Mars is the fourth planet from the sun. It is a reddish planet covered with rocks and craters, or big holes. Mars was named after the ancient Roman god of war. Some scientists believe life may have existed on Mars billions of years ago, but there is no proof that anything is alive on the planet today. Image to left: The planet Mars, like Earth, has clouds in its atmosphere and a deposit of ice at its north pole. But unlike Earth, Mars has no liquid water on its surface. The rust-like color of Mars comes from the large amount of iron in the planet's soil. Credit: NASA/JPL/Malin Space Science Systems

Mars travels around the sun in an elliptical, or oval-shaped, orbit. It takes about 687 Earth days for Mars to go all the way around the sun. Mars has two small moons called Phobos and Deimos. The surface of Mars looks more like Earth's than the surface of any other planet does. Even so, the plants and animals that live on Earth could not live on Mars. Its ground is too cold -- it is usually below 32 degrees Fahrenheit (zero degrees Celsius), the freezing point of water. Also, the air of Mars has almost no oxygen. People and animals need oxygen to breathe.

cientists have studied Mars through telescopes for many years. In 1965, the United States spacecraft Mariner 4 flew near Mars and took pictures of its surface. In 1976, the U.S. Viking 1 and Viking 2 were the first spacecraft to land on Mars. They took pictures and collected soil samples. The U.S. Pathfinder probe landed on Mars in 1997. All these spacecraft were controlled by radio signals from Earth. As yet, no people have set foot on Mars. How to cite this article: To cite this article, World Book recommends the following format: "Mars." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.