From Halopedia, the Halo wiki
Topography and ecology
A small, terrestrial planet, Mercury is a barren world with virtually no atmosphere. Mercury's close proximity to Sol makes for an often hot environment on the planet; the planet's heat and small size prevent Mercury's gravity from retaining any significant amount of atmosphere.
Government and society
Due to Mercury's harsh nature, the planet is of little use to the United Nations Space Command. Despite this, humanity has established antimatter production and solar research facilities at Mercury. The planet has additionally hosted some mining complexes over the centuries, though by 2559 none are active. All of these facilities are operated by rotating crews of workers, with no permanent human habitation anywhere on the world.
List of appearances