The only remaining Orokin Towers are hidden here. The Grineer patrol this area to prevent Tenno incursion.
Mercury is a planet under Grineer control, one of the two planets unlocked at the start along with Earth. Unlike all other planets, Mercury does not have Dark Sector missions and cannot have any Invasions occur.
Apollodorus (Survival) is one of the best places to farm Ferrite, yielding approximately 1,000 of them every five minutes.
Odin (Interception) is good way to farm affinity for new player, since it can yield 300 affinity per captured objective and 500 for total control. Consider allowing the enemy to capture the objectives more often for more affinity gain.
The Larunda relay on the PC platform is the earliest available relay, new players can enter this relay and potentially interact with more experienced players, as well as find other players who may be willing to play missions with them.