Mercury in the Morning

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The planet Mercury -- the planet closest to the Sun -- is just peeking into view in the east at dawn the next few days. It looks like a fairly bright star. It's so low in the sky, though, that you need a clear horizon to spot it, and binoculars wouldn't hurt.

Mercury is a bit of a puzzle. It has a big core that's made mainly of iron, so it's quite dense. Because Mercury is so small, the core long ago should've cooled enough to form a solid ball. Yet the planet generates a weak magnetic field, hinting that the core is still at least partially molten.

The solution to this puzzle may involve an iron "snow" deep within the core.

The iron in the core is probably mixed with sulfur, which has a lower melting temperature than iron. Recent models suggest that the sulfur may have kept the outer part of the core from solidifying -- it's still a hot, thick liquid.

As this mixture cools, though, the iron "freezes" before the sulfur does. Small bits of solid iron fall toward the center of the planet. This creates convection currents -- like a pot of boiling water. The motion is enough to create a "dynamo" effect. Like a generator, it produces electrical currents, which in turn create a magnetic field around the planet.

Observations earlier this year by the Messenger spacecraft seem to support that idea. But Messenger will provide much better readings of what's going on inside Mercury when it enters orbit around the planet in 2011.

Script by Damond Benningfield, Copyright 2008

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