Mercury is the planet in our solar system closest to the sun. Because Mercury is small and near the sun, it is often hard to see from Earth without a telescope. A telescope is a tool that makes faraway objects seem closer and bigger. At certain times of the year, Mercury can be seen just after sunset. At other times, it can be seen just before sunrise. Image to left: The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. Credit: NASA

Mercury moves around the sun faster than any other planet does. The ancient Romans named it Mercury after the speedy messenger of the gods in ancient Roman stories. Like the moon, Mercury is covered by a thin layer of minerals. It also has wide, flat areas of land, steep cliffs, and many deep craters, or holes, like those on the moon. The inside of Mercury is probably like that of Earth. Many scientists think the insides of both planets are made up mostly of iron.

Mercury is dry, very hot, and has almost no air. The sun is much stronger on Mercury than it is on Earth. Scientists believe there is no life on Mercury. The United States Mariner 10 was the first and only spacecraft to reach Mercury. The spacecraft took pictures of Mercury in 1974 and 1975. These pictures tell us all we know about the surface of the planet. How to cite this article: To cite this article, World Book recommends the following format: "Mercury." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.