An X-ray scattering technique for characterising Mercury’s carbon enrichments with BepiColombo

Abstract Spatially resolved observations of carbon abundances on Mercury’s surface are of great scientific importance for the origin and evolution of volatile elements and for the planet’s reflectance properties, notably for Mercury’s Low Reflectance Material (LRM) and with broader impacts for constraining Mercury’s history. However, such measurements are extremely challenging, even with BepiColombo’s state-of-the-art instrument suite. To overcome the issues with direct measurement, we have created an entirely new, indirect method for characterising localized carbon enrichments on Mercury with the Mercury Imaging X-ray Spectrometer (MIXS). Elevated abundances of low atomic number elements increase the spectral intensity of scattered, high-energy X-rays (8-10 keV). This provides a proxy for graphite enrichment, which is the currently preferred hypothesis for the dominant darkening phase in the LRM. We report experimental X-ray measurements, including scattered Xâ-rays and XRF, for a library of reference materials and graphite-spiked samples of varying abundances using the MIXS Ground Reference Facility. By modelling the results using established theory and databases of X-ray fundamental parameters, we validate the method and enable reliable extrapolation to future MIXS observations. Using modelled solar flares, fitted to solar X-ray observations from the MESSENGER mission, we estimate that integration times of hundreds of seconds during intense flares can resolve increases in scatter intensity created by various expected carbon abundances. Through detailed analysis we find that it may be possible to place an upper bound on graphite content on localised scales, strengthening the scientific rationale for dedicated ‘stare’ observations of LRM rich regions during BepiColombo’s extended mission.