This Saturn moon would also hide an ocean under its surface

Several years ago, the hypothesis was raised. Under his crust of ice, Mimas, a giant name for a very small satellite of Saturn – its diameter is only around 400 kilometers –, would hide an ocean. The hypothesis was supported by measurements returned by the Cassini probe (Nasa). But the astronomers were puzzled. Nothing. There was absolutely nothing on the surface of the satellite to confirm this.

“The surface of Mimas is riddled with craters, one of which is particularly huge – some even see a resemblance to theStar of Death from Star Wars. We thought it was just a block of ice”, says Alyssa Rhoden, specialist in the geophysics of this type of celestial objects, in a statement of Southwest Research Institute (SwRI, USA). “Nothing like the surfaces we know from other ‘inner ocean worlds’ that are fractured and show signs of geologic activity. » And it is precisely by wanting to demonstrate this that astronomers have discovered new evidence that the satellite does have an internal ocean liquid.

What betrayed Mimas’ secret was the release recorded a few years ago now, still by the Cassini probe. A libration is the name that astronomers give to slow movements of oscillation that a satellite can have when it is seen from the body around which it is in orbit. But the physicists know that the tidal phenomenon — part of which is caused by the libration in question — tends to dissipateenergy in the form of heat in a satellite. And in this case, enough for a liquid ocean to persist beneath the icy crust of Mimas.

Go see on Mimas what’s going on there

According to the models developed by the researchers, the ice crust on the surface of Mimas would thus be between 20 and 30 kilometers thick. And it would completely cover a liquid ocean. However, the conditional remains in place. Because as astronomers recognize, “it is difficult to reconcile the characteristics orbitals and geology of Mimas with our current understanding of its evolution”. The hypothesis of a nucleus in the form of ball rugby also formulated to explain recorded libration remains possible. Only a mission sent on the spot seems able to decide. It could at the same time study, if it exists, the composition of this ocean, undoubtedly filled with water trapped there for billions of years.

Better understanding the Mimas model could enlighten astronomers on the number of “inner ocean worlds” they can expect to find. Not only in our Solar system. Maybe on the side ofUranus and of Miranda, Ariel Where Umbriel. And on smaller objects than what researchers had previously imagined. But also around more distant stars.

What also open a little more the field of possibilities for life. Indeed, the range defined by the researchers as “habitable” around a star could thus be seen rather largely enlarged. And no longer confined to an area located around an equivalent of the Earth-Sun distance. Thanks to a underground water which would be much more common than surface water.

Around Saturn, Mimas would hide an ocean… or a flattened core

Based on observations made by the Cassini space probe, an international team of researchers has measured the rotation of Mimas, one of many moons of Saturn, and detected oscillations there. Not consistent with predictive models, they suggest that this small satellite could harbor either a strongly flattened core or, under its layer of ice, an ocean.

CNRS article published on 27/10/2014

Mimas’ rotation was measured from Cassini images (Esa, NASA), using a little-known technique called stereophotogrammetry. It makes it possible to reconstruct in three dimensions a portion of the surface of an object when it is observed at least twice from different angles. Just like the Moon around the Earth, Mimas is rotating synchronous around Saturn. This means that this satellite rotates on itself at the same speed that it performs a revolution around the giant planet, thus always showing the same face to his planet.

However, this uniform average movement is superimposed by oscillations. These are called librations, because they result from the gravitational force couple exerted by Saturn on the satellite of about 400 km in diameter. The work carried out by an international team involving French researchers from the Institute of Celestial Mechanics and Ephemeris Calculation (IMCCE) of the Paris Observatory (CNRS, Pierre et Marie Curie, Université Lille 1) and the AIM laboratory (CEA, CNRS, Université Paris Diderot) and scientists from the Royal Observatory of Belgium, the University of Namur (Belgium) and the University of Cornell (United States) have enabled to highlight two types of librations: one at low frequency, the other at high frequency. However, of these two types, the high frequency one has an amplitude twice as large, incompatible with the rotation model of a satellite solid, at hydrostatic equilibrium. This amplitude is indicative of the distribution of mass inside the body and the presence or absence of liquid layers.

Mimas could hide an ocean

These observations are therefore surprising and reveal an intriguing internal structure. After having explored several hypotheses, it appears that this strong amplitude can be explained either by the presence, under the coat ice cream Mimas, of a very elongated rock core, or by the existence of an internal ocean hidden between its icy surface and its core. Indeed, planetary scientists assume that the core of the small satellite must be in hydrostatic equilibrium (where the forces of gravity, centrifugal and pressure balance each other in the body), hence from a very early formative age. However, the high amplitude of the high-frequency libration could indicate a core with an elongation of 20 to 60 km greater than in the hydrostatic case.

If the core of Mimas is indeed elongated, then it would have frozen since its formation and would have largely retained its initial shape. On the other hand, if this moon has an ocean, then it would join the club of ” internal ocean satellites ” from Solar system where several moons of Jupiter (Europe, Ganymede…) and, around Saturn, Titan and Enceladus. Such a global ocean would be a real surprise, as the surface of Mimas shows no signs of recent geological activity. Additional comments from cassini will refine the interior models of Mimas.

Whether it is one or the other of these two solutions, we now know that Mimas, despite its apparently ancient surface riddled with craters and its small size, is not the cold and inert star that we imagined. Uncovering the secret of its interior will surely shed light on its formation, and thereby on the formation of the Saturn system as a whole.