Why Mars is called the "red planet"?

Mars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance as seen from Earth. Mars is covered by a stony desert that contains lots of iron-oxide, which makes it appear a rusty-red color. At one time, Mars had an atmosphere containing oxygen, and it has valleys through which water may have flowed. Now the water and oxygen are locked up in the rusty iron deposits, and the planet has hardly any atmosphere.

Mars has small polar ice-caps that grow larger during the Martian winter. These probably contain water and frozen carbon dioxide. It is possible that there is enough water in these ice-caps to support a human space mission to Mars.