Saturn's satellites have puzzled astronomers since the invention of the first telescope. But while some of their enigmas have been solved, others are still perplexing scientists.
Moons of mystery
Astronomy is the oldest of the sciences, and over the millennia it has amassed a rich trove of mysteries. While such grand conundrums as the origin of the universe tend to attract most attention, there is no shortage of them much closer to home, in our own solar system. Now astronomers believe they may have solved a centuries-old enigma which has a starring role in a classic work of science fiction.
While observing the heavens with his telescope one night in 1671, the Italian-born French astronomer Giovanni Cassini detected a faint star-like object to the west of Saturn. Suspecting it might be a new moon of the ringed planet, Cassini kept the object under observation - or at least tried to. For as the object moved to the east of the planet, it disappeared, only to reappear again as it came round to the other side.
Cassini concluded that the object was indeed a moon of Saturn, which he named Iapetus, after one of the titans of Greek mythology. As for its strange disappearing act, he suggested: "One part of [Iapetus's] surface is not so capable of reflecting to us the light of the sun which maketh it visible as the other part is." Around 300 years later, the science fiction author Arthur C Clarke put forward a rather more exotic explanation. In his celebrated novel 2001: A Space Odyssey, Iapetus takes on the role of an interplanetary lighthouse, its light variations being due to the presence of a huge elliptical black area put on its surface by aliens to attract our attention. When a manned mission to Saturn arrives at Iapetus, it finds a huge black monolith at the centre of the patch, which proves to be - well, read the book. (Don't expect to see any of this in the film, though, which for technical reasons focused on Jupiter instead).
The true explanation for the behaviour of Iapetus finally emerged in 1980, when Nasa's Voyager 2 probe showed that the leading hemisphere of the otherwise icy-white surface of Iapetus is covered with some kind of pitch-black material. Cassini was thus right all along - and his shrewd deduction is commemorated in the name of the enigmatic area, Cassini Regio. Yet despite observations by Earth-based telescopes and a very close fly-by by Nasa's Cassini probe in 2004, the nature and origin of the black stuff has remained a mystery.
Now a team of astronomers think they may have found its source. Anne Verbiscer and her colleagues at the University of Virginia made their discovery while investigating the theory that the black stuff is being dumped on Iapetus by another of Saturn's moons, Phoebe. Circling the planet at a much greater distance than Iapetus, and on a bizarre, highly-tilted orbit, Phoebe seems to have been a wandering chunk of ice and dust that became trapped by Saturn's gravity. Stuck out in deep space, poor Phoebe has fallen prey to all kinds of indignities - including collisions with passing comets, rogue moons and meteors.
Astronomers have found that Phoebe appears to be made of stuff similar to the dark material on Iapetus - prompting the idea that it ended up there after being blasted off the surface of Phoebe by meteor impacts. Dr Verbiscer and her colleagues decided to put this idea to the test by training Nasa's orbiting Spitzer space telescope on the orbit of Phoebe and looking for signs of debris. And as they report in the current issue of the journal Nature, they have found a vast ring of the stuff strewn around Phoebe's orbit.
It's not exactly jam-packed with rubble: the team estimates there may be as little as 20 grains of dust in each cubic kilometre of the ring. Even so, over billions of years it might be enough to create the dark patch covering one side of Iapetus. Frustratingly, just weeks after the ring was discovered, the Spitzer telescope used by the team ceased functioning. They will now have to wait until 2014, and the launch of Nasa's James Webb Space Telescope, to find out more about this huge new ring of Saturn.
In the meantime, astronomers have no shortage of other Saturnian mysteries to ponder. Even now, almost 400 years after they were first seen by Galileo, the origin of the famous and far more spectacular inner rings of Saturn remains unclear. Over 70,000km wide but barely 10 metres thick, they are made up of billions of chunks of ice typically around the size of a suitcase. How they got there, however, is unknown. One possibility is that they are the remnants of an icy moon that wandered too close to Saturn and was torn apart by a combination of the planet's gravity and centrifugal force. Another theory is that the moon was smashed to smithereens in some primordial collision.
Something similar may well be needed to explain the bizarre appearance of another member of Saturn's family of moons, Hyperion. Close-up images taken by the Cassini probe have shown that it looks remarkably like a gigantic bath sponge - indeed, Hyperion is so porous that it would float if put in a big enough bathtub. How it came to look like that is anyone's guess, but extreme violence seems likely, not least because the entire moon is constantly tumbling, as if reeling from some cosmic upper-cut.
But there is one mystery about Saturn's moons that no scientist would be keen to theorise on. It centres on Mimas, the innermost of the major moons of Saturn. When Nasa scientists were sifting through the first detailed images of the Saturnian system sent back from Voyager, they felt a distinct sense of déjà vu when looking at the images of Mimas. The photographs revealed a nice, round moon horribly scarred by one gigantic crater.
It took the team a few moments to recall where they had seen such an object before: at the cinema. Mimas looks astoundingly similar to the Death Star, the Galactic Empire's terrifying space weapon that features in the 1977 movie Star Wars. Suddenly, all that evidence for titanic violence among Saturn's moons no longer seems so inexplicable. Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England