An Icy Moon of Saturn May Be Hiding a Vast Ocean Under Its Crust, Surprising Astronomers

Saturn’s smallest moon, called Mimas, may have a hidden ocean buried deep beneath its frozen surface, according to new research published Wednesday in the journal Nature. Though moons with global oceans have been sighted in the solar system before, the idea that water could be lurking under Mimas’ crust was unexpected. 

“It’s quite a surprise,” says Valery Lainey, an astronomer at the Paris Observatory and lead author of the study, to the Guardian’s Ian Sample. “If you look at the surface of Mimas, there’s nothing that betrays a subsurface ocean. It’s the most unlikely candidate by far.”

Nicknamed the “Death Star moon” due to its massive surface crater that makes it resemble the Star Wars space station, Mimas has a thick, icy crust and a dilapidated, crater-ridden surface. One of at least 146 moons orbiting Saturn, Mimas is notable for its unusual motion: The planet-facing side of the moon rocks back and forth during orbit. This movement, called libration, led astronomers to two possible explanations: Mimas either has a frozen, oblong core, or it is harboring a subsurface ocean. 

Lainey and his team asserted the latter as early as 2014 in a paper published in the journal Science. But astronomers widely disregarded that theory. After all, Enceladus, a moon of Saturn known to have a subsurface ocean, spurts geysers of water out of fractures in its crust—but the frigid, quiet Mimas stands in stark contrast to that.  

“Given how different Mimas looks from confirmed ocean moons like Enceladus, most of us figured that the frozen model was probably correct,” says Alyssa Rhoden, a planetary scientist at the Southwest Research Institute who was not involved in the study, to Kimberly M. S. Cartier of Eos. 

The team, however, remained steadfast in their research, analyzing thousands of images taken by NASA’s Cassini spacecraft, both of Mimas and of 18 other Saturnian moons. Finally, they had a breakthrough: The rotation and orbital motion of Mimas around Saturn shifted slightly over 13 years. Simulations determined that kind of movement could not be sustained by a solid core, unless the moon was extremely elongated, which it isn’t. 

“There is no way to explain both the spin of Mimas and the orbit with a rigid interior,” says Lainey to the Guardian. “You definitely need to have global ocean on which the icy shell can slip.” 

The global ocean in question is believed to lie under approximately 15 miles of ice and reach a depth of 45 miles. Powerful tidal forces radiated by Saturn heat the moon’s interior, and the friction of the water hitting the core also generates warmth, preventing the liquid ocean from cooling into a solid form. 

“At least 50 percent of the volume of Mimas is filled by liquid water,” says Lainey to Space.com. “This is a huge amount of liquid water for the size of the satellite.” 

The study indicates that Mimas’ ocean could be anywhere between 2 million and 25 million years old, which, by cosmic standards, is young. As such, the ocean hasn’t breached through the surface in the form of geysers or visibly altered the moon’s exterior, according to Eos. However, its sheer existence provides unprecedented insight into conditions of habitability, as its warm waters and supply of raw chemicals could foster life, reports National Geographic’s Tom Metcalfe. 

“Mimas is a small object that looks extremely cold, with no geological activity, and you would never expect any geophysical activity inside like heating, or contact between water and with silicates in its rocky core,” says Lainey to Space.com’s Robert Lea. 

Still, some scientists point out that Mimas’ young ocean remains concealed deep beneath its crust. Other solar system moons with subsurface oceans, such as Europa and Enceladus, have revealed geysers or water vapor above their surfaces that may present an easier avenue to track traces of life.

“[On Mimas,] 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 what we have done in the plumes of Enceladus, and hope to do on the surface or in plumes at Europa,” says David Rothery, a planetary geoscientist at the Open University in the United Kingdom who was not involved in the study, to the Guardian. “If there were life inside Mimas, it would be hidden by more than 20 kilometers of unbroken ice.” 

But the discovery of Mimas’ unexpected global ocean suggests astronomers could benefit from expanding beyond conventional thoughts about what kind of object can hold vast amounts of water—and which worlds can support life.

“I would emphasize that Mimas definitely doesn’t look like the sort of object that could have habitability,” Lainey tells Space.com. “So maybe the conclusion is that if this object can be habitable, who knows what other kind of object may be habitable?” 

Scientists speculate that subsurface oceans may exist in other unorthodox places throughout the solar system, such as on the moons of Uranus or Kuiper Belt objects orbiting Pluto. 

“It’s a bit different, but yes—you can expect to have liquid water there after all, on many objects,” Lainey says to National Geographic. “Even Mimas, the most unlikely place in the solar system, has a global ocean.”

Catherine Duncan | READ MORE

Catherine Duncan is an intern with Smithsonian magazine.