Mars is the planet after Earth from the Sun, named after the Roman god of war. It is 142 million miles from the Sun, with surface temperatures ranging from -184 to 80 degrees Fahrenheit. While there is frozen water and ice on the surface, scientists have discovered evidence that water once flowed in the valleys. The thin atmosphere is made up of nitrogen, oxygen, argon and carbon dioxide, and scientists want to try building it up over years. Notable features include Olympus Mons, the tallest solar system mountain at over 15 miles high, and two moons named Phobos and Deimos.