Access to Phobos can be obtained by completing the Junction located on Mars.
Phobos was originally inhabited by a civilization that worshiped the golden 'Skymen'. The Grineer eventually invaded the moon and exterminated the entire population, with the exception of Baro Ki'Teer. It is also the possible origins of the Inaros Warframe.
An immense object can be seen above the clouds in space; the object appears to be a Grineer Asteroid base and possibly Mars' other moon Deimos. This is quite remarkable for how large the object is seen in space, perhaps making it the largest Grineer mining operation to date.
Although the Grineer are supposed to have recently occupied the moon (or that it is one of their most recent occupations at any rate) there are signs of very old Grineer-like equipment scattered and rusting in some places, perhaps suggesting that the Grineer were previously here before but had to evacuate at some point.
The planet is littered with massive wind generators, providing what appears to be the majority of the planet's power, and various pumps and water bays are used to filter and process water.
Phobos is named after the Greek personification of fear and one of the sons of Ares, who is Mars' Greek equivalent. It has a twin moon called Deimos (strife), the latter currently is not available in Warframe and there are no plans on adding it.
In contrast to how Phobos is depicted in-game, in real life, it is irregular in shape. It is widely believed to be a captured asteroid.
The nodes on Phobos are named after actual named geological features of Phobos. Other than Monolith and Iliad, those in turn are named either after astronomers (e.g. Asaph Hall, who discovered Phobos) or after characters in Gulliver's Travels (as its author, Jonathan Swift, predicted the existence of Mars' moons).
The boss level, Iliad, is named after an ancient Greek epic poem, which tells of the battles and events during the weeks quarrel between the King Agamemnon and the Warrior Achilles.
This planet was unlocked as a reward for the community completing the Arid Fear Event. Much of the event involved the Grineer and the Corpus trying to hide Phobos' existence from the Tenno.
Up until The Gradivus Dilemma Phobos was the third planet in the game that was completely under the control of a given faction (besides the indigenous species), following the (then) Infested-controlled planets of Eris and Jupiter.
Phobos is the first planet to have an Assassination mission involving more than one entity, the second being the first phase of Lephantis.
Looking into the distance from the extraction point, one will be able to see what looks like a Fomorian ship on its side in the far background.
Phobos' geology appears to resemble Mars in reality, having a reddish appearance to the soil and a desert like environment.
Phobos is the only planet in which Research components do not drop.