Stage 2
Europa
The first ocean, under a young lid of ice
A global ocean under ice that is geologically young. Dry Io to this much water is the first hard riddle of the sequence.
What missions and papers show
Young ice, ridges, and chaos. Almost no large craters. A global ocean is the published state under that shell.
How this site reads it
This site reads Europa as the first wet lid after fire. The ocean is measured. The predecessor is the hypothesis.
What would kill this stage
If Europa’s ocean is native ice from birth, with no volcanic predecessor, Stage 2 comes off the table.