Mercury is the bowling ball of planets. It's the densest planet of all, and it has the largest metallic core. In fact, the core probably accounts for about 75 percent of the planet's diameter -- more than twice the ratio of Earth's core.
Planetary scientists aren't sure why that's the case, but they hope to find out soon. A spacecraft called Messenger is scheduled to enter orbit around Mercury on Friday. Its instruments will map the planet's entire surface. They'll also measure Mercury's magnetic field, and chart the composition of its surface -- and that's the key to understanding the planet's structure.
There are three leading ideas to explain the unusual heft of Mercury's core. One says it was born that way. Another says that Mercury was born with thicker outer layers of lightweight rock, but they were boiled away by the hot Sun. And a third says the rock layers were blasted away by a collision with a giant asteroid.
Each of these explanations would leave its own unique chemical fingerprint in the rocks at Mercury's surface. So by measuring the composition of those rocks, Messenger should go a long way toward explaining the birth and evolution of this heavy, dense little planet.
Look for Mercury quite low in the west shortly after sunset. It looks like a fairly bright star. It's to the upper right of much brighter Jupiter, so it's pretty easy to pluck from the fading evening twilight.
Script by Damond Benningfield, Copyright 2011