Mimas, which is the smallest and closest to Saturn of the ringed planet's 82 moons, may contain a liquid internal ocean.
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"If Mimas has an ocean, it represents a new class of small, 'stealth' ocean worlds with surfaces that do not betray the ocean's existence," said study author Alyssa Rhoden, a scientist Southwest Research Institute in Boulder, Colorado, in a statement.
The study published last week in the journal Icarus.
Mimas was first discovered in 1789 by English astronomer William Herschel as a tiny dot near Saturn. The Voyager probes imaged the small moon in 1980, and NASA's Cassini mission conducted flybys of it while studying Saturn between 2004 and 2017.
The moon is only 115,000 miles (186,000 kilometers) from Saturn and takes just over 22 hours to complete one orbit around the planet. Mimas is covered in craters, but the largest one is 80 miles (130 kilometers) across and gives the moon its distinctive Death Star appearance.
Scientists have long been intrigued by Mimas because it's likely made almost entirely of ice. Craters scattered across the moon suggest that its surface has remained frozen for a long time.
However, before the Cassini mission came to an end in 2017, it detected an oscillation in the moon's rotation that suggested Mimas may contain a subsurface ocean.
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Our solar system is home to multiple ocean worlds, or moons where oceans exist beneath thick ice shells, including Jupiter's moon Europa and Saturn's moons Titan and Enceladus. These differ from Earth, which is at the right distance from the sun to include liquid water oceans on its surface.
Interior water ocean worlds, or IWOWs, are much further from the sun -- but they could still support life within their oceans.
"Because the surface of Mimas is heavily cratered, we thought it was just a frozen block of ice," said Rhoden, who is also the co-leader of NASA's Network for Ocean Worlds Research Coordination Network.
"IWOWs, such as Enceladus and Europa, tend to be fractured and show other signs of geologic activity. Turns out, Mimas' surface was tricking us, and our new understanding has greatly expanded the definition of a potentially habitable world in our solar system and beyond."
Mimas is tidally locked in its orbit around Saturn, meaning that the same side of the moon always faces the planet -- like our moon as it orbits Earth. The researchers believe a phenomenon called tidal heating allows for the subsurface ocean to exist on Mimas.
Tidal heating causes an internal increase of temperature in a moon due to its gravitational relationship with a planet.
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To re-create the oscillation detected in Mimas' rotation by Cassini, the researchers used models to show that tidal heating occurring in the small moon is enough to maintain an ocean beneath an ice shell that is between 14 to 20 miles (22.5 to 32 kilometers) thick.
This finding could be useful as future spacecraft study ocean worlds in our solar system, but it also shows that Mimas and Saturn's other moons may be worth observing more moving forward. Future spacecraft could confirm that Mimas is indeed one of these interior water ocean worlds.
"Evaluating Mimas' status as an ocean moon would benchmark models of its formation and evolution," Rhoden said.
"This would help us better understand Saturn's rings and mid-sized moons as well as the prevalence of potentially habitable ocean moons, particularly at Uranus. Mimas is a compelling target for continued investigation."
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