Mars – Mars, Terrestrial Planets, Inner Planets – Mars is the fourth planet from the Sun and is often called the "Red Planet" due to its reddish appearance, caused by iron oxide (rust) on its surface. Mars has a thin atmosphere composed primarily of carbon dioxide, with traces of argon, nitrogen, and water vapor. The planet is home to the largest volcano in the solar system, Olympus Mons, and the deepest canyon, Valles Marineris. It also has two small, irregularly shaped moons, Phobos and Deimos, believed to be captured asteroids. Mars' surface shows evidence of ancient water flows, including river valleys, deltas, and lake beds, suggesting that it may have supported microbial life in its distant past. Mars has been a primary target for exploration, with numerous missions aimed at studying its geology, climate, and potential for human colonization.