Mars is a planet in the solar system where Earth is located.
Background[]
William J. Blazkowicz was once transported to a Union Aerospace Corporation base on or near Mars by Nazi time travel technology. The Nazis were stealing technology from the UAC and bringing it back to the past in an attempt to win the war.
B.J. Blazkowicz III and Stan Blazkowicz are his descendants. The former also fought demons at a UAC facility on Mars on multiple occasions, and later Phobos, Deimos, and even Earth.
The ceremony to awaken Heinrich I with the three "Dark Knight hosts" took place during an alignment of Saturn and Mars.
Behind the scenes[]
Note an image in Mission 3 appears to have 'solar flares' which indicates that it is an image of the sun, rather than the red planet. However it does have certain surface characteristics similar to photos of the red planet (but these are actually sun spots). This same image of the sun appears in Doom, and can be seen as early as the first level, the hanger bays.