Europa, one of Jupiter’s moons, is sealed beneath a shell of ice, but the ocean hidden below may contain twice as much liquid water as every ocean on Earth combined — and Jupiter’s gravity may have kept it liquid for billions of years

Europa looks, at first, like a frozen world that should be finished. It orbits far from the Sun, locked in the cold outer Solar System, its surface crossed by long reddish fractures and pale plains of ice. There are no blue seas visible from space, no clouds,…