Planet.

Mars is the fourth planet from the sun and is roughly half the size of the planet Earth. Mars is the last of the inner planets before the asteroid belt. It is named after the Roman god of war. It is a terrestrial planet and, called the 'red planet' because well, its soil is red. This is due to the levels of iron. Mars does have an atmosphere, but it is extremely thin. The surface of Mars is covered with craters, mountains, volcanoes, deserts and even polar ice caps. Mars can be seen from Earth by the naked eye.

The planet has two moons, Phobos (Fear) and Deimos (Dread) which may have once been asteroids. Rei's crow companions are named after these moons, and, in the manga, Mars' palace is also named after the moons. Phobos and Deimos were, in mythology, demigods that accompanied Ares (Mars) into battle.

Of all the planets, Mars is the most Earth-like. It has seasons due to a similar tilt of its axis. Though for many years scientists thought it was likely that life could have possibly been supported on Mars this theory has been debunked.