Ganymede is wider than Mercury, holds more liquid water than all of Earth’s oceans combined, and is the only moon in the solar system that generates its own magnetic field. This deep dive explores why Jupiter’s largest satellite acts like a fully fledged planet. It begins with the 1610 discoveries by Galileo and Simon Marius, the puzzling 365 BC Chinese record of astronomer Gan De seeing a reddish companion to Jupiter with the naked eye, and the density figure of 1.9 grams per cubic centimeter that reveals a roughly 50-50 mix of rock and water ice.

The episode explains how Ganymede formed in about 10,000 years, trapping enough heat to fully differentiate into an iron core, rocky mantle, and icy shell, unlike slowly formed Callisto. It details the club sandwich model of stacked oceans and high-pressure ice phases, the internal heat from formation and radioactive decay that keeps water liquid, and the 2015 Hubble aurora observations that proved the ocean exists through Faraday induction damping. It also covers the dynamo in the liquid iron sulfide core, ghost craters called palimpsests, sputtering on the trailing hemisphere, salts detected by Juno, the thin radiation-made oxygen atmosphere, the 1:2:4 Laplace resonance with Europa and Io, and the upcoming JUICE and Europa Clipper missions.

• How to see a moon of Jupiter without a telescope, and why its color remains a mystery
• Why formation speed determines whether a moon separates into layers or stays mixed
• Ice that sinks: the physics behind alternating layers of ocean and dense ice
• How a two-degree aurora wobble instead of six revealed a salty conductive ocean
• Tidal kneading from orbital resonance and the tectonic grooves it left behind