Cassini on the trail of a runaw...( Scientists are on the trail of Iapet...)

Scientists are on the trail of Iapetus' mysterious dark side, which seems to be home to a bizarre 'runaway' process that is transporting vaporised water ice from the dark areas to the white areas of the Saturnian moon.

This 'thermal segregation' model may explain many details of the moon's strange and dramatically two-toned appearance, which have been revealed in exquisite detail in images collected during Cassinis recent close fly-by of Iapetus.

Infrared observations from the fly-by confirm that the dark material is warm enough (approximately -146C or 127 Kelvin) for very slow release of water vapour from water ice, and this process is probably a major factor in determining the distinct brightness boundaries.

"The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process," said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, USA.

Using multiple instruments on Cassini, scientists are piecing together a complex story to explain the countenance of Iapetus. But yet to be fully understood is where the black material is coming from. Is it native or from outside the moon" It has long been hypothesised that this material did not originate from within Iapetus but was derived from other moons orbiting at a much greater distance from Saturn in a direction opposite to Iapetus.

Scientists are now converging on the notion that the darkening process in fact began in this manner, and that thermal effects subsequently enhanced the contrast to what we see today.

"It's interesting to ponder that a more than 30 year-old idea might still help explain the brightness difference on Iapetus," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany. "Dusty material spiraling in from outer moons hits Iapetus head-on, and causes the forward-facing side of Iapetus to look slightly differe
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