Though it is the smallest of the official planets and, in fact, smaller than some of the larger moons in Earth's solar system, Mercury does have some notable features. For starters, thought to be composed of almost 70% metallic material, it is the densest solid planetary body in the solar system. Complementing the abundance of metal is an overly large core, weighing in at approximately 42% of the planet's entire volume. Earth's core, by comparison, accounts for only 17% of its volume. Mercury's core is expected to be mostly iron due to the sheer mass of the planet. Finally, Mercury is the only other terrestrial planet, aside from Earth, with a global magnetic field.
This small scorched ball of rock and iron has kept scientists boggled with its weak magnetic field since its discovery by the Mariner 10 probe in 1974. Mercury's magnetic field is a mere one percent as powerful as Earth's, while the planet itself is about five and a half percent as massive. Various scientific models have been at a loss to explain the low strength of the field, though observations of an orbital wobble pointed towards and active, liquid core as a likely culprit.
Researchers as the University of Illinois at Urbana-Champaign have produced a model which was subsequently tested and found to have a high probability of explaining the magnetic field. To replicate conditions in Mercury's core, mixtures of iron and sulfur, in approximation of what scientists speculate actually exists in the planet's core, were heated to various temperatures and pressures. The samples were rapidly quenched to preserve the geological processes that may be taking place and then split and observed with a scanning electron microscope.
The study's lead author, graduate student Bin Chen explains "rapid quenching preserves the sample’s texture, which reveals the separation of the solid and liquid phases, and the sulfur content in each phase. Based on our experimental results, we can infer what is going on in Mercury’s core."
Almost fantastically, their evidence points at snow. As the super heated mixture of iron and sulfur rises towards the planet's crust, it cools. As it does, iron particles form cubic flakes that precipitate back towards the core, pushing more heated iron-sulfur mix towards the outside to cool. The researchers feel this iron "snow" serves to help power convection currents in the core, creating Mercury's weak magnetic field.
NASA's second space craft sent to Mercury, MESSENGER, arrived safely to the planet's vicinity on January 14th of this year and preformed a flyby, mapping approximately 30% of its surface. After another two flybys, planned for later this year and late 2009, the probe will see orbital insertion in March of 2011. Using the data from UI group's tests, scientists may have a better idea of what to have the craft look for as it continues to map the surface and collect data on the field itself.