A moon of Saturn that looks like 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, a ball of ice 250 miles wide, becomes the youngest member of an exclusive club, joining Saturn’s Titan and Enceladus and Jupiter’s Europa and Ganymede as moons known to harbor subsurface oceans.
“It’s quite a surprise,” says Valéry Lainey, an astronomer at the Observatoire de Paris in France. “If you look at the surface of Mimas, there is nothing to betray an underground ocean. It is by far the most unlikely candidate.”
Features in Mimas’ orbit have led astronomers to entertain two possibilities: either it contains an elongated core encased in ice, or an internal ocean that has allowed its outer shell to move independently of the core.
By analyzing thousands of images of NASA’s Cassini mission to Saturn, Lainey and his colleagues reconstructed the exact spin and spin motion of Mimas as it orbited the gas giant. Their calculations showed that Mimas must have a hidden subsurface ocean to move the way it does.
“There is no way to explain both the spin of Mimas and the orbit with a rigid interior,” Lainey said. “You definitely have to have a global ocean on which the icy shelf can slide.”
Their calculations suggest that an ocean 45 miles deep lies beneath Mimas’ 15-mile-thick icy shell, with near-seafloor temperatures reaching tens of degrees Celsius. The sea would make up more than half of Mimas’ volume. Details are published in Earth.
By astronomical standards, Mimas’ ocean appears to be relatively young, having formed in the past 25m years when powerful tidal forces exerted by Saturn deformed Mimas’ core, heating it like a massaged squash ball. The heated core then melted overlying ice and created an ocean in the Saturnian moon. The surface remains heavily battered, with one giant impact producing the Herschel Crater, named after the astronomer William Herschel who first identified Mimas in 1789.
The discovery of global oceans in moons around Saturn and Jupiter has sparked a flurry of interest from space agencies eager to explore their potential to host life. More than 100 geysers have been spotted on Enceladus where vapor blows through surface fractures. If life ever developed on the small moon, the plumes could drive extraterrestrial microbes into space where they could be detected by visiting missions.
Lainey said that since Mimas contains water in contact with hot rock, he would not rule out the existence of life there. But if the hidden ocean is only tens of millions of years old, life may not have had a chance to emerge. “Whether it’s too young, nobody knows,” he said. “I would say: why not?”
David Rothery, a professor of planetary geosciences at the Open University, said that even if Mimas hosted an underground ocean, there were easier places to look for life beyond Earth. “There is no indication of a connection between the internal ocean, where life can survive, and the surface or space where traces of life can be detected and sampled, as we have done in the plumes of Enceladus, and hope to surface to do. or in plumes at Europe,” Rothery said. “If there was life inside Mimas, it would be hidden by more than 20 km of uninterrupted ice.
“If the ocean only existed for 25 million years, that might not have been enough time for life to start and establish itself. Europa and Enceladus are much more promising candidates.”