Hidden oceans on icy moons may be boiling beneath the surface

Several frozen moons in our solar system hide vast oceans beneath their icy surfaces. Despite the extreme cold, these worlds—like Saturn’s Mimas and Enceladus, or Uranus’s Miranda—maintain liquid water. Internal heating from their parent planets’ gravitational pull keeps these oceans from freezing solid. Tidal forces from the gas giants warm the moons’ interiors, allowing water to stay liquid under thick ice shells. Water behaves unusually: it expands when freezing but contracts when melting, creating empty spaces in the ice. This causes pressure to drop inside the ocean.

For moons smaller than about 300 kilometres in radius, the pressure can fall to a point where water reaches its ‘triple point’. At this stage, it exists as solid, liquid, and gas at once, making the ocean effectively boil. The boiling triggers gas release, which pushes material upward through the ice, possibly feeding eruptions like those seen on Enceladus.

Larger moons experience a different process. Their thick ice shells press down as the ocean tries to depressurise. The shell cracks before boiling can occur, preventing the same mechanism from taking place. Even geologically quiet Mimas may have a depressurising ocean, though its surface has yet to fracture. This boiling process could be vital for astrobiology. It may supply energy and nutrients to potential life in these hidden oceans. The mechanism also helps explain how samples from Enceladus’s plumes reflect the deep ocean’s chemistry.