A recently formed ocean inside Saturn's moon Mimas.

Moons potentially harbouring a global ocean are tending to become relatively common objects in the Solar System 1 . The presence of these long-lived global oceans is generally betrayed by surface modification owing to internal dynamics 2 . Hence, Mimas would be the most unlikely place to look for the presence of a global ocean 3 . Here, from detailed analysis of Mimas's orbital motion based on Cassini data, with a particular focus on Mimas's periapsis drift, we show that its heavily cratered icy shell hides a global ocean, at a depth of 20-30 kilometres. Eccentricity damping implies that the ocean is likely to be less than 25 million years old and still evolving. Our simulations show that the ocean-ice interface reached a depth of less than 30 kilometres only recently (less than 2-3 million years ago), a time span too short for signs of activity at Mimas's surface to have appeared.