Stage 4
Callisto
A quiet ice lid — still, cratered, outside the tight dance
The outer Galilean world. Thick ice. A surface that has been largely still. The most cratered of Jupiter’s large moons.
What missions and papers show
Callisto sits outside the 1:2:4 lock of Io, Europa, and Ganymede. The ground is heavily cratered. Interior models often leave it only partly layered.
How this site reads it
In the sequence this is the last Jovian-system face: a thick, still lid before the first thick air at Titan. Crater counting dates how long the lid has been quiet, not the last time it was warm.
What would kill this stage
If Callisto is only a cold first-generation ice moon that never had to become Titan, Stage 4 comes off the table.