Mercury's surface looks similar to the surface of the Moon. It has plains that look like mares and has lots of craters. Mercury was hit by a lot of comets and asteroids 4.6 billion years ago. Mercury was also hit during a period called the Late Heavy Bombardment. Mercury has lots of craters because it does not have any atmosphere to slow objects down. Images gotten by "MESSENGER" have shown that Mercury may have shield volcanoes. 