Prior to the release of Curse of Osiris, Mercury was featured only through the Altar of Flame PvP Map. With the expansion's release, it can now be visited directly by expansion owners which have made sufficient progress through the expansion's Story Missions. There is only one landing zone for Mercury, and it is located at the north end of The Lighthouse, the only open-world sector of Mercury's patrol zone, Fields of Glass.
Fields of Glass
Fields of Glass, Mercury's patrol zone, is divided into two main parts: the Infinite Forest, an activity-exclusive region which also encompasses the Simulant Past, Simulant Present, and Simulant Future regions; and the Lighthouse, an open-world sector which serves as Mercury's patrol zone proper.
The Infinite Forest
The Infinite Forest is the name given to the reality engine built by the Vex inside of Mercury. It cannot be accessed freely, only letting players on certain activities (such as Mercury Adventures) through its entrance.
The Lighthouse is a sector on Mercury. It comes with a new vendor, Brother Vance, hidden away within the Lighthouse proper along with The Forge - a Vex device that allows the creation of Mercury-themed Gear.
The area outside The Lighthouse proper is a playable one, with enemies and Public Events, including the Vex Crossroads Public Event, happening outside. This combination of safe zone and combat zone is not unusual - the Church in the Trostland sector of the European Dead Zone is another example of this concept in action.