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 the cameras and papers show

Ganymede is larger than Mercury. Interior models give it stacked water layers and a metallic core that can run a dynamo.

Galileo measured a field of a few hundred nanotesla at the equator — unique among moons. Dark ancient terrain sits next to younger grooved terrain that looks as if the crust was pulled apart.

Papers collected here in 2016 argued Ganymede should have gone quiet about 4 billion years ago if it were a simple first-generation ice moon.

JUICE is on the way to weigh that interior.

How this site reads it

Compared with Earth, this looks like a world whose inside has already organized: layers, a field, younger crust over older crust.

The hypothesis is stronger: Ganymede is Stage 3 of the present cycle, after fire and a first ocean. It can also be a large ice moon that formed this way from the start.

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.