Mimas hides a young global ocean beneath its ancient ice

A new analysis of Cassini-era data shows Saturn’s small moon Mimas has a global ocean about 20–30 km below an ice shell of similar thickness. Formed roughly 5–15 million years ago (likely under 25 million), this conclusion, derived from precise measurements of the moon’s libration and orbital drift, is a geophysical inference rather than a direct observation. The surface appears old and inert, yet the interior contains liquid water, illustrating that oceans can exist beneath cratered, quiet-appearing worlds and highlighting the potential for hidden oceans in other small icy bodies. Future missions would be needed to confirm chemistry and persistence of the ocean.