Mimas is distinguished from Saturn’s 145 other moons by its appearance; it has a huge crater approximately 130 km in diameter which makes it resemble the Death Star, the famous military space station from the Star Wars franchise. But very recently, astronomers made another discovery there which made it even more interesting: it is very likely that an ocean is hidden under its thick crust of ice.
This is a pretty incredible find for astronomers, because until now, no one expected that it could host liquid water. Valery Lainey, researcher at the Paris Observatory and co-author of this work, even considers that this is a discovery “ amazing “. Indeed, it took a little luck, a lot of perseverance and years of investigation to get your hands on this hidden treasure.
It all started in 2010 with the illustrious Cassinithe famous NASA probe which studied Saturn and its moons between 2004 and 2017, the date of its big final dive into the bowels of the gas giant.
The research team was interested in the Cassini Division, a nearly empty region that separates the gas giant’s rings into two distinct groups. At the time, the origin of this division was not yet clear; the researchers assumed that it could be hollowed out by the gravitational influence of Mimas, or more precisely by a change in the parameters of its orbit.
Two contradictory hypotheses
Astronomers have therefore collected piles of data to verify this hypothesis. And when analyzing the results, they observed strange oscillations in the satellite’s rotation. There were only two explanations consistent with the observations: either the rocky core of Mimas had a particularly strange shape, or its icy crust was sliding on a vast layer of liquid.
However, at the time, this second hypothesis seemed highly improbable. Indeed, Mimas is a small object which seems extremely cold, and above all devoid of geological activity. As such, it seemed very different from its cousin Enceladus, another moon of Saturn which is famous for its cryovolcanic activity. The latter regularly ejects large plumes of water, dust and ice. On the other hand, no one has ever observed this type of phenomenon on Mimas.
The nickname Death Star was therefore all the more appropriate; everything indicated that the core of Mimas was virtually inert, with no geophysical activity that could provide sufficient heat to melt the ice.
This interpretation was further reinforced by his appearance. When a celestial body is active, that is to say when different tectonic or hydrological mechanisms come into play, its surface tends to be relatively regular, because it is renewed regularly (all things considered). The craters do not have time to accumulate. We see this for example on Enceladus, where cryovolcanism regularly deposits layers of fresh snow. But on Mimas, it’s quite the opposite. It is dotted with craters, which strongly suggests that its surface stopped evolving a very long time ago.
An overwhelming majority of researchers therefore favored the rocky core hypothesis. But this track wasn’t entirely satisfying either. Indeed, to match the observations, this central structure should be relatively flat and elongated, which would be very surprising. Frustrated, the researchers looked for a way to sort things out.
A story of trajectory
The key to the mystery came when the team began to focus not on Mimas’ rotation, but on the subtle variations in its trajectory around Saturn. More specifically, it was the precession of its orbital plane that betrayed the presence of the ocean. Elliptical orbits like that of Mimas around Saturn are not perfectly fixed; over time, they slowly rotate around the parent body while remaining in the same plane, as in the third example on the right of this video.
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The theoretical speed of this precession depends on the internal structure of the planet. By comparing the trajectory calculated using Cassini’s observations to simulations of both theories, the team was ultimately able to rule out the trail of the elongated heart, and conclude with a very high degree of certainty that Mimas does indeed shelter an underground ocean.
Using this data, the researchers were also able to estimate the age and volume of this body of water. And there again, some great surprises awaited them. Mimas would be at least 50% liquid water, which is pretty huge for a celestial body of this size. And above all, he would be particularly young. The study concluded that he would be aged 25 million years at most. A trifle compared to the age of the solar system (a little over 4.5 billion years).
A gravitational massage provided by Saturn
It remains to be seen how this ocean was formed. The researchers believe that the process started when Mimas’ orbit was disrupted by the gravitational forces of two other Saturn moons, Enceladus and Tethys.
About fifty million years ago, its circular trajectory would have suddenly become more elliptical, with phases where it is very far from the planet and others where it passes much closer than before. However, this distance affects the gravitational influence exerted by Saturn itself. Because of its enormous mass, the latter gradually distorts Mimas when she approaches it. The satellite then returns to its original shape when it moves away from it, and so on.
These cycles had the effect of generating a friction between the ice mantle and the rocky core. However, this friction generates heat, just like our hands when we rub them to warm up in the middle of winter. This energy would therefore have contributed to gradually melting this solid mass, causing this large underground ocean to appear.
A new candidate for the search for extraterrestrial life
Until now, Mimas has not been a priority target for future exploration programs. But this study could completely change the situation. Indeed, these interactions between rock and ice are suspected of having played an important role in the emergence of life on Earth.
Like Enceladus, Mimas suddenly became a candidate for the search for traces of extraterrestrial life. And the good news is that missions that pursue this goal will have a tremendous opportunity to kill two birds with one stone, given that Enceladus and Mimas are close neighbors.
In the short and medium term, NASA’s priority remains Martian exploration. But over the coming decades, it will be very interesting to follow the different programs which will conquer these frozen worlds.
The text of the study is available here.
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