Saturn’s ‘Death Star’ moon was hiding a secret: an underground ocean

Evidence of a deep global ocean beneath the crust of Saturn's moon Mimas—sometimes called the "Death Star"—has surprised astronomers because it's such an unlikely place to find one.

A new analysis of observations by the Cassini spacecraft, which explored the Saturn system from 2004 to 2017, indicates that the rocking motion made by Mimas as it orbits—a phenomenon known to astronomers as libration—is caused by a liquid ocean beneath its surface, rather than a completely solid core.

The new discovery adds to the handful of verified subsurface oceans in our solar system. It also raises the possibility that life could have evolved there as well.

“This is a huge surprise, to be honest,” says astronomer and lead author of the new study Valéry Lainey, who studies the dynamics of Saturn’s moons at the Observatoire de Paris in France.

An unlikely ocean world

Mimas has been called the "Death Star" because a giant impact crater on one side makes it look like the space station from Star Wars; a crater on Earth of comparable size would be wider than Canada. One of the many moons surrounding Saturn—146 at last count—Mimas is unusual because it rocks heavily from side to side during its orbit around the planet.

(Earth’s oceans may hold the key to finding life beyond our planet.)

Such libration could be explained in one of two ways: either Mimas had an extremely elongated core, shaped like a flattened football; or it had a global ocean below its surface.

Lainey was part of a team that first proposed Mimas could have a hidden ocean in a 2014 paper in Science.

But the idea was largely dismissed, in part because there are no signs of such a thing on its surface—unlike Enceladus, another moon of Saturn, which sprays water from its inner ocean into space.

A new paper by the same team, however, published today in Nature, closely studied how the libration changes the orbit of Mimas—and establishes that the moon indeed has a subsurface ocean.

The researchers suggest the ocean is kept from freezing by heat from tidal forces during the moon’s orbit around Saturn.

And it is a substantial body of water, too: Mimas is relatively small, but its subsurface ocean makes up about half its mass, Lainey says.

“Many people will now look at this and say: ‘Oh my gosh, there is truly a global ocean there,’” Lainey says.

The hunt for extraterrestrial life

Astronomers had previously found clear signs of subsurface oceans on only two of Saturn’s moons—Enceladus and Titan—and on Jupiter’s moons Europa and Ganymede.

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As earlier studies noted, where you find liquid water you often find life—and so subsurface oceans are some of the best places to look for extraterrestrial life, which scientists speculate might have evolved around hydrothermal vents from the moons’ cores.

(If alien life exists in our solar system, it may look like this.)

The latest research suggests that the ocean on Mimas must be young—between 2 and 25 million years old, which is almost no time at all in celestial terms.

That might seem too short a time for life to have evolved there.

But Lainey says the ocean on Mimas, which is relatively warm and may have ample supplies of raw chemicals, might be as good a place as any for it to have evolved.

He acknowledges, however, that it would be difficult to drill there and find out: although the ocean on Mimas seems very deep—perhaps more than 40 miles deep in some places—its top lies up to 18 miles beneath an outer crust of rock and ice.

A hint at ocean evolution

Mimas could also help scientists understand how other alien oceans developed.

The moon is only a little smaller than Enceladus and consists of the same types of rock and ice, Lainey says, which implies that their chemistry and geology are similar.

But while the subsurface ocean on Enceladus is about a billion years old, the ocean on Mimas is much younger—perhaps giving scientists a window into the early development of Enceladus, he says.

The giant impact crater that makes Mimas look like the Death Star is also a sign that its ocean must be relatively young, says planetary scientist Alyssa Rose Rhoden of the Southwest Research Institute in Boulder.

(Explosions may have created weird lakes on Saturn's largest moon.)

The crater—named Herschel after astronomer William Herschel, who discovered Mimas in 1789—is thought to have formed hundreds of million years ago when an object several miles across crashed into the moon.

Rhoden, who wasn’t involved in the latest study but is a co-author of an article in Nature about it, says the Herschel impact would have punched right through the crust of Mimas if a subsurface ocean had existed; and so the fact that Herschel looks like it does means there was no ocean at the time.

The search for more alien oceans

The discovery also strengthens the idea that subsurface oceans might exist elsewhere in our solar system, particularly on several moons of Uranus and even on some Kuiper Belt objects, which circle the sun beyond Pluto.

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