Closest to the sun
Mercury, named for the Roman messenger of the gods, is the smallest planet in our solar system and the closest to the sun. It's also one of our nearest neighbors — the planet can come as close as 77.3 million kilometers to Earth.
In many ways, it looks like our moon with a cratered surface, rocky body and very little atmosphere. But unlike the moon, Mercury has an iron core and a dense surface.
It's ironic that we know so little about this planet, though that's changing. This depiction of Mercury comes courtesy of the the MESSENGER mission and it's MASCS instrument, which studied the exosphere and the surface of Mercury for several years.
Editor's note: This gallery has been updated since it was published in June 2011.