The only remaining Orokin Towers are hidden here. The Grineer patrol this area to prevent Tenno incursion.
Mercury is the first planet accessible to players, beginning with the mission Terminus.
The Grineer are the controlling faction on this planet.
The vital target of the planet is Captain Vor, who is located in the mission Tolstoj and has a chance to drop blueprints for the Cronus longsword and his personal sidearm, the Seer. He also has a higher than average chance to drop Morphics.
It seems Mercury is the only planet in the Origin system that has never been in faction conflicts ever since the faction wars has been implement. Venus, on the other hand, is only raided by the Infested, and never the Grineer.