Saturn’s moon Mimas looks like a dead, cratered rock — its surface is so inert that scientists assumed it was frozen solid — but in 2024 astronomers confirmed a global ocean hidden beneath the ice, formed just 5 to 15 million years ago

Valéry Lainey and his colleagues found that Mimas’s heavily cratered ice shell is best explained by a global ocean about 20 to 30 kilometres beneath the surface. Lainey’s team examined an additional signal: the slow drift of the point in Mimas’s orbit where it comes closest to Saturn. Why a cratered moon fooled usNASA describes Mimas as the smallest and innermost of Saturn’s major moons, about 394 kilometres across. Mimas changes the search imageEuropa and Enceladus taught astronomers to look for disrupted ice, young terrain and material escaping from below. The contrast is especially clear beside my recent piece on Europa’s ocean beneath an estimated 29 kilometres of ice.