Scientists detect something really unexpected beneath Saturn's 'Death Star' moon

Mimas, Saturn's cryptic-looking moon, is awfully deceptive.

The small moon is dominated by an 80-mile-wide crater, giving it the appearance of the grim Star Wars Death Star — a space station equipped with a planet-destroying weapon. But otherwise, Mimas appears as a frozen chunk of ice.

Other moons, like Saturn's Enceladus and Jupiter's Europa, have cracked surfaces or telltale plumes shooting from the ground, which are compelling evidence for sub-surface oceans. Mimas doesn't outwardly offer hints of an ocean.

"When you look at Enceladus and Europa, there's clearly an engine in these moons that is running," Alyssa Rhoden, a planetary scientist who researches ocean worlds, told Mashable. "When you look at Mimas it's the opposite — it can't possibly be an ocean world."

Or so Rhoden thought. Looks are deceiving.

In new research published in the planetary science journal Icarus, Rhoden and her coauthor describe how they unexpectedly found evidence for an ocean beneath Mimas' icy shell. The moon may not be a frozen chunk of ice, after all.

The giant crater on Saturn's moon Mimas. It's called the Herschel Crater. Credit: NASA / JPL / Space Science Institute

Plumes of water ice on Saturn's moon Enceladus, captured by the Cassini spacecraft. Credit: NASA / JPL / Space Science Institute

Mimas does have an attribute that could allow it to harbor an ocean. Its orbit around Saturn is highly eccentric, meaning it gets tugged and stretched as it swings close to the powerful gravitational force of the planet and then orbits farther away. (Each orbit takes just 22 hours and 36 minutes!) This process, called "tidal heating," creates vast amounts of heat in ocean worlds like Europa.

With this reality in mind, Rhoden, a principal scientist at the Southwest Research Institute, a science organization, followed up on an earlier observation of a tiny wobble in Mimas' orbit around Saturn. An icy sea sloshing inside Mimas could potentially trigger this wobble. Might tidal heating have melted enough ice inside Mimas to create a sea? If so, there couldn't be too much heat (that would melt through the icy shell) nor too little heat (then the ocean would freeze).

The researchers calculated that if there was indeed an ocean inside Mimas large enough to trigger its wobble, the water would exist beneath an icy shell some 14 to 20 miles thick. So they ran computer simulations of how the heating (from tidal heating) would impact the ice on Mimas. Unexpectedly, it showed an ocean under 14 to 20 miles of solid ice.

"We came up with exactly the right number," Rhoden said.

This isn't, Rhoden emphasizes, nearly certain proof that Mimas harbors an ocean. But there's now compelling evidence that an ocean could exist there, with the information available.

Oceans, as we know on Earth, are immensely diverse places, brimming with life. "Water is at the top of the list of ingredients that make life possible," writes NASA. And on ocean worlds like Europa, tidal heating may ultimately allow life to thrive, though there's still zero evidence of life outside Earth. "Tidal heating could be powering a system that cycles water and nutrients between the moon's rocky interior, ice shell, and ocean, creating a watery environment rich with chemistry conducive to life," NASA added.

Sometimes, the search for life gets narrowed down to "habitable zones" in solar systems, which are the relatively narrow regions where liquid water could exist on a planet's surface. Earth, for example, exists in our solar system's habitable zone.

Europa and Enceladus, located in the frosty regions of our solar system, are well outside the habitable zone. But these worlds harbor oceans. And potentially, life may have emerged there.

"Habitability is not one swath of a solar system," said Rhoden. "There's a lot of different ways life might be able to emerge."