The liquid outer core of Mercury is fundamental to the sustainability of its dynamo, which depends strongly on the composition and related physical properties of the iron alloy. Core composition is influenced by inner core crystallization and interactions with the adjacent reduced mantle. Chemical transport between core and mantle analogs was investigated using impedance spectroscopy at 5 GPa and up to 1973 K and electron microscopy. Results support an outer core that contains significant amounts of alloying agents (>~20 at.% Si, Ni, C, S). Bulk diffusion coefficients across the silicate-core interface are 1-5.10-8 m2/s at 1973 K. Electrical resistivity of Fe-Ni-Si-C/S liquids was measured at 5 GPa and up to 2123 K to estimate the thermal conductivity of the outer core, which ranges from 17 to 31 W/m.K. These low thermal conductivity values, compared to Fe-Si liquids, could increase the power available to the dynamo during core cooling. An Fe-Ni-Si-C/S outer core could enhance dynamo efficiency until carbides and Fe-Ni crystallize. Diamond is not expected in Mercury's core.

Support: the Carnegie Endowment.

The thermal state and chemistry of the metallic core of terrestrial planets and moons governs their evolution and in particular, the intrinsic magnetic field. The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet’s internal structure are consistent with the presence of a FeS layer overlying a silicon-bearing core. The planet is characterised by unusual chemical characteristics and a weak magnetic field generated in a large metallic core, and its early evolution was also marked by the presence of a magnetic field, widespread volcanism, and global contraction. 

In Pommier et al. (2019), we performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si or Fe-Si-S core is at least 1 log unit at pressure and temperature conditions relevant to Mercury’s interior. If present, a thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if the temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and might impact the generation and sustainability of a magnetic field.  To test the latter, modeling the planet's evolution is required.

Therefore, in Davies et al. (2024), we developed a parameterized model of coupled core-mantle thermal and magnetic evolution considering a layered Fe-Si(-S) core structure with chemical and physical properties of the mantle and the core based on previous laboratory studies. We seek successful solutions that are consistent with observations of Mercury’s long-lived dynamo, total global contraction, present-day crustal thickness, and present-day interior structure. Successful solutions have a mantle reference viscosity > 10^21 Pa s, a silicon concentration in the core > 13 wt.%, a present inner core radius of ∼ 1000 − 1200 km and a thermally stable layer ∼ 500 − 800 km thick below the core-mantle boundary. Our results show that if present, a molten FeS layer atop the core has minimal effect on Mercury’s long-term thermal and magnetic evolution. Predictions from our models can be tested with upcoming Bepi-Colombo observations.

Support: NSF CAREER, NSF NSF-COMPRES IV EOID, and NSF-NERC grant.

Alkaline earth sulfides could be abundant in Mercury’s mantle, and knowledge of their melting and transport properties, such as electrical conductivity, is needed to study the inner structure. Defining the conductivity of the interior is a scientific objective of the ESA-JAXA BepiColombo mission. We conducted electrical experiments at pressures in the range 2 – 5 GPa and at temperatures up to ~2400 K on analogs of natural sulfides, i.e., Ca1-xMgxS with minor impurities (CaS, Ca0.8Mg0.2S, Ca0.6Mg0.4S, Ca0.4Mg0.6S, Ca0.2Mg0.8S, and MgS).

Electrical conductivity increases nonuniformly with temperature, with no systematic dependence on cation composition. At relatively low temperatures (near 1100 K, the conductivities span a wide range, whereas at higher temperatures the values converge within ~0.5 – 7 S/m at 1800 K and 5 GPa. The conductivity trends are complex, and likely reflect contributions from divalent cations, alkali metal and carbon impurities, which would similarly contribute to the conductivity of Mercury's crust and mantle. Melting is identified by a sharp increase in conductivity between ~1850 and 2100 K at 5 GPa. These transition temperatures are consistent with the presence of impurities. 

Using electrical studies on relevant silicate minerals and petrological observations, we developed electrical conductivity-depth profiles of Mercury's mantle. Depending on the interconnectivity of the sulfide phase, the conductivity at the base of the mantle containing 8 vol.% sulfide ranges from ~0.2 to > 8 S/m. Our results can be tested with future observations from the ESA-JAXA BepiColombo mission.

Support: the Carnegie Endowment.

 Thermal profile (left) and corresponding electrical conductivity-pressure profiles (middle and right) across the silicate portion of Mercury. Left: Temperature from Davies et al. (2024). Middle: Effect of sulfide content on the bulk conductivity of a silicate mantle made of 70 vol.% olivine (forsterite) and 30 vol.% orthopyroxene (enstatite). The conductivity of the sulfide phase is from this study (Mg0.8Ca0.2S), and the bulk conductivity is calculated using the geometric mean. Right: Effect of sulfide interconnectivity on the bulk conductivity of the silicate mantle. 8 vol.% sulfide is considered, and bulk conductivity is calculated using the modified Archie's law. m corresponds to the cementation exponent. A value of 1 for cementation exponent m corresponds to an interconnected sulfide phase.