Mars

Mars
Inhabitants	Martians
Moons	2
Notable Facts	Zim tries to pilot Mars to squish all of humanity on Earth with the planet.
First Appearance	Battle of the Planets
[Source]

Mars is a planet in the Milky Way Galaxy, found in the Solar System. It is the fourth planet from the Sun and comes directly after Earth when orbiting the Sun.

In the Invader Zim series, Mars is a planet featured in the episode "Battle of the Planets". It contains a mysterious formation known as the "Face of Mars" on a region called Cydonia.

This planet was converted into a "giant spaceship" by the aliens that lived there. They first tested that technology on another, nearby planet. When the holographic interactive instruction manual (the only remnants of the Martian race that can communicate) is asked why they did it, it will simply respond "Because it's cool."

Zim tried to use the planet to annihilate the human race by rolling Mars over Earth's surface, but Dib, who was piloting Mercury via the "Butt on Mercury", flies in and tries to stop him. After a lengthy battle, Zim escapes from Mars in his Voot Cruiser after flying into the Asteroid Belt while Mars hurtles into the Irken Armada (which goes completely unnoticed).

It's unknown what became of Mars after it destroyed a big chunk of the Irken Armada but it could have been destroyed after colliding with another planet, it could have relocated to another Solar System or the reset button was pushed, putting the planet back in it's original orbit.

Martians

Mars was at one time inhabited by a species known as the Martians, who worked themselves into extinction converting the planet into a giant spaceship for only one reason: because it's cool.

Nothing else is known about the Martians prior to their extinction but it's evident they at least mastered space travel to be capable of going to Mercury and presumably Earth in the distant past.

Gallery

The Martian instruction manual.

Zim approaching Mars.

Mars in flight.

Facts of Doom

• The Face of Mars is a real-life structure that is, along with the rest of Cydonia, supposedly part of an ancient, extraterrestrial civilization. It was first photographed by the Viking 1 orbiter, but in 2001, the Mars Global Surveyor took another picture of the face, and it turned out to be an optical illusion. The phenomena has since then drawn considerable controversy as well as both scientific and popular interest.
• In the episode the scale of the planets are way off: Mars and Mercury are the same size, despite Mars being 133% the size of Mercury,  and Mars is about an eighth of the size of the Earth, despite Mars only being one-third the size of Earth.
• The idea of going to Mars and meeting Martians is based off of the popular belief that Mars may contain life, due to only being rather close to Earth, and has tempatures that can be suitable for some life.
• Interestingly it took Zim minutes to accidentally discover that there had been life on Mars, something humans haven't been able to prove in years.
• Technically, since Zim was the first Irken to set foot on Mars, he legally could have claimed ownership of the planet in the name of the Irken Empire.
• The Martians turning their homeworld into one big vessel is similar to the Irken tradition of giving a planet one specific purpose.