Solar system

Recipe for making Saturn's rings

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Simulations show that the still-mysterious origin of Saturn's vast, icy rings could be explained by the 'peeling' by Saturn's tides of the icy mantle of a large satellite migrating towards the planet. See Letter p.943

Since Christiaan Huygens realized in 1655 that the planet Saturn is encircled by a ring, this “jewel of the Solar System” has defied researchers' best efforts to explain its origin. Saturn's rings are made of centimetre- to metre-sized boulders of almost pure water ice1 — a unique characteristic among Solar System bodies (comets and planetary satellites contain about 50% silicates and metals). The total mass of Saturn's rings is thought to be equivalent to that of a satellite about 500 kilometres across2,3. But how and when the rings formed, and why they are so clean of silicates, is not understood. Several mechanisms for their formation have been proposed4,5,6, but none has provided a convincing explanation for their observed peculiarities.On page 943 of this issue, Canup7 offers an attractive solution to the problem that answers several questions at once.

Saturn's rings are in a location that is dominated by tides. Generally, boulders in circular orbits, such as those making up the rings of Saturn, merge and grow to form larger bodies because of their own gravity — this process is thought to be the way in which planets and asteroids form around the Sun and satellites form around giant planets. But because Saturn's rings are so close to the planet (below the planet's Roche limit, which is 2.5 times the planetary radius), the tidal forces that the planet exerts on them prevent the boulders from accreting material. Just as the Moon's tides stretch the oceans on Earth, Saturn's tides stretch any boulder aggregates in the rings, and separate their constituents. Similarly, if a pre-existing icy body were to be placed within Saturn's Roche limit, it would be destroyed by the planet's tides.

These considerations suggest a simple recipe for making Saturn's rings: take a large body and put it into a close orbit around the planet; tides should then destroy it, and the resulting fragments should form the rings. But what sort of body? A large, differentiated satellite — with a core composed of silicates and iron, and a lighter mantle made of water ice — would be a good choice. The rings could then be formed simply by 'peeling off' the satellite's icy mantle from the core using the planet's tides as a knife4,5,6. Although appealing, however, this recipe has never been investigated owing to computational limitations.

Enter Canup7, who describes the details of a numerical and analytical model for tidal splitting of a differentiated satellite around Saturn. She demonstrates that the planet's tidal forces can indeed be strong enough to take water ice away from the satellite, but not to tear its dense silicate core. But how can the differentiated satellite be brought so close to the planet, within the Roche limit? When could this have occurred? And what mechanism can get rid of the silicate core that has not been broken by the tides? Canup solves these problems using a single phenomenon: planetary migration.

The planets of our Solar System formed 4.5 billion years ago in a disk of gas and dust that surrounded the young Sun for a few million years. At the same time, the satellites of the giant planets formed in gaseous and dusty disks around their hosts. But when a body orbits inside a gaseous disk, its orbit shrinks and the body spirals gradually towards the planet as a result of the gravitational interaction of the body with the surrounding gas. In the circum-Saturn disk, satellites grew and fell into the planet in this way, until the last generation formed and escaped inward migration because the disk had vanished8. Canup suggests that the last migrating differentiated satellite, about the size of Titan (Saturn's largest moon), had its icy mantle pulled to pieces by Saturn's tides as it crossed the Roche limit. Beyond this limit, the satellite's silicate core carried on migrating inwards and eventually disappeared into Saturn, leaving behind the ice boulders that make up the rings (Fig. 1). This process would have produced icy rings about 1,000 times more massive than the rings are today.

Figure 1: Canup's model for the formation of Saturn's icy rings7.
figure1

a, A differentiated satellite in the gaseous circumplanetary disk around Saturn migrates towards the planet. When the satellite crosses the planet's Roche limit, its icy mantle starts to be pulled into pieces by the planet's tidal forces. b, The silicate core carries on migrating towards Saturn and eventually falls into it, leaving behind the ice boulders that give birth to the rings and icy satellites of the planet.

This elegant model7 could provide the missing links between a suite of observational and theoretical results that have changed our understanding of Saturn's rings. Such massive rings would be less sensitive to the darkening effect of meteoroid bombardment3,9, thus explaining their brightness today. In addition, because of their mass they should spread more rapidly than the present ones, leading, over the age of the Solar System, to lighter rings like those seen today7,10. During this spreading, the material expanding beyond the Roche limit may have given birth to satellites; such a mechanism has recently been proposed for the formation of Saturn's small moons11. Indeed, observations made with the Cassini spacecraft have shown that accretion processes are still active at the outer edge of Saturn's rings12,13 and on the satellites Pan and Atlas14. Canup suggests that the inner satellites of Saturn, up to and including Tethys, could also have formed by accretion of spreading ring material beyond the Roche limit.

Canup's model7 offers, for the first time, a convincing starting point for a consistent theory of the origin of Saturn's rings and satellites. It shows that the rings and satellites are intimately linked, and that Saturn's system, despite being made of ice, is not frozen but is constantly evolving. The origin of the rings and satellites must be understood in the wider framework of models of planet formation, and this work is one step in that direction. One may question whether the specific conditions required for such rings to form have also been met around other Solar System planets and exoplanets. The details and the consequences of the formation of the satellites at the outer edge of the rings, and their outward migration to their present positions, are still to be explored. Establishing these details could change our understanding of Saturn's satellites, and more generally of giant planets and their environments.

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This article and the paper under discussion7 were published online on 12 December 2010.

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Crida, A., Charnoz, S. Recipe for making Saturn's rings. Nature 468, 903–905 (2010). https://doi.org/10.1038/nature09738

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