Why is Mars called "the Red Planet"?

Mars has been given the moniker "the Red Planet" because it has a bright red color. This reddish colors probably marks the areas that are like deserts on the Martian landscape; the grayish areas are the liquid parts.

The element iron is behind the redness of the planet. On Earth, when iron rusts, it takes on an orange-red color. The same thing happens on Mars. When the iron on the Martian surface "rusts" or becomes oxidized, the result is an orange-reddish hue. When oxygen and iron meet, rust forms.

On Mars, the source of oxygen for oxidizing iron comes from carbon dioxide and water vapor. Both these molecules are abundant in the Martian atmosphere.