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Iapetus, one of the biggest satellites of Saturn, has a peculiarity, that since its discovery by Cassini in 1671 has made it one of the most curious objects in the solar system: one half of its surface is much darker than the other; so much so that viewed from Earth it shows brightness variations of two magnitudes.
Cassini discovered Iapetus while it was in the western half of its orbit, but was surprised not to manage to observe it on the eastern side of the planet. In fact he didn't succeed in observing it on the eastern side of Saturn until 1705, when, equipped with a larger telescope, he finally managed to follow it along all of its orbit.
In fact, Cassini himself was the first to suggest that Iapetus has one hemisphere much darker than the other, and that having the same face always pointing towards Saturn, presented either the darker or lighter side, depending on the position in its orbit.
The first scientific explanation for this strange phenomenon was given in the '70s by Steven Soter, who suggested that the leading hemisphere of Iapetus, that facing the direction of travel, was being bombarded by micrometeorites (and more in general by dust) coming from another of Saturn's satellites in a larger, retrograde orbit, Phoebe.
In support of Soter's hypothesis was the fact that the darkest region, called Cassini Regio, is located exactly at the centre of the leading hemisphere, just where one would expect the bombardment rate to be highest. Also, the discovery of a new ring of Saturn in 2009, located very close to the orbit of Phoebe and pushed inwards by this satellite, seemed finally to confirm Soter's scenario.
But in 2010 a group of astronomers re-examined a series of images from the Cassini mission and found some inconsistencies. The transition from dark hemisphere to light hemisphere is not gradual as one would expect in the case of bombardment along its orbit, but was instead rather abrupt. Also, the poles are too light to have undergone the deposition of dark material. Finally, from a spectroscopic point of view, Cassini Regio and Phoebe are just too different to support the idea of a link between the two.
Now the Cassini mission team, led by Daniel Tamayo of Cornell University, has solved the mystery, demonstrating that the darker hemisphere absorbs more solar energy and so heats up more, causing the sublimation of more reflective ices, and leaving darker, more heat absorbing material behind. Once initiated by the deposition of dust, the process is therefore self-perpetuating.
The fact that dark face of Iapetus also extends somewhat into the trailing hemisphere is explained via eccentricities in the orbit of the dust particles. The poles are largely excluded from this heating process as they receive relatively little bombardment and certainly much less solar radiation.
Tamayo and colleagues have also managed to explain the spectroscopic differences, demonstrating that Phoebe is not the only satellite to "dirty" Iapetus. In fact, even the tiny satellite Ymir and probably other irregular moons are a source of dust, mopped up by their "big brother".
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