Earth's atmosphere protects Earth's surface by burning up most objects and pieces of space debris as they fall through it. Because Mercury doesn't have a true atmosphere, its surface is riddled with thousands of craters. None of them quite compare, however, to Mercury's largest crater. Astronomers believe that a 100 km wide asteroid hit Mercury about 4 billion years ago, creating a area of impact 1,525 km wide. This crater, called the Caloris Basin, is so large that the entire U.S. state of Texas (about 1,244 km wide) would fit inside it, with plenty of room to spare.