Revealed by Mariner 10
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. And yet, relatively little is known about this small planet. In many ways, it looks like our moon with a cratered surface, rocky body and very little atmosphere. But unlike the moon, experts point out that Mercury has an iron core and a dense surface. Here it is as revealed by the Mariner 10 space probe in 1975.