Iapetus: Moon of mystery

The puzzle of Saturn's most intriguing satellite may finally have been solved. Marcus Chown reports
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The origin of Iapetus's Janus faces is one of the longest standing mysteries in the solar system, one that has persisted since the moon's discovery by the astronomer Giovanni Cassini in 1671. But the mystery may at last have been solved. The key turns out not to be an ancient encounter with an enigmatic extraterrestrial race but an ancient encounter with Saturn's spectacular system of rings.

The most important event in unravelling the mystery of Iapetus was the flyby of the moon by the Cassini space probe on 31 December 2004. The onboard cameras captured images of the crater-strewn moon in unprecedented detail. What those images revealed stunned planetary scientists back on Earth.

Stretching for 1,300km - almost a third of the way around the moon - was an extraordinary ridge. In places, it is 20km high - more than twice the height of Everest - and this is on a moon only 1,436km across, much less than half the diameter of our own Moon. The ridge follows the equator closely. And there is nothing like it anywhere else in the solar system.

At first sight, the existence of the ridge appears to be a second major puzzle to add to the outstanding mystery of Iapetus's two-tone colouring. But, earlier this year, an American planetary scientist made a remarkable claim. Carolyn Porco, head of the Cassini imagining team, suggested that the two features are linked. The evidence for this is the fact that the dark region of Iapetus, Cassini Regio, is perfectly bisected by the giant ridge.

But how could the two features possibly be related? This is where Saturn's rings come in. According to the astronomer Paulo Freire, of the Arecibo Observatory in Puerto Rico, at some time past, Iapetus wandered too close to Saturn's ring system. Saturn's rings - themselves believed to be the relic of one or more disintegrated moons - are barely 100m thick but crammed with pieces of tumbling rock and ice, ranging in size from dust grains to office blocks. When Iapetus ran into the outer edge of the rings it would have come under a bombardment of unimaginable ferocity along the line where the rings intercepted the moon. "Millions of craters would have been created each and every second," says Freire.

But it isn't the craters themselves that are important. It is the sheer volume of ring material that a collision like this can deposit in a relatively short time on the surface of a moon like Iapetus. To give some idea of how much, Freire assumes that Iapetus and Saturn's ring system were in contact for three hours, a small fraction of the time it takes the moon to go round Saturn. From the known density of material in Saturn's rings and the speed with which the moon and ring system would have collided - about 10 times the speed of a jumbo jet - Freire calculates that, in those three hours, something like 25 million cubic metres of ring material would be dumped on every metre of the line of the ridge. "This is enough to create a ridge five kilometres high with a width at the base of 10 kilometres," says Freire.

According to Freire, Iapetus merely grazed the edge of Saturn's rings. It did not penetrate the ring system completely. "If it had done so, the resultant ridge would have extended half-way round the moon rather than a third of the way," he says.

The cratering would naturally have been symmetrical on either side of the ridge, which is seen in Cassini photos of Iapetus. But this still does not answer the question: how would the face centred on the ridge have been blackened?

In a paper, Freire points out that the rings contain a perfect source of blackening material - tiny dust grains. The difficulty is in understanding how this dust could have been transported from the ridge to cover almost a complete hemisphere of the moon. The key to this, says Freire, is the ice - such as carbon dioxide, or "dry" ice - which is also present in the ring material. In the violence of the collision with Iapetus, this material would have turned into vapour. "This would have created a temporary atmosphere on the moon," says Freire.

The thin but super-fast winds in the temporary atmosphere screamed out from the ridge and deposited dust over a large area of Iapetus, according to Freire. He points out that this should cause the amount of dust - and the blackening of Iapetus - to drop gradually with distance from the ridge, which is exactly what is observed. The transport and deposition of dust by these winds can also cause the streaks observed near craters at the edge of Cassini Regio.

Another prediction is that the size of dust particles should get smaller and smaller ever farther from the line of the ridge, as winds cannot transport large particles as far as small ones. For Iapetus to have hit Saturn's ring system, it must once have orbited in the same plane as the rings. The ridge would have been created along its equator. Nowadays, Iapetus no longer orbits in the plane of the rings. Something must have hit the moon - most likely another moon - and ejected it into its present orbit. "However, that is an unlikely event," says Freire.

Scientists despise unlikely events. But such a circumstance could have been more likely if there were once many moons in similarly chaotic orbits. This turns out to be a fundamental prediction of our current understanding of the origin of the solar system. The rocky planets were built up by the collision and sticking together of many smaller objects in chaotic orbits around the Sun. Similarly, the moons of Saturn were built up by the collision of many small objects in similar orbits around Saturn. "In the Saturnian system, all were disintegrated, incorporated into other moons or ejected from Saturn," says Freire. "All except a relatively large one - Iapetus."

Iapetus, it turns out, is now one of a large complement of Saturnian moons. After the discovery of another 12 moons, ranging in size from three to seven kilometres across, the total number of satellites orbiting Saturn has reached 46 - 47 if you include a tiny artificial satellite called Cassini.

Marcus Chown is the author of The Magic Furnace, Vintage, £7.99

Iapetus: the facts

Discovered by Giovanni Domenico Cassini

Date of discovery 1671

Diameter 892 miles (1436 km)

Mean distance from Saturn 2,212,889 miles (3,561,300 km)

Rotational period 79.33 days

Orbital period 79.33 days

In Greek mythology Iapetus was a Titan, the son of Uranus, the father of Prometheus and Atlas and an ancestor of the human race. According to legend, Iapetus was later imprisoned by Zeus in Tartarus after the rebellion of the Titans against the gods.