Mars is the fourth planet from the Sun, and its reddish glow comes from iron oxide — literally rust — coating its rocky surface. Despite the cold and thin atmosphere, Mars has seasons, polar ice caps, canyons, and weather much like Earth.
It's home to Olympus Mons, the largest volcano and highest known mountain in our entire solar system — roughly three times taller than Mount Everest. Because it's so close and rocky, Mars has been visited by more rovers and orbiters than any other planet, as scientists search for signs that life may have existed there billions of years ago.