Stage 5
Titan
The first thick air — nitrogen, rain, lakes
Nitrogen air. Methane rain. Lakes and seas. The first thick atmosphere in the sequence.
What missions and papers show
Mostly nitrogen, about 1.5 bars. Methane plays the part water plays here. Huygens landed in 2005. Cassini watched equatorial rain. Polar seas are liquid hydrocarbons. Sunlight would empty the present methane column in tens of millions of years.
How this site reads it
This site reads Titan as the first-atmosphere stage of the present cycle, after Callisto’s quiet lid. A late sky is already on the table in the papers. That is compatible with a late chapter.
What would kill this stage
If Titan has always belonged to Saturn, with no quiet ice-lid predecessor, Stage 5 comes off the table.