The planet Mercury has a very weak magnetic field, equivalent to 1% of the Earth’s magnetic field in intensity. The current state of knowledge, according to which this planet should no longer emit a magnetic field, struggles to explain this particularity. Researchers from the Magmas and Volcanoes Laboratory (CNRS – IRD – University of Clermont Auvergne) have been interested in this “singularity” and have worked on the thermal properties, in particular the heat conduction, of the rocks likely to make up the Hermean mantle. Their research has shown a link between the presence and intensity of the magnetic field and the internal physical state of the planet (solid, liquid or partially molten). These results were published in the journal Physics of the Earth and Planetary Interiors in March 2021.
Abstract: Remanent magnetization and active magnetic fields have been detected for several telluric planetary bodies in the solar system (Earth, Mercury, Moon, Mars) suggesting the presence of core dynamos active at the early stages of the planet formation and variable lifetimes. Among the factors controlling the possibility of core dynamos generation, the dynamics of the surrounding silicate mantle and its associated thermal properties are crucial. The mantle governs the heat evacuation from the core and as a consequence the likeliness of an early thermally driven dynamo. In the case of planets with a thick mantle (associated with supercritical Rayleigh numbers), the core heat is efficiently removed by mantle convection and early thermally-driven dynamos are likely. At the opposite, planets with a thin mantle (associated with subcritical Rayleigh numbers) might evacuate their inner heat by diffusion only, making early thermally-driven dynamos difficult. Within the Solar System, Mercury is a potential example of such a regime. Its small mantle thickness over the planet radius ratio might be inherent to its small orbital semi-axis and hence, might be ubiquitous among the terrestrial objects formed close to their star.
To constrain the likeliness of a thermally driven dynamo on “Mercury-like” planets (i.e. with large Rc/R), we present new thermal diffusivity measurements of various solid, glassy and molten samples. We applied the Angstrom method on cylindrical samples during multi-anvil apparatus experiments at pressures of 2 GPa and temperatures up to 1700 K. Thermal diffusivities and conductivities were estimated for solid and partially molten peridotites, with various melt fractions, and for basaltic and rhyolitic glasses and melts. Our study demonstrates that melts have similar thermal properties despite a broad range of composition investigated. The melts reveal much lower thermal conductivities than the solids with almost an order of magnitude of decrease: 1.70 (±0.19) to 2.29 (±0.26) W/m/K against 0.18 (±0.01) to 0.41 (±0.03) W/m/K for peridotites at high temperatures and various melts respectively. Partially molten samples lie in between and several predictive laws are proposed as a function of the melt fraction and solid/melt texture.
Using our results into forward calculations of heat fluxes for dynamo generation for Mercury-like planets, we quantify the effect of mantle melting on the occurrence of thermally driven dynamos. The presence of a mushy mantle and partial melting could significantly reduce the ability of the mantle to evacuate the heat from the core and can prevent, shut or affect the presence of a planetary magnetic field. The buoyancy and fate of molten material in such bodies can thus influence the magnetic history of the planet. Future observations of Mercury-like planets accreted near their star and the detections of their magnetic signatures could provide constraints on their inner state and partial melting histories.