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Even Tiny Mimas Seems to Have an Internal Ocean of Liquid Water

Data from the Cassini mission keeps fuelling discoveries. The latest discovery is that Saturn’s tiny moon Mimas may have an internal ocean. If it does, the moon joins a growing list of natural satellites in our Solar System that may harbour liquid water under their surfaces.

Worlds with interior oceans are called Interior Water Ocean Worlds (IWOWs). If the new paper announcing this discovery is correct, tiny Mimas will join worlds like Europa, Enceladus, Titan—and maybe Pluto—on the growing list of IWOWs in our Solar System. How did scientists discover that Mimas is a potential IWOW?

It’s all thanks to the Cassini mission to Saturn, a collaboration between NASA, the ESA, and the Italian Space Agency. That mission ended with a plunge into Saturn in 2017, called the Grand Finale. But before it concluded its mission with a purposeful plunge into the gas giant’s atmosphere, the spacecraft’s instruments detected an unusual libration in Mimas’ rotation.

Librations like the one found in Mimas’ rotation are often indicators of a geologically active world with a subsurface ocean.

A new paper published in the journal Icarus explains the findings. Its title is “The case for an ocean-bearing Mimas from tidal heating analysis,” and it’s available online at Science Direct. The authors are Alyssa Rose Rhodena and Matthew E. Walker. Rhodena is an expert in the geophysics of icy satellites at the Southwest Research Institute, and Walker is an Associate Research Scientist at the Planetary Science Institute.

If Mimas is an IWOW, it would be different from others in the Solar System like Europa and Enceladus. Those ocean-bearing moons show geological activity that Mimas lacks. The authors point out that the physical librations Cassini sensed “… can be explained by either a non-hydrostatic core or a global, liquid water ocean beneath a 24–31 km thick ice shell.”

The moons Enceladus (L), Europa (M), and Mimas (R,) not to scale. While the surfaces of Enceladus and Europa show clear signs of geological activity like fractures and troughs, Mimas’ surface is different. It’s covered in craters and shows no signs of activity. Image Credits: Enceladus: By NASA/JPL/Space Science Institute. Europa: By NASA/JPL/DLR. Mimas: By NASA / JPL-Caltech / Space Science Institute.

“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 SwRI’s Dr. Alyssa Rhoden in a press release. Rhoden studies the geophysics of icy satellites, particularly those containing oceans, and the evolution of giant planet satellites systems.

The word ‘stealth’ certainly applies to Mimas compared to other IWOWs like Enceladus and Europa. Mimas’ cratered surface gives no hint of an ocean underneath, though researchers suspected it might be a large chunk of ice rather than rock. Its density is too low to be all rock, though it may contain some rock.

“Because the surface of Mimas is heavily cratered, we thought it was just a frozen block of ice,” Rhoden said. “IWOWs, such as Enceladus and Europa, tend to be fractured and show other signs of geologic activity. Turns out, Mimas’ surface was tricking us, and our new understanding has greatly expanded the definition of a potentially habitable world in our solar system and beyond.”

Mimas is close enough to massive Saturn to be shaped by the planet’s powerful gravity. Saturn’s gravity is strong enough to stretch the small moon into an ellipsoidal shape. Those same tidal forces cause heating, along with heating from rotational energy. There’s a delicate balance here; the heating must be powerful enough to melt the interior into liquid but not so powerful that the surface melts.

The researchers developed numerical models based on tidal heating models to develop a plausible explanation for the librations that Cassini sensed. They concluded that an icy shell 24 km to 32 km (14 to 20 miles) thick covers Mimas’ interior ocean. Compare that to Europa’s icy shell, which is probably 10–30 km (6–19 miles) thick, and to Enceladus’ which is probably 30 to 40 km (19 to 25 miles) thick.

“Most of the time, when we create these models, we have to fine tune them to produce what we observe,” Rhoden said. “This time evidence for an internal ocean just popped out of the most realistic ice shell stability scenarios and observed librations.”

This is still a preliminary discovery, and a visit from a spacecraft can confirm it. It needn’t visit Mimas itself; a visit to any IWOW would clarify the critical issue of heat flow against ice shell thickness. From the press release: “The team also found that the heat flow from the surface was very sensitive to the thickness of the ice shell, something a spacecraft could verify. For instance, the Juno spacecraft is scheduled to fly by Europa and use its microwave radiometer to measure heat flows in this Jovian moon.”

NASA’s Juno will perform a flyby of Europa near the end of 2022 at a distance of about 320 kilometres (200 miles). Data from the flyby will help researchers understand how heat flow affects icy shells. The dynamics between heat flow and frozen shells on IWOWs will grow in importance when NASA’s Europa Clipper mission launches in 2024. The spacecraft will reach the Jovian system in April 2030 and enter a long looping orbit around Jupiter, performing repeated flybys of Europa.

The Europa Clipper will study Europa in detail, and its findings will tell us a lot about IWOWs. Will data from that mission help scientists understand Mimas better? Probably. But for now, the data from the Cassini mission is what scientists have to work with. According to Rhoden, there are still some unreconciled aspects in that data regarding Mimas. If Rhoden and other researchers can reconcile some of the challenges in the data, they’ll learn more about other ocean moons and the systems they belong to. It’s like facing a locked door. Behind the door is a greater understanding of IWOWs, and Mimas is the key that unlocks it.

“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,” Rhoden said. “Evaluating Mimas’ status as an ocean moon would benchmark models of its formation and evolution. This would help us better understand Saturn’s rings and mid-sized moons, as well as the prevalence of potentially habitable ocean moons, particularly at Uranus. Mimas is a compelling target for continued investigation.”

Uranus has five large moons, and images from Voyager 2’s flyby in 1986 showed that they’re roughly equal parts ice and rock. Voyager 2 didn’t have the same capable instruments as Cassini, so more detailed data isn’t available. But the images did show evidence of cryovolcanism, where liquid erupts through the surface and then freezes. Enceladus has the same eruptions, sending plumes of material from the subsurface ocean out into space.

This image is a false-colour artist’s illustration of the cryovolcanic plumes erupting from Saturn’s moon Enceladus. Image Credit: NASA/ESA

The Solar System’s gas giants might all have IWOWs. The outer giant planets could have enough mass to induce tidal heating in their moons, and that’s something scientists are keen to examine. Neptune has 14 moons, with Triton by far the most massive. Triton’s crust is mostly water ice, and the moon is geologically active like Europa and Enceladus. It’s likely a captured Kuiper Belt Object (KBO.) When Voyager 2 approached the moon in 1989, it saw geyser-like plumes 8 km (5 miles) high.

These are three images of a cryovolcanic plume on Triton taken about 45 minutes apart, from left to right, by Voyager 2 on August 26th, 1989. They show the geyser-like volcanic plume spewing an 8km (5 miles) tall cloud of fine, dark particles into Triton’s thin atmosphere. The cloud is drifting downwind to the right for a distance of roughly 150 kilometres (about 100 miles). Image Credit: NASA/JPL

Each time scientists discover another IWOW, it brings up the possibility of life. For life to start, evidence shows that there needs to be an active interface between water and rock. Mimas’ density suggests it could have a rocky core. If there’s heat, then interactions between water and rock can supply the necessary ingredients for life.

Mimas is small, with a surface area about the size of Spain. It’s only 396 kilometres (246 mi) in diameter, and it’s the smallest known body that’s rounded due to self-gravity. If a world this small can harbour a subsurface ocean, then that expands the list of other objects that could have them, and our understanding of potentially habitable worlds will grow, too.

Does Mimas have an ocean? Could it harbour life? Are IWOWs far more common than we expected?

For now, we don’t know.

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Evan Gough

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