Stage 3
Ganymede
An organized interior — a field of its own
The largest moon in the solar system — bigger than Mercury. Layered inside. The only moon with a magnetic field of its own. Dark old land and younger grooved land.
What missions and papers show
Dark terrain covers about 35 percent of the globe — about the area of Europa. Grooved ice cuts that older lid. Galileo mapped both. JUICE will map them again.
How this site reads it
This site reads the dark patches as leftover lid after Europa. The area is the measurement. Analog is not identity.
What would kill this stage
If Ganymede formed already layered and magnetized, with no path through fire and a first ocean, Stage 3 comes off the table.