What is the planet Mercury like?

Mercury is a small rocky planet closest to the Sun, with a battered gray surface and almost no atmosphere. It races around the Sun quickly but spins slowly, so days feel strange. In sunlight it bakes; in darkness it becomes bitterly cold.

It is only a bit larger than our Moon, yet much of its inside is iron. That oversized metal center is one reason scientists keep studying it. Spacecraft such as MESSENGER have mapped its craters, cliffs, and plains so we can know this harsh world without standing there.