Meet Mimas, The Newest ‘Ocean World’ In Our Solar System That Instantly Widens Our Hunt For Life

That’s no moon, it’s an ocean world—and it could greatly expand the number of potentially habitable worlds thought to exist.

Published in the journal Icarus this week is a new paper that reveals that Mimas—the innermost moon of Saturn and a lookalike of the Death Star space station in Star Wars—may have a liquid internal ocean.

If it’s true then the tiny moon, which measures 123 miles in diameter, joins a bevy of other “ocean worlds” in the outer Solar System that are thought to contain underground oceans.

However, Mimas is special—so special that it could greatly increase the chances of habitable worlds existing elsewhere in our galaxy.

“Ocean worlds”—typically moons but also dwarf planets—with underground seas beneath layers of rock appear to be relatively common in our Solar System. They’re also referred to as interior water ocean worlds (IWOWs).

Others IWOWs in the Solar System are thought to include:

• Enceladus and possibly Titan, also moons of Saturn.


• Europa, Callisto and Ganymede, moons of Jupiter.


• Triton, moon of Neptune.


• Ceres and Pluto, dwarf planets.

However, Mimas could be a special case.

“If Mimas has an ocean, it represents a new class of small, “stealth” ocean worlds with surfaces that do not betray the ocean’s existence,” said lead author Dr. Alyssa Rhoden, a specialist in the geophysics of icy satellites at the Southwest Research Institute (SwRI) in San Antonio, Texas.

The discovery pivots on the detection of a curious libration—a wobble—in the rotation of Mimas. The newly analyzed archive data comes from NASA’s probe Cassini, which orbited Saturn from 1997 through 2017.

Such an oscillation often indicates a geologically active world that’s able to support an internal ocean.

“Because the surface of Mimas is heavily cratered, we thought it was just a frozen block of ice,” said Rhoden. “IWOWs, such as Enceladus and Europa, tend to be fractured and show other signs of geologic activity. Turns out, Mimas’ surface was tricking us.”

So what’s the explanation? Rhoden thinks that tidal heating within Mimas must be just enough to keep any underground ocean from freezing, but not enough to melt its icy shell. The team’s models indicate that the icy shell on Mimas could be 14 to 20 miles thick over a liquid ocean.

However, there is uncertainty. “Although our results support a present-day ocean within Mimas, it is challenging to reconcile the moon’s orbital and geologic characteristics with our current understanding of its thermal-orbital evolution,” said Rhoden.

Only a spacecraft at Mimas could confirm it. Such a mission would also have other benefits. “Evaluating Mimas’ status as an ocean moon would benchmark models of its formation and evolution [but it would also] help us better understand Saturn’s rings and mid-sized moons,” said Rhoden.

An SwRI scientist has discovered that Saturn’s small moon Mimas (left) likely has something in ... [+] common with its larger neighbor Enceladus: an internal ocean beneath a thick icy surface. Thought to be a frozen inert satellite, Mimas is now considered a “stealth” ocean world with a surface that does not betray what lies beneath. This discovery could greatly expand the number of potentially habitable worlds thought to exist.

NASA / JPL-Caltech / Space Science Institute

She added that a close-up study of Mimas would also help planetary scientists understand the prevalence of potentially habitable ocean moons in general, but particularly at Uranus where its moons Titania, Oberon, Umbriel, Ariel and Miranda are also thought to possibly host underground oceans.

The discovery of underground oceans in the outer Solar System has arguably been one of the biggest findings in planetary science in the 21st century.

It has profound consequences.

It was once thought that to be habitable a planet or moon needed to have liquid water on its surface—like Earth—and therefore within a certain range of the host star.

Since ocean worlds are found over a wide range of distances from our Sun it could be that the number of worlds in the Milky Way galaxy that contain liquid water in some form—and so could be habitable by some kind of life—are far greater than once thought.

“Our new understanding has greatly expanded the definition of a potentially habitable world in our Solar System and beyond,” said Rhoden.

Wishing you clear skies and wide eyes.

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