Media release

From: Springer Nature

Evidence for a subsurface ocean on Saturn's moon Mimas *IMAGES & VIDEO* (N&V)

Saturn’s moon, Mimas, may have an ocean hiding beneath an icy cratered surface, a paper in Nature suggests. Analysis of observations from the Cassini spacecraft indicates that the ocean is relatively new and still evolving. Further studies of Mimas might teach us more about the formation of icy worlds.

Evidence that some moons may have oceans beneath their surface is growing, but the detection of such watery worlds is challenging. Mimas, one of Saturn’s small moons, is an unlikely candidate, owing to differences in its surface properties compared with other icy moons, such as Enceladus. This theory is challenged by Valery Lainey and colleagues, who assess observations of the small moon made by Cassini.

Previous research has suggested two possibilities for Mimas’s interior: either an elongated rocky core or a global ocean. The latest analyses by Lainey and colleagues reveal changes to the rotational motion and orbit of the small moon that are affected by its interior. Applying the solid-body model would require the rocky core to be elongated, almost pancake-shaped, which does not match the observations. Instead, the measurements of Mimas’s position suggest that the evolution of its orbit is better explained as being influenced by an internal ocean. The authors calculate that the ocean lies beneath an icy shell approximately 20–30 km deep. Simulations suggest that it appeared between 25 and 2 million years ago. As such, signs of such a subsurface ocean would not have had time to make a mark on the surface.

These results imply that the recent processes on Mimas may have been common in the early stages of formation for other icy worlds, the authors note. “Lainey and colleagues’ findings will motivate a thorough examination of mid-sized icy moons throughout the Solar System,” write Matija Ćuk and Alyssa Rose Rhoden in an accompanying News & Views.