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Saturn’s "Death Star" Moon Might Have A Secret Internal Ocean


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockJan 20 2022, 15:20 UTC
That is definitely a moon! Image Credit: NASA / JPL-Caltech / Space Science Institute

That is definitely a moon! Image Credit: NASA / JPL-Caltech / Space Science Institute 

Titan and Enceladus might not be the only moons of Saturn with an ocean. Mimas, the death-star-shaped moon of the ring planet might also hide an internal ocean under a layer of ice and rock. The potential discovery come about as researchers were trying to prove the opposite, that Mimas was just an inert frozen rock.

Mimas is the innermost of the large moons of Saturn and the smallest object in the solar system to have a spherical shape. During the late part of NASA’s Cassini mission, the spacecraft got close enough to study the little world to measure its libration – a wobble in the way the Moon rotates during its orbit.


This measurement, first taken in 2014 has led to speculation of a possible ocean but the lack of a fractured surface like we see on Enceladus or Jupiter’s moon Europa threw astronomers off the scent. Different ideas were then suggested. Now, work published in the journal Icarus shows a model where Mimas has both an uncracked thick surface and a deep hidden ocean.

“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,” lead author Dr. Alyssa Rhoden, a specialist in the geophysics of icy satellites at the Southwest Research Institute, said in a statement.

The model is based on the fact that tidal processes keep the moon heated inside. But it has to contend with two facts: The tidal heating has to be big enough to keep the ocean liquid and, at the same time, small enough to keep the surface unfractured.

When the team plugged in numbers that are reasonable for a world like Mimas, they found that it is possible for a thick shell of 24–31 kilometers (14 to 20 miles) to exist over a liquid ocean.


“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.”

An important result from the model is that the heat flow from the surface was linked to the thickness of the ice shell. A measurement such as this could be taken by a spacecraft studying an icy moon. When Juno flies by Europa in September, or when upcoming missions to Europa and Ganymede get there over the next decade, they would be able to make such measurement. This might provide more indirect evidence of an ocean on Mimas. Although it won’t be confirmed until we go back there.

“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.”

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