Mars is the fourth planet from the Sun and is made up of rock and a small amount of water. It has a diameter of 6,752 km and two small moons. Mars is an outer planet with a thin atmosphere composed mostly of carbon dioxide, so it is cold and does not support life. The surface appears red in color and contains many large volcanoes and craters from impacts. Humans have sent robotic spacecraft to explore Mars but have not yet traveled there.