Lethe is the largest moon of Mnemosyne, massive enough to retain its own thin atmosphere of methane and nitrogen, and heated by the brown dwarf to relatively moderate temperatures. While nearly the size of Earth, its overall density is low, suggesting a paucity of valuable heavy metals. It is tidally locked to Mnemosyne, one hemisphere always bathed in the brown dwarf's heat and dim red light.
The moon experiences constant weak tectonic activity, driven by the tidal fluxes of Mnemosyne's gravity rather than Lethe's own internal heat. Several large, ancient volcanoes release wide-ranging flows of molten silicate.