Mars, sometimes called the red planet, is the fourth planet from the sun. Its surface is made up of iron, which is why it looks red. Mars is only about half of the size of Earth and is too cold for humans to live on, although in some parts of Mars, it can be warm during the day. Like Earth, Mars has been found to have ice at its north and south poles. Unlike Earth, Mars has not one, but two moons! Its moons are named Phobos and Deimos. Mars is also the home of Olympus Mons, the highest mountain in the solar system. It is three times higher than the highest mountain on Earth.