Saturn's moon Mimas appears to have a vast global ocean beneath its icy shell, according to precise measurements of its orbit. If other icy worlds had similar oceans, this could increase the number of planets hospitable to life.
Mimas is the smallest of Saturn's seven major moons. It was long thought to be mostly ice and solid rock, but in 2014, astronomers observed that its orbit around Saturn was wobbling in unexpected ways, which could only be explained by a core shaped like rugby ball or by a liquid ocean.
Many astronomers rejected the oceanic explanation because the friction necessary for the ice to melt should also have produced visible marks on Mimas' surface. However, recent simulations suggest that this ocean could exist without such marks.
To look for more clues, Valéry Lainey at the Paris Observatory in France and colleagues analyzed observations of Mimas' orbit made by NASA's Cassini space probe. They found that its orbit around Saturn had drifted by about 10 kilometers in 13 years.
According to the team's calculations, this orbital drift could only have been produced by the oscillations of an icy shell sliding across an ocean, or by a core with a physically impossible pancake shape.
The Moon's oval 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’s very, very recent,” Lainey says. “We are more or less witnessing the birth of this global ocean. »
In addition to explaining the lack of surface markings, this recent activity could help explain why the Moon is so markedly different from neighboring moons. Enceladus, which has a shape and orbit similar to those of Mimas, has a global ocean but also a very active surface and a giant waterspout. That difference could be just a matter of time, Lainey says, and millions of years from now, Mimas' melting ice could make it look like Enceladus.
“It’s remarkable if it’s true,” says William McKinnon at Washington University in St. Louis, Missouri. But there are still some things that don't really add up, he says, like the vast 139-kilometre-wide Herschel crater, which formed following a huge impact. If Mimas' icy shell is really only tens of kilometers deep, then we would have seen evidence of it during the impact and afterward, like a distorted crater floor, McKinnon says. Furthermore, it is unlikely that we will be at the forefront during such a short and unique period in the long history of Mimas, he says. “I remain a skeptic of Mimas oceans,” says McKinnon.
But if Mimas does indeed have a hidden ocean, it could suggest that other icy planets and moons in our solar system or elsewhere could be similar, which would also expand the possibilities for life. “It expands our view of what is a habitable world and what is not,” says Lainey. “Mimas shows you that even a corpse that doesn't seem to harbor anything could have life one day.”