X-ray Spectroscopy at Mercury with the Mercury Imaging X-ray Spectrometer

In December 2025, the joint ESA and JAXA BepiColombo spacecraft will enter orbit around Mercury to begin a two-year mission. Among the 16 instruments aboard BepiColombo is the Mercury Imaging X-ray Spectrometer (MIXS), which offers high-resolution (~10 km per pixel) imaging capabilities for low-energy (0-10 keV) X-ray photons. This information will be used to determine the elemental composition of Mercury’s surface.

The novel imaging capability, requires a new data analysis method to help maximise the instrument’s science return. In order to prepare for this, a ground-reference facility (GREF) has been built and the first tests of its performance are reported here, demonstrating that this facility is ready to enable further characterisation and calibration of the instrument and data analysis methods in the coming years.

This thesis presents the initial work done to develop and test a new “fundamental parameters” model for X-ray fluorescence. Testing the model against laboratory data from the GREF revealed the next steps in the development of both the model and the ground testing needed to demonstrate that it will meet the scientific goals of MIXS.

MIXS requires solar flares for high-resolution imaging, therefore targets of particular scientific interest need to be prioritised, to avoid valuable data being deleted onboard the spacecraft, due to mission-level telemetry constraints. Prioritisation began with cataloguing Mercurian complex craters, revealing a correlation between peak-ring basins and the high-magnesium terrane (HMT), implying the basins may be responsible for the elevated magnesium/silicon ratio. Investigating the correlation determined a list of fifteen targets that MIXS should prioritise, in order to demonstrate whether the correlation is indicative of causality or not. MIXS will determine the composition of basin features, improving our understanding of the origin of the HMT, a feature on Mercury’s surface that is the source of much debate in the community.