Saturn’s moon Mimas may be hiding a vast global ocean under its ice

Saturn’s moon Mimas appears to have a vast global ocean underneath its icy shell, according to close measurements of its orbit. If other icy worlds have similar oceans, it could increase the number of planets that are hospitable to life.

Mimas is the smallest of Saturn’s seven major moons. It was long thought to be mostly composed of solid ice and rock, but in 2014 astronomers observed that its orbit around Saturn was unexpectedly wobbling, which could only be explained by either a rugby ball-shaped core or a liquid ocean.

Many astronomers rejected the ocean explanation because the friction needed to melt the ice should also have produced visible marks on Mimas’s surface. However, recent simulations have suggested that this ocean could exist without such marks.

To look for more clues, Valéry Lainey at the Paris Observatory in France and his colleagues analysed observations of Mimas’s orbit made by NASA’s Cassini spacecraft. They found that its orbit around Saturn has drifted around 10 kilometres over 13 years.

According to the team’s calculations, this orbital drift could only have been produced by wobbles from an icy shell sliding over an ocean, or a core with a physically impossible pancake shape.

The moon’s oval-shaped orbit and lack of surface marks also suggest that the ocean is around 30 kilometres deep and formed less than 25 million years ago. “It’s very, very recent,” says Lainey. “We are more or less seeing the birth of this global ocean.”

As well as explaining the lack of surface marks, this recent activity could help explain why the moon is so markedly different from neighbouring moons. Enceladus, which has a similar shape and orbit to Mimas, has a global ocean but also a very active surface and a giant water spout. This difference might just be one of time, says Lainey, and in millions of years Mimas’s melting ice could make it look similar to Enceladus.

“It’s remarkable if it’s true,” says William McKinnon at Washington University in St. Louis, Missouri. But there are still things that don’t quite add up, he says, like the vast 139-kilometre-wide Herschel crater, which was formed from an enormous impact. If Mimas’s icy shell really is only tens of kilometres deep, then we would have seen evidence of this in the impact and aftermath, like a warped crater floor, says McKinnon. Also, it is unlikely that we would have a front-row seat for such a short and unique time in Mimas’s long history, he says. “I remain a Mimas ocean sceptic,” says McKinnon.

But if Mimas does have a hidden ocean, then it could suggest that other icy planets and moons in our solar system or elsewhere could be similar, which also expands the possibility for life. “It’s extending our vision of what is a habitable world and what is not,” says Lainey. “Mimas shows you that even a dead body that doesn’t look like it’s harbouring anything could have life one day.”