Mars

Mars is a location from the animated short, "Adventure Time." When Pen (later changed to Finn) gets frozen by one of Ice King's frozen lightning bolts, his mind gets transported back in time and to Mars by Abraham Lincoln, who gives him the courage to believe in himself. Earth appears normal in the background, which is explained by the fact that this scene takes place "back in time."

In "Too Young," Peppermint Butler tells the Earl of Lemongrab that "food comes from Mars." However, before he can explain further, Lemongrab silences him.

In "Sons of Mars," Finn and Jake encounter Martians, who are the inhabitants of the planet. The episode reveals a deity named Grob Gob Glob Grod and introduced Abraham Lincoln, the King of Mars.

Description
Only a small portion of Mars is shown in Sons of Mars. The portion that was shown is an amphitheater which was used as a courthouse. The courthouse is surrounded by a force field which can be opened and closed. It is currently inhabited by Martians and was formerly ruled by Abraham Lincoln.

History
After his assassination, Abraham Lincoln made a trade with Death to become immortal and he eventually became the king of Mars. More than 200 years before the start of the series, Magic Man created havoc in Mars by doing multiple horrible acts like turning water into hair, bringing shadows to life and joining the limbs of all Martian citizens. Magic Man was banished to Earth. 200 years later, Magic Man turned Jake into a copy of himself to fool the Martians. Jake (currently transformed into Magic Man's image) is convicted of execution. After Lincoln finds what really happens, he made a deal with Death in which Jake will be brought out to life in exchange for Lincoln's immortality. Due to this Lincoln dies.

Trivia

 * There are Mars symbols on many of the structures. The symbol is a circle with an arrow emerging from it, pointing at an angle to the upper right, interpreted as the shield and spear from the war god Mars.
 * In "The Lich," it is revealed that Mars is the third planet from the sun because the Sun's expansion resulted in the destruction of Mercury.
 * It also looks similar to Cowboy Bebop's Mars, where humans also live in domes because the rest of Mars cannot be terraformed.