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