Evidence that some moons may have oceans beneath their surface is growing, but the detection of such watery worlds is challenging.

Mimas — the innermost, and smallest (radius = 198.2 km, or 123 miles), regular moon of Saturn — is an unlikely candidate, owing to differences in its surface properties compared with other icy moons, such as Enceladus.

This theory is challenged by Sorbonne Université researcher Valery Lainey and colleagues, who assess observations of the small moon made by Cassini.

Previous research suggested two possibilities for Mimas’ interior: either an elongated rocky core or a global ocean.

The new study reveals 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’ position suggest that the evolution of its orbit is better explained as being influenced by an internal ocean.

The researchers calculate that the ocean lies beneath an icy shell approximately 20-30 km deep.

Their 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.

The results imply that the recent processes on Mimas may have been common in the early stages of formation for other icy worlds.

“Mimas is a small moon, and its heavily cratered surface gave no hint of the hidden ocean beneath,” said co-author Dr. Nick Cooper, a researcher at Queen Mary University of London.

“This discovery adds Mimas to an exclusive club of moons with internal oceans, including Enceladus and Europa, but with a unique difference: its ocean is remarkably young.”

Sources:
Published today in the journal Nature; V. Lainey et al. 2024. A recently formed ocean inside Saturn’s moon Mimas;
doi: 10.1038/s41586-023-06975-9

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