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Illustration of Saturn's moon Mimas

Photo: Frederic Durillon / Animea Studio / Observatoire de Paris / PSL / IMCCE / AFP

There is a global ocean of liquid water about 20 to 30 kilometers below the icy surface of Saturn's small moon Mimas: At least that is what an international research team has concluded from analyzing old data from the US Saturn probe Cassini. Until now, experts assumed that the interior of the celestial body was solid. What is also surprising is the astronomically young age of the ocean: it was formed a maximum of 25 million years ago, according to the scientists in the journal Nature.

"Moons with a global ocean under a thick layer of ice are quite common objects in the solar system," explain Valery Lainey from the Sorbonne University in France and his colleagues. These oceans reveal themselves through structures on the surface that arise from breaks and cracks in the ice sheet. Examples of this are Jupiter's moon Europa and Saturn's moon Enceladus.

Saturn's moon Mimas, which is almost 400 kilometers in diameter, shows nothing of the sort: its surface is littered with old impact craters. The Herschel crater is particularly striking, with a diameter of 139 kilometers. That's why Mimas is "the most unlikely place to look for an ocean," says Lainey.

First, other explanations

Measurements by the Cassini probe, which orbited Saturn from 2004 to 2017 and also explored its rings and moons, showed that Mimas oscillates slightly back and forth as it rotates. This phenomenon, called libration, could indicate a "sloshing" ocean inside the moon. However, since there were no signs of this on the surface, astronomers preferred a different explanation: the hard core of the moon was slightly deformed, i.e. elongated by Saturn's gravitational pull.

However, such a deformed core should influence the moon's elliptical orbit around Saturn - the orbital ellipse should show a slight rotation, called precession. Lainey and colleagues looked for this effect in Cassini data. The big surprise for the researchers: If one assumes that Mimas is a solid, frozen celestial body, the measurements for the moon's libration and precession cannot be reconciled.

The only solution that is compatible with all data is to assume a global liquid ocean beneath the frozen surface. According to the researchers' calculations, it is around 70 to 80 kilometers deep. “That means: About half of Mimas’ volume consists of liquid water,” emphasizes Lainey.

But why are there no signs of the ocean on the surface of the moon? According to Lainey, the orbit of Mimas also provides an answer to this question - again a surprising one: such a large ocean causes an elliptical orbit to become circular in an astronomically short time. As the team's calculations show, the ocean can therefore be a maximum of 25 million years old. According to the researchers, this period of time is not long enough to leave traces on the surface.

The discovery by Lainey and colleagues is likely to change the way planetary scientists view the many small ice moons of the large planets in our solar system. There may be other large oceans hidden beneath the surfaces of some of these inconspicuous celestial bodies.