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Saturn's 'Death Star' moon Mimas may have an underground ocean scientists never believed could exist

Astronomers have discovered that a tiny moon of Saturn, named Mimas, may harbor a hidden liquid ocean beneath its thick icy shell and may thus have the conditions for habitability.

This shocking finding radically changes the definition of what an ocean moon can be, and could ultimately redefine our search for alien life on moons in the solar system. That's because, at first appearance, Mimas — nicknamed the 'Death Star' because a large crater means it resembles the Empire's space station in Star Wars — doesn't look like the kind of body scientists would expect to support an ocean. In fact, it doesn't even look capable of supporting such a vast body of liquid at all.

The team behind the watery discovery estimates the ocean is located around 12 to 18 miles (20 to 30 kilometers) beneath the ice crust surface of Mimas; the researchers also believe it is relatively young, having only appeared between 2 million and 25 million years ago. Yet, despite remaining hidden for millions of years, the ocean appears to comprise at least half of the moon's volume.

"The major finding here is the discovery of habitability conditions on a solar system object which we would never, never expect to have liquid water," Valery Lainey, discovery team member and a scientist at the Observatoire de Paris, told Space.com. "It's really astonishing."

The discovery makes Mimas even more similar to its Saturnian moon sibling, Enceladus, which scientists had already known has a subsurface ocean. Both moons exist at similar distances from the gas giant planet and are similar in size, with the ice-covered Enceladus measuring around 310 miles (500 kilometers) across and the similarly frigid Mimas sitting slightly smaller at 246 miles (396 km) across.

One key difference between the two moons is that, whereas the ocean of Enceladus breaches its surface in the form of massive jets and plumes, the sea of Mimas has yet to crack its icy crust.

That means that, while NASA's Cassini spacecraft was able to fly through plumes of ice spraying from Enceladus to confirm its oceans and even discover some of the complex molecules contained within, the spacecraft was not able to do any such thing for Mimas.

"It's really surprising we haven't seen anything, but the thickness of the icy shell of Mimas is enough to maintain this ocean without any significant activity betraying it for millions of years," Lainey continued. "That's why Cassini did not find anything at the surface of Mimas."

However, that doesn't mean Cassini, which spent 13 years in Saturn's system before purposefully crashing into the gas giant in 2017, wasn't key in assisting in the discovery of the ocean of Mimas.

Mimas' 'wobble'

Lainey and colleagues discovered their initial hints of Mimas having a buried liquid ocean when they used Cassini data to investigate a break in the infamous rings of Saturn called the "Cassini division."

In 2010, while aiming to discover if a change in Mimas' orbit could have caused the Cassini division, the team noticed a strange shift in both the moon's rotation and its orbit. The team determined, in 2014, that these large librations were either the result of the Saturnian moon having a misshapen, solid rocky core — or, a subsurface ocean that would allow its outer shell to oscillate independently of its core.

The breakthrough came when the team finally modeled the motion of Mimas, and determined a rocky core could only be responsible for the observations if it was elongated and flat like a pancake. Obviously, this did this not match what the team was seeing in real life, but moreover, the way Mimas' orbit has evolved since 2014 also offered support to the global subsurface ocean hypothesis.

"There is no rigid interior that can be compatible with the rotation and the orbital evolution of Mimas," Lainey said. "It is a relief to eventually succeed in demonstrating that that is the solution."

A waterlogged moon

Not only was the team able to determine that the oceans have only been present for a few million years (due to the fact that Mimas' orbit remains flattened, or eccentric) but the crew was also able to calculate just how much water is likely present in the moon's oceans.

"At least 50% of the volume of Mimas is filled by liquid water," Lainey said. "This is a huge amount of liquid water for the size of the satellite."

This water appears to be grinding against the rocky core of Mimas, simultaneously getting heated by the friction this action generates. This interaction also gives rise to what Lainey describes as "interesting chemistry" likely developing on the Saturnian moon — right now.

Water and rock interactions are thought to have played a vital role in the origins and continued existence of life on Earth, meaning such chemistry on Mimas is indeed an exciting prospect for investigations of life and habitability in the solar system.

"Mimas is a small object that looks extremely cold, with no geologic activity, and you would never expect any geophysical activity inside like heating, or contact between water and with silicates in its rocky core," Lainey said. "Finding this happening is really astonishing."

For future investigations, Lainey said he would love to land a spacecraft on the surface of Mimas, or even Enceladus for that matter.

"I'm pretty sure any space mission to Enceladus will also visit Mimas as they are extremely close, and they are extremely similar ocean systems but at different times in their evolution," he explained.

But with NASA's planned Orbilander set to leave Earth in 2038 and arrive at the latter moon in 2050, such a project is still a long, long time away.

In the meantime, Lainey intends to investigate Mimas from Earth to learn more about how its temperature has evolved, how the presence of this ocean has impacted the Saturnian moon's orbit, and what effect this has had on the rings of Saturn and the gas giant's other moons. This could help better calculate the age of Mimas' oceans.