Phobos (moon)

Phobos is the larger and innermost of the two moons of the planet Mars, the second being Deimos. It is the scene of the first Doom episode, Knee-Deep in the Dead. Phobos is named for the ancient Greek god of fear, and a son of Ares.

Doom Classic
According to the Doom Manual, Phobos is used to store nuclear waste, and is also the home of Teleportation experimentation with Deimos. Sometime before the game however, an experiment goes wrong, Deimos disappears from sight, the Phobos base is attacked by an unknown force, and the player's squad is sent up to investigate. The squad ends up being defeated sometime later, and the Lone Survivor ventures into the base, eventually defeating the Bruiser Brothers and supposedly dying (or knocked unconcious) after being teleported to Deimos. In some conversions sections of the base appear to have grass or mossy loam within the perimenter of the buildings, rather than hard rock (as in the original).

Doom 64
Though not mentioned by name, what appears to be another of Phobos or Deimos bases is visited during the game. The manual discusses it being location of the classified "Doom Episodes" described as a 'space installation' he once saved, and in Nintendo Power, Our Hero mentions he had returned to the location of his 'first mission'. In addition it refers to 'installations' plural in a another seciton which could be a reference to both Phobos and Deimos. However, it could be Mars Base as well as it is described to be 'planetary' base twice, and the second level actually takes place in a terraforming facility. It is highly unlikely that either of the moons could be 'terraformed'...

If it is set on Mars, it actually could be a reference to the Doom II's manual's story, which offered a different 'escape' for the Marine (Doom II), after he fought the forces of hell on Mars Base itself.

Whatever the case, the sky on the "Martian" levels of Doom 64 simply a starlit background.

Doom RPG
In Doom RPG, the moon Phobos is seen when the Doom Marine is seen landing at the old Mars Outpost (Mars Research Facility). Deimos orbit trajectory is seen to pass near it (though the moon cannot be seen).

Doom 3
While the moons of Doom are not seen, the Marine visits two labs named after the moons of Mars. One of these is Phobos Labs.

Doom (2016)/Doom Eternal
''Orignally established as a mining and communications relay, the UAC outpost on Phobos began immediate expansion following the destruction of the Argent facility on Mars. The UAC board of directors, intent on retaining their control of the red planet and its surrounding airspace, began the constructon of an expansive defense platform. Using technology derived from the BFG-9000, the desigh plan called for a massive annon to be built on the orbiting moon. The result was the BFG-10000, the largest manmade weapon platform ever conceived, wielding enough firepower to defend against even the most sophisticated capital-grade FTL cruisers in existence-or against the eventual possiblity of a space-bound demonic threat.''

In Doom (2016) Phobos (and Deimos) are used as mining stations, although it is never stated, or implied that any outbreaks, or military operations occurred there.

The Doom Slayer shows up on Phobos after the demon invasion has begun, and there are still living human scientists and guards in the bases command center. Doom Slayer is shown taking a weapon from a scared guard, and exits the command center to kill the invading Demons. Other elements of the base include large defense cannons (BFG-10000).

Behind the scenes
In Doom, Phobos is depicted with Earth-like gravity, a thick atmosphere, and tall, seemingly vegetation-covered mountains; the sky texture for the episode was derived from a photograph taken of Yangshuo Cavern in China. However the map of the base shows that there is no vegetation and its set in a crater. Strangely enough PSX doom changes some of the gorund textures giving parts of Phobos grey ash/sand like dune textures outside some of the windows (see outside window where the first green armor is found in the first level), and in some levels even marshy/moss-like loam or even grass near the base (see level 2).

In reality, Phobos is a asteroid, some 20 kilometers in diameter with gravity less than a thousandth of that on Earth, and no atmosphere (even if an atmosphere could be generated artificially, the moon's gravity would be insufficient to hold it in place without its own artificial gravity). Future releases (Doom 3, and 2016's DOOM) moved the plot to Mars. However, while the character can seemingly walk around outside the base's buildings in some maps. It is known that the character is generally portrayed as wearing a breathing mask as seen in the box art (which may allow/protect him from the low-atmosphere of the moon), or sections of the base have shields keeping air inside of them (which might explain the moss or loam in some conversions). Phobos returns as a location in Doom Eternal.

In the Doom 64 backstory it mentions that a 'planetary' base was sealed, and bombarded with "apocalyptic amounts of radiation" which allows the Mother Demon in a crippled state to resurrect the dead demons, a nearby satellite detects this and in a dying message relays this back to Earth, and the Doom Marine is sent back to this facility defeat the demons. Some interpret this facility to be Phobos although the reference to 'planetary' (twice) could also be a reference to the UAC Mars Base from Doom II backstory in which the Marine stopped an invasion. At least the backstory given in Nintendo Power from the Our Hero's perspective he call it his 'first mission' which does seem to suggest that its either Phobos or Deimos (assuming Deimos returned to Earth space after the orignal doom).

Phobos and its base are mentioned as a mining colony in the 2016 reboot DOOM, and is set to return in Doom Eternal.