Experimental constraints on the internal structure of Mercury

Measurements of the surface of Mercury by the MESSENGER spacecraft (NASA) indicate that it is covered with basaltic lavas unusually rich in sulfur (up to 4 wt.% S). Such values are significantly higher than those reported for any basaltic composition on Earth. Understanding the geochemical cycle of sulfur on Mercury and how these lavas are produced is essential to constrain large-scale processes of planetary differentiation due to volcanic activity, crust-mantle formation, and segregation of a Fe-S-Si core. In this project, the primary magmatic history of Mercury will be investigated experimentally and theoretically. Experiments using furnaces and presses will be performed at high temperature (> 1000°C) and for pressure conditions relevant to the Mercury’s crust and mantle (1 bar to 40 kbar). A special design of experimental capsules will be developed to control equilibration under highly reduced conditions similar to those prevailing on Mercury. The key objectives of the project are (1) to precisely constrain the sequence of crystallization of the Mercurian magma ocean and (2) to determine phase equilibria during melting of mantle cumulates. Experimental data and theoretical models produced during this project will allow understanding the processes of Mercury’s differentiation on a global scale as well as determining the likely mantle source of the basalts observed at the surface of the planet. This will be of crucial interest to fully interpret the geochemical and spectral data of MESSENGER, and those that will be acquired in the near future by the BepiColombo spacecraft of the European Space Agency.