Mars is the fourth planet in distance from the sun. It was named after the Roman God of war. There are two moons that orbit Mars, Phobos and Deimos. Mars has short, relatively warm southern summers and long, relatively cold southern winters. The planet's polar regions are marked by bright caps, that are apparently made of frost or ice. The Martian atmosphere consists mainly of carbon dioxide. The maximum summer temperatures may reach 63° F, but average daily temperatures do not exceed -27° F. Mars could be compared to a very cold, high altitude desert. The surface temperatures are too cold for liquid water to exist. U.S. National Aeronautics and Space Administration (NASA) has proposed a manned voyage to Mars early in the 21st century.