the magnetosphere and neutral exosphere with measurements from its Fast Imaging Plasma Spectrometer (FIPS) and the Mercury Atmospheric and Surface Composition Spectrometer UV spectrometer (MASCS/UVVS). Although FIPS routinely observed large variability in solar wind protons and ions (e.g. Na+) on short times scales. Variability observed in the UVVS neutral observations, by contrast, is dominated by a repeatable seasonal and local time pattern with no large episodic variability. To untangle this discrepancy, we conducted a coordinated search to find evidence of episodic variability in the neutral exosphere (using MASCS/UVVS data) in order to correlate it with the state of the magnetosphere (using FIPS data). We modeled UVVS observations using with the appropriate geometry and line of sight to reflect the actual instrument pointing and spacecraft location (see attached figure, left panel). When comparing UVVS data with model scenarios from previously established impactor sources (for calcium and magnesium), observations show excess emission above the northern cusp region. See attached figure, right panel. We conducted a series of model runs to determine the location and source needed to reproduce the UVVS data. The resulting source is much larger than expected for solar wind sputtering. We will discuss the resulting implications for space weathering at Mercury.