Possible Vast Global Ocean Discovered Beneath Ice on Saturn’s Moon Mimas

Saturn's moon Mimas appears to have a vast global ocean beneath its icy shell, according to detailed measurements of its orbit. If other icy worlds have similar oceans, the number of planets that can support life could increase.

Mimas is the smallest of Saturn's seven major moons. For a long time, it was thought that most of it was composed of solid ice and rock, but in 2014 astronomers observed that the orbit around Saturn was unexpectedly wobbling, suggesting that this could only be explained by either a rugby ball-shaped nucleus or a liquid ocean.

Many astronomers rejected the ocean explanation, as the friction required to melt the ice would have caused visible marks on Mimas's surface. However, recent simulations suggest that this ocean may exist even without such traces.

Looking for more clues? Valerie Rainey Researchers from France's Paris Observatory analyzed observations of Mimas' orbit by NASA's Cassini spacecraft. They found that the orbit around Saturn has shifted by about 10 kilometers over 13 years.

According to the team's calculations, this orbital drift could only have been caused by an ice shell sliding over the ocean, or by wobbles from the physically impossible pancake-shaped core.

The moon's elliptical orbit and lack of surface markings also suggest that the ocean is about 30 kilometers deep and formed less than 25 million years ago. “It was very recent,” Rainey says. “We are more or less witnessing the birth of this global ocean.”

This recent activity could help explain not only the lack of traces on the surface, but also why the moon is so different from its neighbors. Enceladus has a similar shape and orbit to Mimas, and has a global ocean, but it also has a very active surface and giant spout. Rainey said the difference is simply a difference in time, and in a few million years Mimas' ice could melt and it could look similar to Enceladus.

“It would be surprising if that were true,” he says. William McKinnon at Washington University in St. Louis, Missouri. But he says there are still things that aren't perfectly aligned, such as the vast 80-mile-wide Herschel crater, which was formed by a giant impact. If Mimas' ice shell was truly only tens of kilometers deep, McKinnon said, we would have seen evidence of a distorted crater floor in the impact and aftermath. It's also unlikely, he says, that you'll be able to get a front-row seat at such a short and unique time in Mimas' long history. “I remain a Mimas ocean skeptic,” McKinnon says.

However, if Mimas has a hidden ocean, it suggests that other icy planets and moons in the solar system and elsewhere may have the same, expanding the possibility of life. “It's expanding our vision of what is and isn't a habitable world,” Rainey says. “Mimas teaches us that even a corpse that seems to have no life in it may someday come to life.”