Ganymede is the largest moon in the Solar System. It's bigger than Mercury, more massive than Pluto, and if it orbited the Sun instead of Jupiter, we'd call it a planet without hesitation.
Beneath an ice crust 150 km thick lies a salty ocean up to 100 km deep — three times deeper than the Mariana Trench. More liquid water than in all of Earth's oceans combined. But the structure is more complex: models suggest a "club sandwich" — alternating layers of ice and liquid all the way down to a rocky core at 800 km depth. All of it sustained by Jupiter's gravity, kneading the moon's interior and keeping it from freezing solid.
The most remarkable thing is the magnetic field. Ganymede is the only moon with its own magnetosphere. It sits nested inside Jupiter's — and auroras glow at Ganymede's poles. Not on Earth. On a moon of another planet, more than half a billion kilometers away.
The surface is a book of two eras. Dark plains scarred with craters four billion years old. Next to them — bright bands of grooves and ridges, traces of ancient tectonics. Ganymede didn't just sit frozen. It lived.
In December 2034, ESA's JUICE probe will enter its orbit — the first time a human-made spacecraft orbits not a planet, but a moon of another planet. We're not going for the views. We're going to look beneath the ice.
Ganymede reminds us: even in Jupiter's shadow, you can be a world of your own.