Mimas, one of Saturn’s moons, is known for its striking resemblance to the Death Star from the Star Wars movies. But on Wednesday, researchers announced that they had discovered a new feature: a liquid ocean beneath its surface, Reuters reported.

Astronomers announced Wednesday that data obtained by NASA’s Cassini spacecraft on the rotational motion and orbit of Mimas confirm the presence of a liquid ocean about 20 to 30 kilometers below its icy surface.

An equally amazing discovery is that the ocean appears to have formed recently, less than 25 million years ago, a burst at the age of the solar system, and most likely 5 or 15 million years ago. For comparison, primitive life appeared in the Earth’s oceans billions of years ago.

“At first sight [Mimas] it’s the most unlikely place in the solar system to find liquid water,” said astronomer Valery Laney of the Paris Observatory, lead author of the study published in the journal Nature.

Mimas “looks old and inactive, with a huge number of craters,” says the astronomer, adding that “at the moment, nothing suggests the existence of an ocean.”

How astronomers determined the presence of a liquid ocean under the surface of Mimas

Laney also explains that the new discovery makes Mimas the smallest moon in the Solar System with compelling evidence of a liquid ocean beneath its surface. Others are Enceladus and Titan, as well as Saturn’s natural moons, as well as Jupiter’s moons Europa and Ganymede.

There is some suspicion that such oceans may exist on other moons in the Solar System, including Callisto, which also orbits Jupiter.

The Cassini probe ended its 13-year mission to study Saturn and its moons in 2017 before plunging into the atmosphere of the planet known for its distinctive ring system.

Cassini’s observations of the icy surface of the moon Mimas did not reveal the deformations that normally signal the existence of a subsurface ocean. But the researchers were able to show that certain aspects of its orbit can only be explained by the existence of an ocean below the surface, and rule out the hypothesis of a solid subsurface.

Mimas is the seventh largest moon orbiting Saturn, a planet that has more than 100 natural moons. Some are smaller than an apartment building on Earth, but others astronomers consider worlds in miniature.

Titan, for example, is larger than the planet Mercury.

The tidal force causes the interior of Saturn’s moon to heat up

The moon Mimas, on the other hand, has an average diameter of about 400 kilometers and is not perfectly round. It also has a synchronous rotation with Saturn, meaning it always points the same hemisphere toward the planet as Earth and our Moon. Earth’s moon, however, is about 2,000 times more massive than Mimas.

Mimas’ most iconic feature is Herschel Crater, which covers a third of its surface and makes it look like the Death Star from Star Wars.

The presence of a liquid ocean beneath the surface suggests that there must be a powerful heat source inside the Moon that heated the water enough to turn it from a solid state into a liquid. Mimas moves in an elliptical orbit around Saturn at an average distance of 186,000 kilometers from the planet.

As the distance between the Moon and the planet changes, Saturn’s gravitational and tidal forces also fluctuate.

“This leads to a periodic deformation of the interior of Mimas, and some of the energy associated with these deformations is converted into heat,” says Gabriel Toby, an expert in planetary science at the University of Nantes and a co-author of the study.

The presence of liquid water inside the satellite of Saturn gives rise to new assumptions about the existence of life

Researchers say liquid water in Mimas makes up more than half of the moon’s total volume, although it’s only 1.2-1.4% of the amount of water found in Earth’s oceans. Scientists also say that the presence of liquid water so close to the moon’s core could facilitate complex chemical processes leading to the emergence of organic life.

The basic ingredients of life – heat, water and organic compounds – are believed to already exist in the Saturn system on the moon Enceladus, known for its geyser-like cryovolcanoes.

But Tobey says the fact that the liquid ocean on Mimas formed so recently “could be a problem for life to develop,” even if all its ingredients exist on the moon.

“But no one knows how long it takes for life to emerge from a favorable environment. Mimas may provide a unique opportunity to explore the first stage of the evolution of life,” he says.