Does Saturn's moon, Little Mimas, have an ocean hidden under ice?

Mimas is a small moon orbiting the giant planet Saturn. Its radius is only 200 km, the same distance between Oslo and Kragero.

By comparison, our Moon has a radius of more than 1,700 km.

The small moon is one of Saturn's many moons, and its most prominent feature is the giant crater. You can see it clearly in the image above, which shows the moon photographed by the Cassini probe in 2010.

It's so huge and prominent that Mimas was quickly compared to the creepy “Death Star” space station from the “Star Wars” movies.

But Mimas could hold something different and unknown beneath the surface: a sub-ice ocean.

As far as we know, liquid water is a very important prerequisite for life as we know it. There is much speculation that such liquid oceans on icy moons may be the most exciting place to look for life elsewhere in the solar system other than Earth.

Ice moons with seas

The solar system contains several icy moons, many of which show signs of a subsurface liquid ocean. One of the most famous is Europa, which orbits Jupiter.

This moon is covered with a relatively smooth layer of ice with distinct dark stripes. Scientists have long suspected that a liquid ocean exists here. You can read more on forskning.no about the possible way to get to the sea under the ice.

Enceladus, another of Saturn's moons, has also been highlighted as one of the moons most likely to contain liquid water beneath the ice.

Now a research group says the same thing could be found in Mimas.

– It's interesting to think about whether Mimas could have had an ocean, says Stephanie Werner forskning.no. Werner is a planetary scientist at the Center for Planetary Habitability at UiO.

– It is not intuitive that such a small moon would have a liquid ocean.

She is not convinced that there is a liquid ocean under the ice after looking at the study, but she thinks this is an interesting description.

Made of ice only

So Mimas is a bumpy little ball. It is too small to form a smooth sphere, and this happens when moons and planets reach a certain size.

But there is something strange about Mimas. The way it moves in its orbit around Saturn indicates that there is something inside the small moon. It may just be because there is ice inside, but some of the movements indicate there may be something else going on.

It is particularly concerned with the movement called libration, a form of rocking and turning that occurs periodically as the Moon orbits around Saturn. You can see an example of our moon's movement in the video below.

Measurements by the Cassini probe, which also took images of Saturn and its moons, showed that this motion was different from what was expected.

In a new study published in the journal Nature, a research team believes that these and other movements in Mimas' orbit can be better explained by the presence of a subsurface liquid ocean.

They have built computer models that attempt to explain Mimas' measurements and observations.

In the new paper, they also try to explain how this ocean formed and that it is a very new ocean on the solar system's time scale.

Researchers say that this sea may be only a few million years old. This is very new in a solar system that is over four billion years old.

Heating by tidal forces

The idea is that there were changes in Mimas' orbit that allowed the sea to form in the interior.

But how could a liquid ocean exist this far away in the solar system, where the Sun shines much weaker than it does here on Earth? Mimas has a surface temperature of -200 degrees, so there is no doubt that there is liquid water on the surface.

Simulations show that changes in orbit may have created stronger tidal forces that began to affect Mimas. This means that the Moon is pulled and pulled by the gravitational forces of the Saturn system.

This creates friction and thus heat within the moon itself. Researchers say the heat is enough to melt some of the ice below the surface. This is also the main reason for the presence of oceans under ice on other moons such as Europa and Enceladus.

Simulations conducted by the research group, which come from the Sorbonne University in Paris, show, among other things, that this sea may exist 20 to 30 kilometers below the surface.

Old roof?

But is there really an ocean beneath the surface? Nobody knows that. The new study relies on computer models that attempt to explain Mimas' observations.

– It's a game with standards, says Werner.

So, it's a hypothesis about a floating ocean, which is important to remember, explains Werner. A computer model of Mimas with a liquid ocean inside explains the observations, but there may be something else going on in reality.

Werner points out that the model doesn't explain why Mimas's surface looked the way it did — for example, the surface wasn't cracked or distorted by a possible sub-ice ocean.

If Mimas had a subglacial ocean, it would look very different from Europa or Enceladus. These moons have smooth surfaces and are relatively young.

The surface of Mimas is likely very old and cratered from millions of years old by meteorite impacts. The giant crater could not have formed if the ice was relatively thin with an ocean underneath.

The researchers' idea is that the sea formed recently and therefore had little time to affect the surface.

Another explanation for the anomalies in Mimas is that the moon has an elongated rocky core and not a liquid ocean, as the researchers describe.

But Werner thinks that a liquid ocean or an extended core are somewhat extreme explanations.

She believes that it is difficult to build a good, well-founded model that explains the existence of a liquid ocean under the ice. At the same time, it's an interesting thought experiment.

The European Space Agency (ESA) recently launched the JUICE probe, whose main mission is to explore the icy moons orbiting Jupiter.

This mission will collect more data and knowledge about these moons, but Werner stresses that there will still be many uncertainties.

– There are still some uncertainties when relying on remote sensing.

To really find out what's going on, one has to land on the ice and dig a hole. It may be a long time until someone does this in Mimas.

reference:

Lenny et al.: A recently formed ocean inside Saturn's moon Mimas. Nature, 2024. DOI: 10.1038/s41586-023-06975-9. summary