Astronomers have found the best evidence of a vast, young ocean beneath the icy outer surface of Saturn’s Death Star-like minimoon.

A French-led team analyzed changes in Mimas’ orbit and spin and reported Wednesday that a hidden ocean 12 to 18 miles (20 to 30 kilometers) beneath the frozen crust is more likely than an elongated rocky core. The scientists based their findings on observations from NASA’s Cassini spacecraft, which observed Saturn and its more than 140 moons for more than a decade before plunging into the ringed planet’s atmosphere and burning up in 2017.

Only 250 miles (400 kilometers) in diameter, the heavily cratered moon lacks the typical fissures and geysers seen on Saturn’s Enceladus and Jupiter’s Europa.

“Mimas was probably the most unexpected place to find a global ocean and liquid water in general,” co-author Valerie Laney of the Paris Observatory said in an email. “So it looks like a potentially habitable world. “But no one knows how long it will take for life to emerge.”

The results were published in the journal Nature.

The ocean is believed to fill half of Mimas’ volume, according to Laney. However, considering the small size of the Moon, it constitutes only 1.2-1.4% of the Earth’s oceans. Despite its small size, Mimas has the second largest impact crater among the Solar System’s moons; This leads to comparisons to the fictional Death Star space station in Star Wars.

“The idea that relatively small icy moons could harbor young oceans is exciting,” Matia Chuk of the SETI Institute and Alyssa Rose Rhoden of the Southwest Research Institute wrote in an accompanying editorial. They were not part of the study.

The overall temperature of this subsurface ocean, whose age is estimated to be between 5 and 15 million years old and is too young to mark the Moon’s surface, would be near freezing, Laney said. However, according to him, the water temperature on the seabed can be much higher.

Co-author Nick Cooper, of Queen Mary University of London, said the presence of an “extremely young” liquid water ocean makes Mimas a prime candidate for studying the origins of life. Discovered by British astronomer William Herschel in 1789, Mimas takes its name from a giant in Greek mythology.