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The Cassini probe is revealing the fine details of Saturn’s rings – details that open a window on the earliest days of the solar system

FOR six years, the has been touring Saturn and its magnificent rings. Unlike previous spacecraft, which have snatched a passing glimpse of the planet on their way to the edges of the solar system, Cassini has beamed back stunningly detailed images year after year.

Such images do much to unravel the mysteries of Saturn, its rings and moons. And that’s not all. “These detailed observations have yielded insights into the formation of the solar system,” says Carl Murray, a member of the Cassini imaging team at Queen Mary, University of London.

Saturn’s ring system is the closest thing we have to the disc of dust and rubble that gave birth to Earth and the other planets 4.55 billion years ago. The protoplanetary disc took shape when a spherical cloud of ultra-cold gas and dust began to collapse under its own gravity. As the spinning cloud shrank, it took the form of a disc, swirling around the newborn sun. “Once the sun had blown away the gas, the disc of orbiting rubble would have resembled the disc of Saturn’s ring system,” says Murray.

From then on, larger bodies would begin to aggregate out of the debris. They would have gradually grown, vacuuming up material from their surroundings, their gravity reaching ever further. Competition between growing bodies would have seen some cannibalised by others.

It would probably have taken about 100,000 years and a complicated sequence of events for a planet to form, but we have very little observational evidence to tell us how it played out. “Planet formation theory is in a state not far from a shambles,” says at the Lunar and Planetary Laboratory in Tucson, Arizona.

This is where Saturn’s rings come in. “The incredible variety of complex structures we are seeing in the rings has much to teach us about planet formation,” says Murray.

Among the most striking are structures shaped like aircraft propellers. They are the wakes created by unseen moons as they plough through the icy rubble orbiting around Saturn. Dozens of propellers have been spotted in the A-ring, a band 14,000 kilometres wide and only 10 metres deep.

The moons themselves are too small even for Cassini to see directly because they are only a few kilometres to a few hundred metres across. In contrast, their propeller-shaped wakes can be thousands of kilometres long, and some have now been observed orbiting Saturn for several years. They form as a result of a moon’s gravity tugging on the surrounding material. The debris inside its orbit, being closer to Saturn, is moving faster, and the perturbation therefore quickly overtakes the moon, creating a long, thin wake ahead of it. This is the leading blade of the propeller. Meanwhile, the material outside the moon’s orbit is moving more slowly, creating the trailing blade. “I have no doubt that propellers like these were the first structures to appear on the way to making the planets,” says Murray.

Saturn’s larger moons represent the next stage in planetary evolution. These growing moons have sufficient gravity to sweep up material and carve gaps between the rings, just as the growing planets would have done. As such bodies in the protoplanetary disc grew bigger, their influence would have grown, and we can watch exactly this process happening today. Saturn’s moon Mimas is medium-sized at 400 kilometres across. It not only sweeps up debris along its orbit but also kicks it out from “resonant” orbits. For example, particles in a 2:1 resonant orbit make one circuit of Saturn for every two orbits of Mimas. They find themselves tugged regularly by the moon.

“Saturn’s moons sweep up material and carve gaps between the rings, just as young planets would have”

While each extra gravitational tug doesn’t amount to much, over many cycles they build up and knock the particles from their orbit to leave a gap. Gravitational tugs by Mimas have sculpted the inner edge of the most prominent gap in Saturn’s rings, a 4700-kilometre-wide void called the Cassini Division.

Further out, in the F-ring, Cassini’s eagle eye has shown that the moon Prometheus creates waves in the debris. That’s because the moon pulls material inside its orbit towards it, but by the time the debris reaches Prometheus’s location, the moon has scudded on past. So the material falls back, setting up oscillations in the ring. This has far-reaching consequences, because the wave compresses the debris and makes it behave more like a solid body. “What we are seeing is a passing moon triggering the formation of a body that has insufficient gravity to come together on its own,” says Murray. If the same happened in the protoplanetary disc, growing bodies like the embryonic Jupiter could have triggered the formation of other planets that otherwise wouldn’t exist.

That’s not the only way the rocky disc could have affected planet formation. Linda Spilker is Cassini’s project scientist at the Jet Propulsion Laboratory in Pasadena, California. What has surprised her about the propeller structures is that their orbits continually change. “We have tracked the larger propellers for about four years and noticed that they are sometimes moving inward and sometimes outward,” she says.

The early years

Perhaps some of the embedded moons are being buffeted by the surrounding ring material and nudged outwards. With others, it may be that gravitational interplay with the rubble robs the moon of its energy, pulling it closer to Saturn. That would certainly help to explain the existence in other star systems of so-called hot Jupiters, gas giant planets that orbit extremely close to their star. Because gas near a star is too hot for gravity to hold onto, such gas giants are thought to have grown up much further from their stars and migrated inwards. “Perhaps this is what we are seeing in Saturn’s rings,” says Murray.

Malhotra is sure there is more to learn. “Saturn’s rings and moonlets provide a very clean natural laboratory to hone our understanding of that detailed physics,” she says.

They will never tell us the complete story, though. “Although we can see an analogue of the early stages of planet formation, we cannot see the later stages,” says Murray. That’s because the icy debris in Saturn’s rings can never aggregate into moons in the same way that the protoplanetary disc turned into planets. Saturn’s rings are more than 300 times closer to the planet than Mercury is to the sun, and their close proximity means that the planet’s gravity will rip apart any budding large moons.

In Saturn’s rings, the forces of destruction always have the upper hand. But that won’t stop us marvelling at Cassini’s beautiful images and all that they can tell us.