Researchers have discovered that Mimas, one of Saturn’s moons, has a hidden ocean buried miles beneath its surface. Despite its small size of 250 miles, it is now a part of a select group of moons known to have subterranean oceans, including Saturn’s Titan and Enceladus and Jupiter’s Europa and Ganymede. Mimas was an unlikely candidate for hiding a subsurface ocean because there was nothing on its surface to suggest it. However, the astronomers found peculiarities in its orbit that led them to believe it had either an elongated core shrouded in ice or an internal ocean that allowed its outer shell to shift independently of the core.
Scientists analyzed thousands of images from NASA’s Cassini mission to Saturn and reconstructed the precise spin and orbital motion of Mimas as it looped around the gas giant. Their calculations showed that Mimas must possess a hidden subsurface ocean to move the way it does. They believe the ocean is 45 miles deep, buried beneath a 15-mile-thick icy shell, and accounts for more than half of Mimas’ volume. The researchers suggest that the ocean formed in the past 25 million years when powerful tidal forces exerted by Saturn deformed Mimas’s core, warming it like a massaged squash ball. The heated core then melted overlying ice, creating the ocean inside the Saturnian moon.
The discovery of global oceans in moons around Saturn and Jupiter has raised interest among space agencies eager to explore their potential for harboring life. More than 100 geysers have been spotted on Enceladus where vapor blasts through surface fractures. If life ever evolved on the tiny moon, the plumes could propel extraterrestrial microbes out into space where they could be detected by visiting missions.
Lainey said that since Mimas contained water in contact with warm 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 did harbour a subsurface ocean, there were easier places to search for life beyond Earth. “There’s no indication of a connection between the internal ocean, where life could survive, and the surface or space where traces of life could be detected and sampled, such as we have done in the plumes of Enceladus, and hope to do on the surface or in plumes at Europa,” Rothery said. “If there were life inside Mimas, it would be hidden by more than 20km of unbroken ice.
“If the ocean has existed for only 25m years, that may not have been enough time for life to get started and established. Europa and Enceladus are much more promising candidates.”