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A false colour image from NASA of Mimas transiting Saturn’s ring shadows.

Striped by its rings’ shadows, Saturn (light blue; artificially coloured) looms behind its moon Mimas (grey sphere), which conceals a liquid ocean underneath its surface.Credit: NASA via Alamy

There’s a newfound ocean in the outer Solar System, and it’s in a very surprising place1. Mimas, a mid-sized moon of Saturn, turns out to have an ocean beneath its icy surface — despite looking too geologically inert to have water sloshing inside.

Mimas joins a growing list of icy moons that are also ocean worlds. The fact that boring-looking Mimas has an ocean means that “you could have liquid water almost anywhere”, says Valéry Lainey, an astronomer at the Paris Observatory.

That’s important because interactions between ocean water and rock, which would occur where a buried ocean meets a moon’s rocky core, can generate enough chemical energy to sustain living organisms. If there are more stealth ocean worlds out there similar to Mimas, there are greater chances of extraterrestrial life.

Peek-a-boo ocean

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The discovery, reported today in Nature by Lainey and his colleagues, largely resolves the long-standing question of whether Mimas has an ocean. Many researchers hadn’t expected it to: Mimas’s geology does not display signs of a possible buried ocean, such as the icy rafts that jostle on Jupiter’s moon Europa or the geysers that spew from Enceladus, another icy moon of Saturn.

But in 2014, a team that included Lainey and that was led by Radwan Tajeddine, an astronomer then at the Paris Observatory, analysed images taken by NASA’s Cassini spacecraft, which explored Saturn and its moons between 2004 and 2017. By studying how the 400-kilometre-wide Mimas wobbled in its orbit around Saturn, the researchers concluded that it had either a buried ocean or a rugby-ball-shaped core2. As more scientists studied how an ocean could have formed and evolved, it became harder to explain the geology of Mimas without invoking an ocean3.

In the 2024 study, Lainey and his colleagues seem to have nailed the case. They went further than they had in 2014, by analysing not just the orbit’s wobble but also how Mimas’s rotation around Saturn changed over time. The team combined Cassini observations with simulations of Mimas’s interior and its orbit to conclude that there must be an ocean 20–30 kilometres below Mimas’s surface.

Solid evidence

The work is the best evidence yet for an ocean in Mimas, says Alyssa Rhoden, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who will report similar conclusions at a conference next month in Texas. “I am happy to move Mimas from the ‘maybe possibly an ocean world’ category to the ‘yeah it really could be an ocean moon’ category,” she says.

But it seems to be a young ocean — having formed in the last 25 million years, compared with almost 4 billion years ago for Earth’s first ocean. If the ocean had been around for longer, it would have begun to exert its influence on Mimas’s icy surface by now, for example by fracturing it. At some point in the recent past, Lainey says, Mimas was probably travelling on a stretched-out orbit that caused it to gravitationally interact with other Saturnian moons. That tidal interaction would have heated up Mimas, melting its interior and creating the ocean.

Ultimately, the pockmarked Mimas could evolve to look similar to smooth Enceladus, which is coated in ice created by water spraying through cracks in its shell. And beyond Saturn, the discovery suggests that several moons of Uranus could also be hiding oceans of their own, despite looking static and frozen on their surfaces.

“There are no boring moons,” Rhoden says.

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