Hidden oceans on icy moons may boil beneath their frozen shells

Several frozen moons in our solar system hide liquid oceans beneath their icy surfaces. Despite bitterly cold temperatures, these oceans remain liquid due to internal heating. The process involves tidal forces from their parent planets, which generate warmth deep inside the moons. The size of a moon determines how its hidden ocean behaves under pressure. On smaller moons, pressure can drop so low that water reaches its 'triple point', existing as solid, liquid, and gas at once. This causes the ocean to 'boil', releasing bubbles of water vapour and dissolved gases.

For larger moons, the weight of the ice shell prevents this boiling effect. As the ocean tries to depressurise, the shell cracks before the water can boil. Mimas, for example, appears inactive but may have a boiling ocean beneath its intact shell.

The mechanism is theoretical, based on simulations rather than direct observation. It suggests that eruptions on moons like Enceladus could carry material more representative of their deep oceans. Water’s unusual property of expanding when it freezes creates voids in the ice, further lowering pressure. This boiling process could have implications for astrobiology. It may provide energy and nutrients to potential life in these dark oceans by driving upward flow and releasing gases. However, the theory remains unproven, as it is based on models rather than observed evidence.