The Earth, Mercury and Ganymede represent an exclusive group of terrestrial planetary bodies with present-day magnetic fields generated within their metallic cores. Though Mars, possibly the Moon and some meteorite parent bodies hosted active magnetic field generation in the past. Dynamo action requires sources of thermal and/or compositional buoyancy capable of driving sufficient convective motions. Modeling has shown that thermal buoyancy, the result of core cooling, typically is incapable of driving the necessary convection by itself. On Earth, compositional buoyancy is thought to derive from crystallization of the inner core, which produces a light element enriched fluid at the inner core boundary. In smaller bodies the crystallization behavior and resulting compositional buoyancy may be quite different, depending on pressure and the identity of the light alloying component(s) in the core. The last decade has seen an explosion in knowledge of the physical and thermodynamic properties of candidate core forming materials. These advances have led to the recognition of an array of possible core crystallization sequences, each with potentially important consequences for planetary evolution and magnetic field generation. For example, the melting behavior of iron-sulfur alloys at pressures up to ~40 GPa suggests that iron "snow" would precipitate in shallow regions of the cores of Ganymede, Mars and Mercury, if sulfur is the dominant light alloying element. Similarly, experiments on the iron-carbon system also suggest shallow precipitation of iron at some pressures. While shallow precipitation of iron snow can provide a buoyancy source for core convection, it can also lead to compositional gradients capable of modifying the scale of, or even suppressing, convection. Liquid immiscibility could also have an influence on convection and magnetic field generation in planetary cores that contain more than one light alloying element. The modest pressures and temperatures in the cores of the smaller terrestrial bodies in the Solar System are readily accessible in the laboratory, and allow systematic study of the petrologic behavior of the wide range of possible core systems. We discuss the potential consequences of core petrologic evolution for magnetic field generation and prospects for future progress in constraining the operation of planetary cores.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- 5430 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Interiors;
- 5440 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Magnetic fields and magnetism;
- 5460 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Physical properties of materials