Mercury

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Data from the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft, the Mariner 10 spacecraft, and decades of ground-based observations have revealed a wealth of information about the geochemistry of Mercury, the inner most planet in our Solar system. These data indicate that Mercury is an endmember among the terrestrial planets with a core mass fraction of ∼71–78% of the planet's mass, a surface that is low in Fe and rich in S, and a surface that is darkened by 1–4% graphitic carbon that may have been sourced from an ancient primary flotation crust. Mercury has an interior oxygen fugacity (fO2) that is more reduced than the other terrestrial planets. In fact, the interior of Mercury is so reduced, the geochemical behavior of the elements on Mercury deviates from the expected behavior based on studies of Earth, Moon, and Mars. Moreover, Mercury is a volatile-rich planet with moderately volatile element abundances similar to Mars and possibly enriched relative to Earth. The origin of Mercury's endmember status is still an open question, but there are two broad classes of hypotheses for the origin of Mercury, including (1) impact erosion models consisting of either a single giant mantle-stripping impact or series of smaller impacts that preferentially removed silicate material from Mercury without re-accretion; and (2) formation of Mercury from highly reduced and broadly chondritic metal-rich precursor materials that are not well represented by chondritic meteorites. Both of these formation models have highly contrasting implications for the thermochemical evolution of Mercury and its bulk composition. However, from the data available, both models remain viable. Many open questions about Mercury's geochemistry remain unanswered. These questions require additional experimental and modeling work that can be completed today, but they also require the continued exploration of planet Mercury. The European Space Agency's BepiColombo spacecraft will reach orbital insertion around Mercury in late 2025, and it has a suite of geochemical instruments that will gather data from across the surface of Mercury. Looking beyond BepiColombo, a Mercury lander could provide detailed insights into the surface of Mercury that are not possible from orbit, and studies of samples, either through the identification of a meteorite from Mercury or from a Mercury sample return mission would revolutionize our understanding of Mercury's geochemistry.