Tartaros
From Multiverses
Tartaros is a moon of Inferno, in the Daemoniverse.
Characteristics
Tartaros is 0.1 Earth masses in mass, large for a moon. Its density is 5.3g per centimetre cubed, very high and especially so for a satellite.
It has a reasonably strong gravity, which also allows it to have a thin atmosphere. This is mainly carbon dioxide, but also a small amount of argon.
Notable Features
It seems Tartaros was colonised by the Veiled Ones. The rubble from several cities and evidence of a stronger atmosphere are present.
Most notable is a giant ring-like structure, 450km across, on Tartaros' northern hemisphere. It only has a thin layer of rock over a far deeper structure of metal, and is definitely artificial.