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

Talk presented at AGU Fall Meeting 2021, New Orleans, online on 2021-12-14

Abstract: Experimental 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. The measured density of near-binary Fe-Si alloys are below those measured by Sanloup et al. (2004, GRL, 31(7), L07604) and Tateyama et al. (PCM, 38(10), 801-807), but consistent with data from Yu and Secco (2008, HPR, 28(1), 19-28) and Terasaki et al. (2019, JGR:Planets, 124(8), 2272-2293). The VP measurements confirm the increase of VP with concentration of Si reported by Williams et al. (2015, JGR:Solid Earth, 120(10), 6846-6855) and Terasaki et al.. However, the moderate increase of VP with Si concentration at these pressures is not commensurate with the high dK/dP of about nine to twelve that Williams et al. show is nessesary to meet the (low) density measurements of Fe-Si-Ni alloys at >30 GPa by Morard et al. (2013, EPSL, 373, 169-178). The experimental 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 ~1200 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-3). Interior structure models with the central obliquity value (2.04 arcmin) and less than 7.5wt%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-3 mantle densities and an inner core radius below 1200 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-3), less than 4wt%Si in the core, and an inner core radius larger than 1600 km.

Keyword(s): Mercury, core, Fe-Si-C
Funding: MSCA-IF-EF-ST/845354/MERCURYREFINEMEN