But now scientists have sent a ship called MESSENGER racing past, and "it was not the planet we expected," says the principal investigator, Sean Solomon. "It was not the Moon."
The Johns Hopkins Applied Physics Lab, which is running the mission for NASA, has been posting material HERE. If you look at most of the pictures, you'll have a hard time telling them apart from Apollo Moon shots.
But Mercury is the only inner planet, other than ours, that has a magnetosphere -- something essential to life on Earth. It's the Earth's magnetic field that protects us from cosmic radiation, stuff that would make life here impossible.
Click HERE for a very large version of the picture above. There are differences the scientists point out. For instance, the Moon has large, dark areas, known as maria, believed to be ancient depressions that were filled in by volcanic material from below. Mercury seems to have the opposite -- there's a large circular feature, called the Caloris Basin, that's actually higher in elevation than what surrounds it.
There were deep furrow-like cuts in various places on the surface, possibly steep cliffs. And one odd spot, nicknamed "The Spider" because it looked like one, had deep troughs radiating out from the center of a large basin called Caloris. Cracks from when magma welled up from beneath? Damage from the formation of a crater? No saying yet.
MESSENGER will settle into orbit around Mercury in 2011, after a long, circuitous journey that trades length for fuel savings. (NASA has wised up over the years about cost; this mission isn't cheap, but it was done on a relatively tight budget.) The mission managers, anticipating the question, have assembled a page labeled "Why Mercury?"
When one thinks of space (or at least when I do), one's tempted to think of the eternal cold that consumes most of it. Mercury is so close to the Sun that MESSENGER's most visible part is a giant sunshade.