Enceladus, showing the tiger stripes at the southern region of the moon.

Credit: NASA

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PARIS, 1 June 2006 - Scientists believe they can explain a mystery that enshrouds Enceladus, a moon of Saturn that could be the best bet for looking for life elsewhere in the Solar System.

Orbiting eccentrically in Saturn's outermost ring, Enceladus is a strange and tiny world of white.

It measures just 504 kilometres across, thus defying its name as a giant of Greek mythology, and has brilliant shell of ice that is pristine except for some odd-looking grooves and pockmarks from recent space impacts.

Just as its surface is a frigid hell, counter-intuitively, beneath the ice Enceladus seems to be relatively balmy.

Flybys by the U.S. probe Cassini have shown plumes of water vapour that vent from its surface, shooting crystal jets upwards for hundreds of kilometres (see COSMOS magazine, Issue 8).

One hypothesis is that these "cryo-volcanoes" are caused by a phenomenon called tidal heating.

Gravitational pull from giant Saturn and the nearby satellites of Dione and Janus squeezes and stretches the moon's geological interior, causing friction that heats the sub-surface water.

But, perplexingly, Enceladus' hotspot is only found in a polar region - at its south pole.
A pair of American space scientists believe they have the answer for this.

Writing in Thursday's issue of the British weekly journal Nature, they suggest that beneath Enceladus' icy surface, tidal heating has caused an upwelling of warm, low-density material.

Spinning bodies are most stable if most of their mass is close to the equator. Any redistribution of mass within a rotating object causes the axis of spin to become unstable.
In the case of Enceladus, the large blob of low-density material - either warm water or hot silicate at its rocky core - would cause the moon to roll over.

The spin axis would remain fixed, but the blob, known as a diapir, would end up on the south pole.

This would explain not only the geysers but so-called tiger stripes, or fault lines, in the ice that emanate from the southern polar region and measure some 130 kilometres long.

"The whole area is hotter than the rest of the moon, and the stripes are hotter than the surrounding surface, suggesting that there is a warm concentration of warm material below the surface," said co-author Francis Nimmo of the earth sciences department at the University of California at Santa Cruz.

Enceladus may not be alone in being reoriented this way. A similar process could have happened on other small moons, such as the Uranian satellite Miranda, according to their theory.

Astrobiologists are hugely intrigued by the combination of heat and water, two of the essentials for life, on Enceladus.

On its surface, the temperature is -193ËšC, while in the "tiger stripes" it warms to -133ËšC, which means the interior must be hotter still.

Bob Brown, a senior scientist working on Cassini, told a conference in Vienna last month that Enceladus has the chemical building blocks, or their precursors, for making life.
Even so, any life is likely to be microbial at best, holed up in what could be a chill sub-surface ocean, Jeffrey Kargel of the University of Arizona said in a commentary in the U.S. journal Science in March.

"Any life that existed could not be luxuriant and would have to deal with low temperatures, feeble metabolic energy, and perhaps a severe chemical environment. Nevertheless, we cannot discount the possibility that Enceladus might be life's distant outpost."