Mars is often called the "red planet." It's red because its surface is covered with rusted particles of iron. Like the Earth, Mars has polar ice caps, mountains, craters, plains, canyons, and summer and winter seasons. Scientists think that liquid water once flowed on the surface of Mars when it was warmer. But today there are no signs of life or water on Mars' rocky surface.

Image credits: main image, NASA/Greg Shirah.