Mars

Mars (aka Risi Soru the Red Planet) is the fourth planet from the sun in the Solar System.

Original Doom


Doom's action takes place on Phobos and Deimos, the two moons of Mars, although in the introductory story the protagonist is initially stationed on Mars itself.

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.

Doom II
According to the backstory the Marine of Doom II stopped a parallel invasion at Mars Base which occurred over the course of a few days. He was the only survivor. He was sent home from Mars on shore leave only to discover that Earth was invaded after his drop pod landed.

Doom 3, Resurrection of Evil, and Doom Resurrection
In Doom 3, the action is set directly on Mars, in the UAC Mars Research Base and 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).

Doom RPG
The backstory treats it as a sequel to Doom 1-3 (or at least in the same timeline, the exact order is not clear), it is set in the Mars Outpost the oldest facility on Mars, older than the Mars City.

In Doom II RPG the main characters (a different set of characters from the character in the first game) transfer from Mars to go to the Lunar Outpost.

Deimos is missing from Mars skies which seems to be a nod to the events of the original Doom/Ultimate Doom.

Doom (2016)
See Mars (2016)