In 2010, our team began to investigate the true nature of Mimas. With a radius of 200 km, the smallest of Saturn’s so-called main moons is easily recognizable by its crater-covered surface. Discovered in 1789 by the British astronomer William Herschel, we had to wait until the space age to obtain the first resolved images of this moon, and thus discover its inert surface.
Studying Mimas to understand the shape of Saturn’s rings
The link between Mimas and our initial work wasn’t immediate, since it involved explaining the most remarkable structure of Saturn’s rings : the Cassini division. Discovered by Jean-Dominique Cassini in 1675 as a young director of the Paris Observatory, he had observed a dark zone in the giant’s rings, which now bears his name. With a span of some 4,500 km, this zone literally cuts the rings into two distinct parts.
Black and white image of a portion of Saturn’s rings, with the moon Mimas in the background. You can see the Cassini division, a darker portion of the rings.
Surprising as it may seem, the scientific community was unable to explain the existence of this division in 2004, when the Cassini probe arrived in Saturn’s vicinity. However, by studying the motion of its moons, we had envisaged the possibility that this peculiar formation could be the consequence of a drastic change in the distance between the small moon Mimas and Saturn. Indeed, it had long been known that the presence of Mimas had a strong gravitational effect on the inner edge of the Cassini division.
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The Cassini space probe, with a portion of Saturn and its rings in the background.
More precisely, we knew that the small ice particles that make up Saturn’s rings, and which lie at the inner edge of the division, rotate exactly twice as fast as Mimas around Saturn. It is precisely this synchronization that allows gravitation to have a major effect on the rings there, as the gravitational disturbance from Mimas builds up, rather than averaging out over time. Our hunch then was that, if in the recent past, Mimas had moved closer to Saturn, the ice particles caught in the resonance should have been driven inwards, following Mimas’ approach to Saturn, and thus opening up the famous split.
**What does the interior of Mimas look like ?
As appealing as this idea was, it came up against a major problem. For Mimas to approach Saturn, the only possible mechanism had to be the tidal effects generated by the planet. Just as the Moon deforms the Earth, including the oceans on its surface, creating tides, Mimas is deformed by Saturn over time. This deformation, known as the tidal effect, is due to the fact that the side of Mimas facing Saturn is closer to it, and therefore more attracted to the planet, than the opposite side.
However, the ice of which Mimas is composed would necessarily have melted in very large quantities due to the internal friction that would have resulted from these tidal effects. This hypothesis was completely at odds with the extremely cratered and inactive surface of Mimas, which implied the absence of internal activity. Indeed, the presence of abundant liquid water beneath the surface of Mimas should have resulted in the existence of surface faults, surface renewal and even geysers, as observed on Enceladus, a neighboring moon of Mimas.
Nevertheless, in 2014, Radwan Tajeddine, then a PhD student in our team, noticed that Mimas’ rotation on itself was affected by an anomaly. Theoretical explanations suggested two possibilities : either Mimas’ interior was rigid, but hid a very elongated rock core ; or there must be a global ocean of water beneath the satellite’s surface. This result, published in the journal Science, was the first mention of a possible ocean of water beneath Mimas.
Two hypotheses : an internal ocean or a long rock core
Over time, the vast majority of the international community adopted the hypothesis of an elongated rock core. This fitted in perfectly with the vision of a cold, inactive moon, in perfect harmony with its impact-riddled surface. Our team had to know for sure, as the explanation for the existence of the Cassini division was only compatible with the presence of an ocean. So we looked for a way to obtain additional information about the satellite’s interior.
We turned to the orbital motion of Mimas around Saturn. This is highly complex, due to the numerous gravitational perturbations it is subject to from neighboring moons. However, a quick calculation suggested that the observation of Mimas’ orbital evolution by the Cassini probe, in orbit around Saturn between 2004 and 2017, should enable us to decide between the two models of internal structure.
Tens of thousands of images taken by the Cassini probe
So we took all the available data (several tens of thousands of images) and fitted the motion of nineteen moons in the orbital system, including of course Mimas, to the Cassini observations. The analysis left little doubt : a rigid interior with an elongated rocky core is not compatible with the observation of the moon’s orbital motion. On the other hand, modelling an interior with three internal layers, i.e. a rocky core topped by an ocean of liquid water, covered by an ice shell, is in agreement with observations if the thickness of the solid ice layer is between 20 and 30 kilometers. These values also help to explain why the moon’s surface shows no activity at present.
We now know that more than half the volume of Mimas is composed of liquid water. Moreover, calculations of the orbital evolution of Mimas under the action of tidal effects imply that this ocean is less than 25 million years old. But what makes the existence of this ocean so interesting is that we would never have imagined that liquid water could ever be detected beneath such an ancient and inert-looking surface. This casts doubt on the possibility of liquid water beneath many other objects in the Solar System. The quest for liquid water in our system and its conditions of habitability has only just begun...