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Mercury is an end member planet, not only because it is the innermost but also because it is the least oxidized planet of the Solar System. The NASA MESSENGER mission to Mercury provided priceless data on the physical and chemical properties of the planet. It allows evaluating previous models of Mercury’s chemical composition, thermal history, internal structure, as well as models of planetary accretion.
Mercury’s bulk planetary volatile content
The volatile depletion of a planet is usually correlated with the temperature of its formation. Prior to the MESSENGER mission, it was expected that Mercury is the most volatile-depleted planet, due to its close proximity to the Sun. Results from the MESSENGER spacecraft have recently shown that Mercury has a volatile-rich surface, with elevated K/Th and K/U ratios, suggesting a weak depletion of the moderately volatile element K relative to the refractory U and Th. This challenges previous models of planet formation. We investigated whether these elevated ratios could be explained by low U- and Th-contents relative to K in the mantle and crust, due to their early sequestration in Mercury’s core. We found that at very reducing conditions and if Mercury contains sulfides in its interior, K/Th and K/U ratios would be as low as those of Venus and lower than Mars (Boujibar et al. 2019, Am. Min.).
Formation of Mercury’s crust
To date, X-ray spectrometer of NASA MESSENGER spacecraft provided the most precise data on major elemental compositions of Mercury’s surface. Produced maps revealed large volcanic plains and abundant pyroclastic deposits, with important chemical and mineralogical heterogeneities that suggested several stages of differentiation and melting processes. Part of my postdoctoral work at NASA JSC was to investigate the processes leading to the various compositions of Mercury’s surface. This included carrying out experiments of partial melting of enstatite chondrites, up to pressures corresponding to core-mantle boundary. Our results show that the majority of the geochemical provinces of Mercury’s surface can be explained by mixing of two melting products of enstatite chondrites. At the reducing conditions of Mercury’s differentiation, the formation of Mg and Ca-bearing sulfide liquids modifies the chemical composition of silicate melts and produces a large range of magma compositions. This indicates that Mercury’s surface could have been produced by melting of a primitive mantle at various depths, down to the core-mantle region (Boujibar et al. 2025, Icarus).
Variation of K/U and K/Th in terrestrial planets as a function of the heliocentric distance. These ratios are used to quantify the depletion of moderately volatile elements in planets. If Mercury contains sulfides and is very reduced (black square), it would be as depleted or more depleted in volatile elements than Venus and Earth (Boujibar et al. 2019, Am. Min.).
Maps of Mercury showing the pressures and degrees of melting of experimental melts that resemble the most Mercury's surface composition (Boujibar et al. 2025 ).
Internal structure of Mercury that possibly contains a sulfide layer between core and mantle (left).
Sample obtained experimentally at high pressure and temperature using multi-anvil press (right), simulating chemical reactions during core-mantle segregation.
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