Stage 6
Triton
A thin leftover sky
Neptune’s large moon. It orbits the wrong way. Capture is granted. The leftover question is the body itself, not the frost on top.
What missions and papers show
Voyager 2 found young patches, nitrogen geysers, a pink south cap. Dominant surface volatile is nitrogen ice, with methane, carbon monoxide, and carbon dioxide. Thin N₂ air, about 14–19 μbar, sits in vapor-pressure balance with that ice.
How this site reads it
This site reads Triton as the lid after Titan. The ices are the same family Titan carries as air, frozen onto water ice. Titan still holds the complex chemistry. Triton holds the frost and the vents.
What would kill this stage
If Triton is ordinary outer-system ice and rock, captured as it is, Stage 6 comes off the table.