We thought the Solar System was a soggier place.
The analysis suggests that one of the moons may have a liquid ocean. The new culprit is the little moon with a big crater, which gives it a resemblance to the Death Star from Star Wars.
A liquid internal ocean could be the cause of a slight wobble exhibited by the moon.
Mimas will join other Solar System moons in the category of 'IWOWs' if this is the case. It's an IWOW of a kind we've never seen before, expanding our understanding of what's possible.
"Because the surface of Mimas is heavily cratered, we thought it was just a frozen block of ice," says Rhoden.
IWOWs, such as Enceladus and Europa, tend to be fractured and show other signs of geologic activity. Our new understanding has greatly expanded the definition of a potentially habitable world in our Solar System and beyond.
There are a few places on Earth where life can thrive in darkness.
One of them is located at the bottom of the ocean and is clustered around hydrothermal vents. Life here relies on chemical reactions to synthesise food.
This became relevant to our search for extraterrestrial life when it was found that Jupiter's moon Europa and Enceladus had liquid oceans beneath their icy crusts.
Geological activity deep within the moons creates enough heat to keep water below the surface from freezing.
Mimas did not seem to belong to this group of moons. It's closer to the moon and has a less active moon, but it should experience stronger tides.
Scientists concluded that Mimas was probably frozen solid, and thus less prone to change.
The problem of its libration, or physical 'wobble', remained. It shouldn't wobble in the same way if Mimas was solid.
The moon's libration suggested that it had a differentiated core or liquid ocean that prevented the core from being rigidly connected to the surface.
Rhoden and her team wanted to find out if there was a liquid ocean. They needed to keep water inside the moon liquid while maintaining a very thick frozen outer shell.
Rhoden explains that most of the time when we create these models, we have to fine-tune them to produce what we see. Evidence for an internal ocean popped out of the most realistic ice shell stability scenarios.
According to their models, the ice shell of Mimas is between 24 and 31 kilometers thick, under which a global ocean swirls. Enceladus' icy shell is between 5 and 35 kilometers in diameter, while Mimas' is just 396 kilometers in diameter.
The team found that the heat flow from Mimas' surface is very sensitive to the thickness of the ice. Future probes should be able to measure this on Mimas and other IWOWs.
In addition to a liquid ocean, the models suggest that Mimas also has a differentiated core. The moon should have produced a different path to the one it has today, according to our previous models. This could be very interesting for further study and exploration.
It is difficult to reconcile the moon's geologic characteristics with our understanding of its thermal-orbital evolution, Rhoden says.
Mimas' status as an ocean moon would be benchmarked. This would help us better understand the moons of the planets, as well as the moons of the ocean. It is a compelling target for continued investigation.
The research has been published.
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