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SATURN鈥橲 rings are among the greatest puzzles in the solar system. And they are certainly the gaudiest of their kind, far outshining the faint trails of dust that circle the other giant planets. The Cassini spacecraft is now giving us a remarkable view of the rings, revealing far more than their static beauty. Over the past 18 months, the spacecraft鈥檚 cameras have captured dynamic changes, watching as the rings are teased into fantastic shapes by passing moons. But it remains to be seen whether Cassini can solve the deepest mystery of the rings 鈥� their origin.

The beginnings of the rings are well hidden because whatever made them was long ago ground to powder. As Cassini has confirmed, the rings are made of ice fragments ranging from a few centimetres up to tens of metres across. Altogether there is about enough material to build a small moon a few hundred kilometres across, though that material is spread out in a wafer-thin plane little more than a hundred metres thick.

Until about 30 years ago, the rings were thought to be just leftovers from the formation of the Saturn system, orbiting rubble that never managed to build a moon. But then planetary scientists realised that if the rings had spent four billion years gathering space dust, their ice ought to be dark and grimy, whereas the rings鈥� material is clean and bright 鈥� about 99 per cent pure ice. It is so clean that it can鈥檛 have been gathering dust for more than a few hundred million years.

So something must have created the rings fairly recently in planetary time. One idea is that an old moon was smashed up in a violent impact. There is evidence of such violence very close by: Saturn鈥檚 inner moon Mimas has a giant crater called Herschel that records an impact nearly powerful enough to have destroyed the body entirely. Perhaps another moon was less lucky.

Alternatively, a vast comet might have been captured and then ripped apart by the planet鈥檚 gravity. Cassini has revealed that Saturn鈥檚 moon Phoebe is almost certainly a captured interloper from the Kuiper belt of objects in the outer solar system (see 鈥淢otley crew鈥�), and if Phoebe had come a lot closer to Saturn, it might have been turned into a splendid set of rings.

Or the forging of the rings may prove to have been a much more complicated affair, one that is not yet finished. The rings may be frequently refreshed by the demise of moonlets or small comets, for example, which is something that Cassini can test with its spectral analysers. These instruments have already shown that some parts of the rings are dirtier than others. 鈥淚f we see a narrow, clean region, that might tell you of a recent event at that location, such as the break-up of some larger ring particles or a mini-moonlet,鈥� says Joshua Colwell of the University of Colorado at Boulder, who works with Cassini鈥檚 ultraviolet imaging spectrometer.

And once scattered into a ring, material might slowly clump together into new moonlets 鈥� perhaps only to be shattered again in later collisions. The late planetary scientist and comet spotter Eugene Shoemaker believed that a lot of Saturnian moons have broken up and re-accreted many times. And the low densities of several inner moons suggest that they might be no more than loose rubble piles, as you would expect with reassembled fragments.

This kind of make-and-break process is something that Cassini might soon be able to test. Among the new images captured by the mission鈥檚 imaging team, astronomer Carl Murray of Queen Mary, University of London, spotted a new ring: a fine line of ice following the orbit of the 40-kilometre-wide moon Atlas, just beyond the outer edge of the A ring. 鈥淚t鈥檚 a microcosm of the whole ring system,鈥� he says. 鈥淚s the ring a relic of the material from which Atlas formed, or is it material coming off the moon?鈥� By watching Atlas and its slender ring over the coming years, we might get to find out how easily moons turn into rings, or vice versa.

And there is a much more subtle exchange that could solve one of the main mysteries of the rings: how old they are. Gravitational interactions mean that the rings are gradually passing their angular momentum to the small inner moons that circle them. That should make the rings shrink towards Saturn, while the moons drift outwards. If Cassini can measure this change in the lunar orbits, scientists will be able to work out just how fast the rings are shrinking and finally get a good idea of their age.

The Cassini team has already found the probable origin of one ring: the faint, outermost fuzzy E ring. It is probably a vapour trail pumped out by a set of volcanic vents at the south pole of the moon Enceladus. But it will be a lot harder to work out just what made the main ring system.

To do that, astronomers may have to look elsewhere, exploring the Kuiper belt and finding out more about other interloping bodies from the outer solar system. That way, they can work out how likely it is that a ring-making mega-comet or a moon-smashing asteroid might have arrived during the lifetime of the rings. And decades hence, astronomers could be studying ringed planets around other stars. Perhaps we will only understand Saturn鈥檚 rings when they no longer seem unique.