9.02.2024

Calculations suggest a 45-mile-deep internal body of water lurks beneath Mimas’s 15-mile-thick icy shell

Peculiarities in Mimas’s orbit had led astronomers to entertain the idea it could be harbouring an internal ocean. Photograph: Frédéric Durillon/Animea Studio/Observatoire de Paris/IMCCE

A moon of Saturn that resembles the Death Star from Star Wars because of a massive impact crater on its surface has a hidden ocean buried miles beneath its battered crust, researchers say.

The unexpected discovery means Mimas, an ice ball 250 miles wide, becomes the latest member of an exclusive club, joining Saturn’s Titan and Enceladus and Jupiter’s Europa and Ganymede as moons known to harbour subterranean oceans.

“It’s quite a surprise,” said Valéry Lainey, an astronomer at the Observatoire de Paris in France. “If you look at the surface of Mimas, there’s nothing that betrays a subsurface ocean. It’s the most unlikely candidate by far.”

Peculiarities in Mimas’s orbit had led astronomers to entertain two possibilities: either it contained an elongated core shrouded in ice, or an internal ocean that allowed its outer shell to shift independently of the core.