MECHANICUM OF MARS
Mars, also known as "The Red Planet," is the first and greatest Forge World of the Imperium of Man located in the Sol System of the Segmentum Solar, producing vast numbers of weapons, vehicles and other advanced technology for the Imperium and its war efforts throughout the galaxy. Mars is also the homeworld and headquarters of the Adeptus Mechanicus and a planet sacred to the faith of the Cult Mechanicus, which is one reason why travel to the Red Planet for Imperial personnel who are not members of the Mechanicus and share its peculiar faith is restricted.
Mars is the twin planet of Terra, and one of many long-inhabited human colony worlds located near the Cradle of Humanity. It is the homeworld and primary power centre of the Adeptus Mechanicus, the ancient and byzantine priesthood of the Machine God, and the heart of their galaxy-spanning monopoly on human science and technology.
The Red Planet is the birthplace of the Cult Mechanicus, and is considered the holiest of all celestial orbs in the galaxy to the adherents of that faith. So vaunted is Mars that across the stars a hundred worlds have been terraformed and settled in exactly the same manner. Millennia of incessant construction have turned Mars into a smog-choked hellscape. The surface is covered with massive forge complexes, sprawling refineries, towering monuments to the glory of the machine and weapons shops that scrape the skies. The massive orbital conglomeration of thousands of drydocks and other starship manufacturing facilities in a geosynchronous orbit that turn above the Martian equator are known collectively as the Ring of Iron.
It was the shipyards of the Ring of Iron that constructed the vast exploratory and Expeditionary Fleets of the Great Crusade in the late 30th Millennium and even today produce the majority of the Imperial Navy's capital ships. Spacecraft are constructed within the Ring's extensive orbital factories, and many of the ships of the Battlefleet Solar are based in its huge floating docks. The moon Deimos is absent from Mars' orbit, seconded to the Grey Knights of Titan in aeons long past.
In ancient times, before the current Age of the Imperium officially began following the end of the Horus Heresy, the Adeptus Mechanicus was instead called the Mechanicum. The Adeptus Mechanicus was formally separated from the larger Mechanicum of Mars by order of the Council of Terra in 012.M31. This occurred after the Red Planet had fallen to the control of the Traitor faction of the Mechanicum loyal to the Warmaster Horus later called the "Dark Mechanicum." The Mechanicus acknowledge the Emperor of Mankind as the ruler of the Imperium of Man, but not the religious truth of the Imperial Cult or the Ecclesiarchy, and are granted an unusual amount of political and religious autonomy within the Imperium's structure, a role protected by the ancient Treaty of Mars. The Mechanicus essentially maintains an empire-within-an-empire in Imperial space, and its interests usually, but not always, coincide with those of the wider Imperium. Instead of the Imperial Creed, the servants of the Mechanicus follow their own dark and mysterious scriptures and worship the strange deity they call the Machine God or the "Omnissiah."