For most of the twentieth century the search for life in the Solar System meant Mars. Then the Galileo and Cassini missions revealed something unexpected: some of the icy moons of Jupiter and Saturn are hiding oceans beneath their frozen shells. Liquid water, kept warm not by the Sun but by tides.
How tides keep an ocean liquid
Europa orbits Jupiter every 3.5 days on a slightly elliptical path. As its distance from the planet changes, Jupiter's gravity stretches and relaxes the moon, flexing its interior. That flexing generates heat, enough to keep a global ocean liquid under 15 to 25 km of ice. The same mechanism turns Io, one orbit closer in, into a volcanic inferno.
The candidates
- Europa (Jupiter): an ocean perhaps 100 km deep, containing two to three times as much water as all Earth's oceans. Its surface is criss-crossed by cracks stained with salts from below. NASA's Europa Clipper arrives in 2030.
- Enceladus (Saturn): only 500 km across, yet Cassini flew through plumes of water erupting from cracks at its south pole and found salt, silica and organic molecules, plus hydrogen gas that suggests hydrothermal vents on the ocean floor. This is the strongest evidence yet of a habitable environment beyond Earth.
- Titan (Saturn): has lakes of methane on the surface and, separately, a water ocean far below the ice. The Dragonfly rotorcraft will explore its surface in the 2030s.
- Ganymede (Jupiter): the largest moon in the Solar System. Its ocean is thought to be sandwiched between layers of ice. ESA's JUICE mission will orbit it in the mid-2030s.
Why it matters
On Earth, life thrives around deep-sea vents in total darkness, feeding on chemical energy rather than sunlight. If Enceladus has vents too, and Cassini's hydrogen finding suggests it does, then the ingredients for that kind of ecosystem exist a billion kilometres from the Sun. Finding it would mean life started twice in one Solar System, which would imply it is common across the galaxy.
