Mars is the fourth planet from the Sun and has a reddish appearance due to iron oxide on its surface. It has a thin atmosphere composed of carbon dioxide and nitrogen. The largest volcano in the solar system, Olympus Mons, is located on Mars. Several robotic rovers have explored Mars, including Curiosity which landed in 2012 and continues to study the planet's environment and geology. India has also successfully sent an orbiter, called Mangalyaan, to study Mars from orbit.