Whether Jupiter's moons Europa and Ganymede or Saturn's moons Enceladus, Dione and Titan - numerous satellites of our solar system have an ocean beneath their surface. Saturn's small moon Mimas has long been considered a further candidate. But so far there has been no conclusive proof that there is actually a large water reservoir hidden beneath its jagged ice crust. Researchers led by Valery Lainey from the Observatoire Paris now provide this in the magazine “Nature”.

Manfred Lindinger

Editor in the “Science” department.

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    The scientists analyzed the observation data from the American space probe Cassin, which also orbited Saturn's moon Mimas as part of its mission from 2004 to 2017. Lainey and his colleagues believe that Mimas' ocean is still relatively young by astronomical standards.

    Until now, the existence of an ocean of liquid water on Mimas seemed rather unlikely. Unlike its big sister Enceladus, Mimas shows no signs of activity either on or below the cratered surface that suggests the existence of a subglacial water reservoir. Furthermore, Mimas is a moon with a modest diameter of 400 kilometers. Due to its size, the satellite should not be able to store the heat inside, which is caused by a liquid ocean, for long. That's why many planetary scientists sympathized with the idea that Mimas's interior is more likely to consist of a rocky core. But Lainey and his team come to a different conclusion based on the analyses.

    Mimas not an isolated case?

    The researchers examined the rotation of the moon during its elliptical orbits around Saturn. They focused on a peculiar wobbling movement that had already been noticed by the scientists on the Cassini mission. They modeled Mimas' orbit and compared their results with observations from the Cassini spacecraft.

    They found that the orbit of Mimas can be explained much better if it is assumed that there is a global liquid ocean beneath the surface of Mimas, which sloshes back and forth as it rotates. This rules out the option of a massive rocky core for the researchers.

    The authors of the study suspect that the ocean lies under a layer of ice about 20 to 30 kilometers thick and is about 70 to 80 kilometers deep. This means that half of the satellite's volume is liquid water. The researchers also have a clear idea about the age of the ocean. It could have been formed between two and 25 million years ago. This period of time is too short for the subsurface ocean to make itself felt through any signs on the surface, such as cracks and breaks in the ice sheet. 

    The results of Lainey's researchers suggest that other icy worlds in the solar system are similar to Mimas, according to the researchers from Paris. “The results of Lainey and colleagues will encourage a thorough study of medium-sized icy moons throughout the solar system,” write Matija Ćuk and Alyssa Rose Rhoden in an accompanying commentary. There may also be large oceans hidden beneath the surfaces of some inconspicuous satellites that now need to be discovered.