Knibbe, Jurrien ; Rivoldini, Attilio ; Luginbuhl, Stefanie ; Namur, Olivier ; Charlier, Bernard ; Mezouar, Mohamed ; Sifre, David ; Berndt, Jasper ; Kono, Yoshio ; Neuville, Daniel ; van Westrenen, Wim ; Van Hoolst, Tim

published in Journal of Geophysical Research: Planets, 126 issue 1

Abstract: Measurements of density by X-ray absorption and of P-wave velocity (VP) by ultrasonic techniques of liquid Fe-(<17wt%)Si-(<4.5wt%)C alloys at pressures up to 5.8 GPa are presented. They are used to develop a ternary Fe-Si-C mixing model, which is implemented in interior structure models of Mercury to characterize the Fe-Si-C liquid outer core. The interior structure models indicate that, unless Mercury’s mantle is dense and the core extremely hot, Mercury can only accommodate an inner core with a radius below 1200 km, consistent with reported dynamo simulations for Mercury’s magnetic field, if S and/or C are present in Mercury’s core at wt% level. Interior structure models with >14wt%Si in the core, inferred for Mercury by metal-silicate partitioning of an EH chondrite like bulk composition, are only possible if the obliquity of Mercury’s surface is near the upper limit of observational uncertainties (2.04±0.08 arcmin) and the mantle density is high (3.43–3.68 g·cm-3). Interior structure models calculated with the central obliquity value (2.04 arcmin) and with <7.5wt%Si in the core, which is within the range inferred from metal-silicate partitioning of CB chondrites, have mantle densities of 3.15–3.35 g·cm-3 and facilitate an inner core radius below 1200 km. Interior structure models with the obliquity of Mercury’s surface near the lower limit (1.96 arcmin) of observational uncertainty margins are only compatible with a low-density mantle (2.88 – 3.03 g·cm-3), <4wt%Si in the core, and >1600 km inner core radius. Implications of C and/or S in Mercury’s outer core for dynamo generation are briefly discussed.

Keyword(s): Mercury ; density ; iron ; silicon ; carbon ; High-pressure
DOI: 10.1029/2020JE006651
Funding: MERCURYREFINEMENT/MERCURYREFINEMENT/MERCURYREFINEMENT