Mars (aka Risi Soru the Red Planet) is the fourth planet from the sun in the Solar System.
It is generally assumed the setting of the game is on Mars, but in fact none of the events take place on the planet. In fact, the player never ventures to the surface of the planet in any of the games or episodes.
In Doom 3, the action is set directly on Mars, in the fictional Mars City. The player is required to venture onto the barren surface of Mars a number of times during Doom 3, and must fight off demons while topping up their oxygen supply via collectible tanks. Although the planet is portrayed as being realistically inhospitable, a decision was made early on to increase Mars' gravity to match Earth's for gameplay reasons (in reality, Mars has barely 38% of Earth's gravity).