Young surroundings in Mimas

About 20 to 30 kilometers below the icy surface of Saturn's small moon Mimas lies a global ocean of liquid water. An international research team has now reached this surprising result after analyzing old data from the US Cassini space probe. Until now, researchers have assumed that the interior of the celestial body is solid throughout. What's also surprising is that the ocean is astronomically young: it formed a maximum of 25 million years ago, scientists report in the journal Nature.

“Moons with large circumferences under a thick layer of ice are very common objects in the solar system,” says Valery Linney of the Sorbonne University in France and his team. These oceans reveal themselves through structures on the surface that arise from cracks and fissures in the ice sheet. Typical examples are Jupiter's moon Europa and Saturn's moon Enceladus. Saturn's moon Mimas, which is approximately 400 kilometers across, shows nothing of the sort: its surface is littered with many ancient craters. This makes Mimas “one of the most unexpected places in the solar system to look for an ocean,” Lainey said.

But measurements by the Cassini space probe — which orbited Saturn from 2004 to 2017 and also explored its rings and moons — have already shown that Mimas oscillates slightly back and forth as it rotates. This phenomenon, called libration, could indicate the presence of a “rough” ocean inside the Moon. However, since no signs of this have been found on the surface, astronomers have so far favored a different explanation for Libra. They hypothesized that the Moon's solid core was slightly deformed, that is, elongated by Saturn's gravity, causing it to wobble.

The movement of Saturn's moon can only be explained by the ocean

However, such a distorted core should also affect the Moon's elliptical orbit around Saturn, where the orbital ellipse should actually show a slight rotation, called precession. Lainey and his team were now looking for this effect in data from Cassini. But the researchers faced a big surprise: If we assume that Mimas is a solid, frozen celestial body, the measurements of the Moon's precession and precession cannot be reconciled. The only explanation consistent with the probe data is the assumption that there is a global liquid ocean beneath the frozen surface of Mimas. According to researchers' calculations, the depth of the ocean is about 70 to 80 kilometers. This means that about half the volume of Mimas is liquid water.

But why are there no signs of the ocean on the moon's surface? The orbit of Saturn's moon also provided a surprising answer to this question. Because such a large circumference causes the elliptical orbit to become circular in a very short period of time. As orbital analyzes have shown, the ocean can be a maximum of 25 million years old. This period of time is not long enough to leave traces on the surface. Lainey and his team's discovery is likely to change the way astronomers view the many small icy moons of the large planets in our solar system. Because there may be other large oceans hidden beneath the surfaces of some of these inconspicuous celestial bodies.