Iron 'Snow'

New scientific evidence suggests that deep inside the planet Mercury, iron "snow" forms and falls toward the center of the planet, much like snowflakes form in Earth's atmosphere and fall to the ground.
The movement of this iron snow could be responsible for Mercury's mysterious magnetic field, say researchers from the University of Illinois and Case Western Reserve University.
Mercury is the innermost planet in our solar system and, other than Earth, the only terrestrial planet that possesses a global magnetic field. Discovered in the 1970s by NASA's Mariner 10 spacecraft, Mercury's magnetic field is about 100 times weaker than Earth's. Most models cannot account for such a weak magnetic field.
Made mostly of iron, Mercury's core is also thought to contain sulfur, which lowers the melting point of iron and plays an important role in producing the planet's magnetic field.
"Recent Earth-based radar measurements of Mercury's rotation revealed a slight rocking motion that implied the planet's core is at least partially molten," said Illinois graduate student Bin Chen, the paper's lead author. "But, in the absence of seismological data from the planet, we know very little about its core."

Full article: http://www.sciencedaily.com/releases/2008/05/080507110712.htm