Implications of Mercury’s core composition on its libration and present-day thermal state

Data from the MErcury Surface, Space ENvironment GEochemistry and Ranging (MESSENGER) spacecraft revealed that Mercury’s surface is volatile-rich and iron depleted. In particular the high sulfur concentration in surface lavas and their low iron content are indicative that Mercury formed under highly reducing conditions. Consequently, its core likely contains a significant amount of silicon along a smaller fraction of sulfur. Additionally, the low surface reflectance and spectral measurements support the presence of substantial amounts of carbon on its surface. Several lines of evidence indicate that Mercury was carbon-saturated early in its evolution and for this reason carbon might be abundant in its core.

Unlike silicon, carbon and sulfur have a strong decreasing effect on the melting temperature of iron and as such, even small amounts of these elements imply a relatively low present-day core liquidus, affecting the inner core radius and magnetic field generation. The partitioning behaviour of light elements between solid inner- and liquid outer impact the density structure of the core and the gravitational coupling strength between the mantle and inner core. As a result, the amplitude of the longitudinal libration of Mercury can be affected by its core composition. Here we study the effect of the core composition on Mercury’s present-day thermal state and assess how its forced and free libration are affected.