Mars is a planet in the solar system where Earth is located.
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.
The ceremony to awaken Heinrich I with the three "Dark Knight hosts" took place during an alignment of Saturn and Mars.
It is possible that Nazis of the New Order have the technology to colonize Mars, if they haven't already.
Considering that in the Wolfenstein II: The New Colossus the player visits a colonized Venus, which requires much more advanced technology to colonize; the colonization of Mars incredibly likely. Additionally, a poster advertising Venus mentions exploring Mars. While this may just may be a typo, considering that the poster is meant to be advertising Venus, it may also be more evidence of Nazi activity on Mars.
Behind the scenes Edit
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.