A UK-built X-ray spectrometer will map Mercury’s entire surface for the first time, revealing the planet’s composition in unprecedented detail. The BepiColombo mission, arriving at Mercury in spring 2026, carries MIXS—an X-ray instrument that will produce a global compositional map at 10-kilometre resolution. This fills a critical gap: existing Mercury data lack the global coverage and high resolution needed to test hypotheses about the planet’s origins, its unusual carbon-rich magma ocean, and how volatile elements moved across its surface. By night, MIXS will detect X-rays from charged particles hitting the surface, tracking how Mercury’s magnetosphere—strongly shaped by the solar wind—couples with the planet’s interior. This will reveal how electrical conductivity changes with depth, testing how Mercury differentiated from metal-rich planetesimals into its current core, mantle, and crust. This is fundamental planetary science with no immediate practical application. But understanding how a metal-rich world formed and evolved in the inner Solar System sharpens models of planetary formation generally—models that underpin everything from interpreting exoplanet data to assessing Earth’s own deep structure. Past fundamental research on Mercury’s odd composition, for instance, reshaped theories of how terrestrial planets acquire their volatile elements.
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Our objectives are to determine the Origins, Evolution and Interactions of Planet Mercury, by testing new and existing hypotheses relating to its constituent planetesimals, magmatism leading to fractionated igneous rock types, the potential role of a carbon-rich magma ocean, and the processes associated with volatile element mobilisation. These objectives are also firmly linked to our aim of providing a new understanding of Mercury’s unique magnetosphere-surface coupling and interior conductivity. The challenge that we have faced with existing Mercury datasets is a lack of global and high-resolution data. These science questions can now be addressed with the ESA-JAXA BepiColombo mission which will begin science operations at Mercury from Spring 2026. The X-ray spectrometer MIXS that we lead will provide both the first global compositional map of Mercury’s surface, and also for the first time, 10 km spatial resolution in its telescope channel, allowing an unprecedented compositional characterisation of geological landforms, e.g., lava flows, volatile-rich hollows, explosive volcanic vents, and crater uplifts from the interior. These hold the key to determining the origin and evolution of the planet. By night MIXS will be sensitive to the direct interaction between Mercury’s magnetosphere, strongly driven by the solar wind, and the surface. This is equally important because resulting nightside X-ray emissions will allow us to track the precipitation of charged particles onto Mercury’s surface and to characterise the coupling between the external magnetic field and the planet’s interior. Ultimately, we will be able to establish how electrical conductivity varies with depth, and test how this relates to the differentiation of Mercury from metal-rich planetesimal precursors into its current core, mantle, and crustal compositions. Integrated with our science aims, we will perform laboratory experiments and X-ray model validation on analogue materials in our Ground Reference Facility. This will benefit our project, and ultimately the wider scientific community, by providing a rigorously determined, quantified compositional planetary dataset from the X-ray fluorescence and electron impact processes detected by MIXS. Our research will be collaborative, for instance we will use complementary mineralogical data from Bepi’s MERTIS thermal-infrared mapping instrument to enhance our interpretation of the compositional data of geological landforms and the geochemical terranes that we will define. We will deliver our objectives through a set of 14 interlinked work packages over 5 years from 2025, led by a team of researchers who are experts in the variety of instrumentation and planetary science topics that this project requires.
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