Saturn's moons shedding mystery

2007-10-12 07:44

Paris - Little by little, two moons of Saturn that rank among the enduring enigmas of the solar system are shedding their mystery.

Using the orbiting US-Italian probe Cassini, scientists are getting astonishing close-up looks of Enceladus and Iapetus, two strange slaves of the Saturnian king.

Enceladus seethes with paradox.

Following an eccentric orbit in Saturn's outer ring, it is named after a giant of Greek mythology yet is just a tiddler, little more than 500km across.

Odder still, Enceladus's surface is a brilliant white shell of ice - yet some believe that beneath it lurks a warm ocean, heated by the tidal friction of water molecules pulled by Saturn's gravity - and if so, this could be a bet for harbouring life.

Enceladus' hotspot is the south pole, where geysers of vapour vent into space.

Astronomers at the Space Science Institute in Boulder, Colorado, have carried out a thermal mapping of this region, and found that the jets correlate with the location of "tiger stripes", the name for fractures that run in parallel across the region.

"This is confirmation that the (vented) material's coming from the tiger stripes, or very near the tiger stripes," said Joseph Spitale, co-author with Carolyn Porco of the paper which appears on Thursday in Nature.

"What we've still to discover is what's causing the jetting," he said.

"Tidal energy is probably the ultimate source of the heat, although the exact orbital mechanism that's maintaining that is not entirely agreed upon. Nor is there agreement on the fundamental physical structure that results in this material coming out."

'Thermal segregation'

Spitale added that there remained rival hypotheses as to whether the material was water or hydrated gases called clathrates, and competing theories as to whether the material lay just beneath the surface or farther below.

Meanwhile, eyes are turning to Iapetus, Saturn's bizarre two-tone moon.

One side of Iapetus is becoming progressively darker and the other side turning progressively whiter.

European and US scientists - again using precious data sent back by the distant Cassini - believe they may have found the explanation for this Janus-like effect.

Dusty material, originating from Saturn's outer moons orbiting on the other side of the gas giant, hit Iapetus head-on.

As a result, one side of the moon gets blanketed in a thin black coating, which absorbs more sunlight and warms up.

It's not exactly balmy on the dark side - minus 146 degrees Celsius - but the warming is enough to cause local water ice to evaporate, goes the hypothesis.

This vapour then circulates to condense on the nearest cold spot, on the icy, bright side of the moon.

As a result, the dark side loses its surface ice, and thus becomes darker, while the bright side accumulates ice, and gets brighter.

The "thermal segregation" theory was first aired more than 30 years ago, but is now gaining support thanks to thermal imaging of the vapour and ultra-violet data sent back from Cassini, the European Space Agency (ESA) said on Tuesday.

- AFP