LAUSR

The unusual bulk composition of planet Mercury, particularly its anonymously large iron (Fe)-fraction of 60–80 wt% that is inferred from the planet's high density (Rabe, 1950; Urey, 1951), has led to a strong interest in Mercury's formation history. It is generally assumed that the proto-Mercury formed by accretion of solar nebula condensates, out of which a metallic core and a silicate mantle segregated under influence of gravity (e.g., Urey, 1951). Planetary formation scenarios that attempt to explain the large Fe-fraction of Abstract Experimental measurements of density by X-ray absorption and of P-wave velocity by ultrasonic techniques of liquid Fe-(<17 wt%) Si-(<4.5 wt%) C alloys at pressures up to 5.8 GPa are presented. These data are used to construct an Fe-Si-C liquid mixing model and to characterize interior structure models of Mercury with liquid outer core composed of Fe-Si-C. The interior structure models are constrained by geodetic measurements of the planet, such as the obliquity and libration of Mercury. The results indicate that S and/or C with concentrations at the wt% level are likely required in Mercury's core to ensure the existence of an inner core with a radius (below ∼1,200 km) that is consistent with reported dynamo simulations for Mercury's magnetic field. Interior structure models with more than 14 wt% Si in the core, estimated for Mercury by assuming an EH chondrite-like bulk composition, are only feasible if the obliquity of Mercury is near the upper limit of observational uncertainties (2.12 arcmin) and the mantle is dense (3.43–3.68 g·cm). Interior structure models with the central obliquity value (2.04 arcmin) and less than 7.5 wt% Si in the core, consistent with estimates of Mercury's core composition from an assumed CB chondrite-like bulk composition, are compatible with 3.15–3.35 g·cm mantle densities and an inner core radius below 1,200 km. Interior structure models with the obliquity of Mercury near the lower observational uncertainty limit (1.96 arcmin) have a low-density mantle (2.88–3.03 g·cm), less than 4 wt% Si in the core, and an inner core radius larger than 1,600 km.