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 the cameras and papers show

Callisto sits outside the 1:2:4 lock of Io, Europa, and Ganymede. It does not get the same tidal workout.

The ground is heavily cratered. Counts are often read as a lid about 4 billion years old. That dates how long the surface has been quiet. Valhalla is a multi-ring impact basin thousands of kilometres across.

A possible ocean under that lid is reported from magnetic induction. This site does not use that ocean as a shortcut to Titan.

Callisto is the least internally heated of the four large Jovian moons. 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.

A cratered lid can also be a cold first-generation ice moon that simply never heated much. That is why this stage is the weakest on the list.

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.