The data from the mission keeps making discoveries. The moon Mimas may have an ocean. The moon may harbour liquid water under its surface if it does, joining a growing list of natural satellites in our Solar System.
The Interior Water Ocean Worlds are called IWOWs. If the new paper is correct, tiny Mimas will join other IWOWs in our Solar System. How did scientists find out about Mimas?
The Italian Space Agency, NASA, and the European Space Agency collaborated on the Cassini mission. The mission ended with a plunge into the moon. Before it ended its mission with a plunge into the gas giant's atmosphere, it detected an unusual libration in Mimas' rotation.
The rotation of Mimas is an indicator of a geologically active world with a ocean.
The paper explains the findings. The case for an ocean-bearing Mimas from tidal heating analysis is available online at Science Direct. The two authors are Matthew E. Walker and Alyssa Rose Rhodena. Rhodena is an expert in the geophysics of icy satellites and Walker is an associate research scientist.
It would be different from others in the Solar System if Mimas is an IWOW. Mimas lacks geological activity, but those ocean-bearing moons show it. The authors point out that the physical librations can be explained by either a non-hydrostatic core or a global, liquid water ocean beneath the ice shell.
The moons are not going to scale. While the surface of Enceladus and Europa show signs of geological activity, Mimas is different. It is covered in craters and shows no activity. The image is from NASA/JPL/Space Science Institute. By NASA and JPL. JPL-Caltech is part of the Space Science Institute.
According to the press release, 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. The evolution of giant planet satellites systems is studied by Rhoden.
The wordstealth is used to describe Mimas compared to other IWOWs. The cratered surface of Mimas gave no indication of an ocean underneath, though researchers suspected it was a large chunk of ice. Its density is too low to be rock.
Rhoden said that they thought it was a frozen block of ice because of the cratered surface. 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.
The planet is shaped by the planet's powerful gravity. The small moon is stretched into an ellipsoidal shape because of the strong gravity. The heating is caused by the same tidal forces. The heating must be strong enough to melt the interior into liquid but not so strong that the surface will melt.
The numerical models developed by the researchers were based on tidal heating models. They found that the Mimas' interior ocean is covered in an icy shell 24 km to 32 km thick. Compare that to Enceladus, which is probably 20 to 25 miles thick, and to the icy shell of Europa, which is 10 to 20 km thick.
Rhoden said that most of the time, when we create these models, we have to fine tune them to produce what we observe. Evidence for an internal ocean popped out of the most realistic ice shell stability scenarios.
A visit from a spaceship can confirm the discovery. The critical issue of heat flow against ice shell thickness would be clarified by a visit to any IWOW. The team found that the heat flow from the surface was very sensitive to the thickness of the ice shell. The Jovian moon of Europa will be the site of the Juno spacecraft's microwave radiometer to measure heat flows.
Near the end of 2022, NASA's Juno will fly by the moon at a distance of about 200 miles. Researchers will be able to understand how heat flow affects icy shells. The dynamics between heat flow and frozen shells on IWOWs will become more important when NASA launches its Europa Clipper mission. The Jovian system will be reached by the spacecraft in April 2030.
The findings of the study will tell us a lot about IWOWs. Will the data from that mission help scientists? Probably. The data from the Cassini mission is what scientists have to work with. Rhoden says there are still unresolved aspects in that data. Rhoden and other researchers will learn more about other ocean moons if they can reconcile some of the challenges in the data. It is like facing a locked door. Behind the door is a better understanding of IWOWs, and Mimas is the key that unlocks it.
Rhoden said that it was difficult to reconcile the moon's geologic characteristics with the current understanding of its thermal-orbital evolution. Evaluating Mimas as an ocean moon would benchmark models of its formation and evolution. 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 images from the 1986 flyby show that the moons are roughly the same size as ice and rock. More detailed data isn't available because the instruments that were used in Voyager 2 weren't as capable as those used in Cassini. The images showed evidence of a phenomenon called cryovolcanism, in which liquid erupts through the surface and then freezes. The same eruptions send material from the ocean out into space.
There is a false-colour artist's illustration of the cryovolcanic eruptions from Enceladus. The image is from NASA/ESA.
The gas giants of the Solar System might have IWOWs. Scientists want to know if the outer giant planets have enough mass to cause tidal heating in their moons. The most massive of Neptune's moons is Triton. The moon is mostly water ice and geologically active. It is most likely a captured Kuiper Belt object. In 1989 the moon saw geyser-like clouds 8 km high.
The images were taken about 45 minutes apart from left to right on August 26th, 1989. They show a cloud of fine, dark particles in the air. The cloud is moving to the right for a distance of about 100 miles. The image is from NASA/JPL.
The possibility of life is brought up when scientists discover another IWOW. Evidence shows that there needs to be an active interface between water and rock. It could have a rocky core. The necessary ingredients for life can be supplied if there is heat.
The surface area of Mimas is about the size of Spain. It is the smallest known body that is rounded due to self-gravity. Our understanding of potentially habitable worlds will grow if a world this small can harbour an ocean.
Does Mimas have a body of water? Is it possible that it could harbour life? Is IWOWs more common than we thought?
We don't know.
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