Internal and tectonic evolution of Mercury

Mercuryʹs geological and internal evolution presents an interesting enigma: are there conditions that allow for both apparently limited radial contraction over the last 4 billion years and sufficiently rapid core cooling at present to permit a hydromagnetic dynamo? To address this question, we simulate the coupled thermal, magmatic, and tectonic evolution of Mercury for a range of parameters (e.g., mantle rheology, internal heat production, core sulfur content) in order to outline the set of assumptions most consistent with these two conditions. We find that among the models tested, the only ones strictly consistent with ∼1–2 km of radial contraction since 4 Ga and a modern magnetic field generated by a core dynamo are those with a dry-olivine mantle rheology, heat production provided primarily by Th (negligible U or K), and a bulk core sulfur content >6.5 wt%. However, because of the limited coverage and resolution of Mariner 10 imaging and derived topography, the tectonic history of an entire hemisphere is unknown. The potential for other mechanisms (e.g., long-wavelength lithospheric folds) to accommodate contraction remains untested, limiting the ability to restrict models on the basis of accumulated strain. Furthermore, Mercuryʹs magnetic field may be a consequence of a thermoelectric dynamo or even crustal remanence; neither hypothesis places strong constraints on current heat flux from the core. Spacecraft observations of Mercury are needed to elucidate further the internal structure and evolution of the planet.