Mercury is the nearest known planet to the Sun.
It is three times closer to the Sun than Earth. Mercury is hot. Its rocky, bare, and cratered surface is 800° Fahrenheit (F) during the day. At night, however, the temperature falls to –280°F. Mercury has been baked by the Sun for 4.5 billion years. Not long after it first formed, the planet may have crashed into a giant asteroid that blew away most of Mercury’s outer covering. Today, Mercury is mostly a ball of hot iron with a thin, rocky outer crust.
