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Astronomy

Debate rages over water on Saturn's icy moon

SPACE.com
13 October 2009

The recent discovery of plumes containing water vapor erupting from the south pole of the frigid Saturnian moon Enceladus set off a firestorm of debate. Many scientists thought the geysers of gaseous water must boil out of liquid water stored under the moon's surface, which would make Enceladus a promising candidate for life.

But a new study challenges that conclusion, arguing that the plumes could just as easily come from ice through the process of sublimation - the direct leap from the solid to gaseous state.

This conclusion may dampen Enceladus' astrobiology hopes, though it does not exclude the possibility of liquid water, and thus life, on the moon.

Enceladus' geysers were first spotted in 2005 by NASA's Cassini spacecraft, which was launched in 1997 on a mission to orbit Saturn. Shortly after the discovery, a research group led by Carolyn Porco, head of Cassini's imaging science team, calculated the ratio of water ice to vapor in the plumes. Cassini wasn't able to directly measure the masses of ice and vapor in the plumes, but the researchers estimated the mass of ice from the observed brightness of the plumes (because ice reflects light, so the more ice, the brighter the plumes). They deduced the mass of water vapor from a measurement of the molecular signature of vapor in the wavelengths of light Cassini observed from the plumes.

The scientists found that there were almost equal amounts of ice and water vapor. If this is the case, the researchers argued, the plumes cannot result from the sublimation of ice. The ice crystals are believed to be re-condensed from the vapor, rather than directly ejected from the surface or subsurface of the moon. A lot of vapor is needed to produce that 50/50 ice and vapor ratio, but the laws of thermodynamics prevent that much vapor from being produced solely by sublimation. Therefore, some of the vapor must be produced by evaporation rather than sublimation. This means a vast reserve of liquid water must exist on Enceladus, perhaps as shallow as 23 feet (7 m) under the surface. Under this theory, the researchers dubbed Enceladus' plumes "Cold Faithful" after the Old Faithful geyser in Yellowstone National Park. Their paper was published in 2006 in the journal Science.

But a new study led by Susan Kieffer of the University of Illinois at Urbana-Champaign challenges that work. She and her colleagues recalculated the ice-to-vapor ratio and produced vastly different results. Under their calculations, vapor is much more abundant than ice, with their final ratio less than 2:10, compared to the previous team's estimate of 1:1.

Full article at SPACE.com