From: Reuters
Published February 6, 2008 02:11 PM

Scientists explain big vapor plume on Saturn moon

By Michael Kahn

LONDON (Reuters) - Scientists on Wednesday said they have an explanation how one of Saturn's moons can spew out a giant plume of water vapor, adding to evidence a source of life -- water -- lies beneath the moon's frozen surface.

Using a computer model, German researchers showed the temperature at the bottom of surface cracks on Enceladus has to be about 0 degrees Celsius, the so-called triple point of water where vapor, ice and liquid water all can coexist.

"This makes this moon very interesting for further study because there is a connection between liquid water and life," Sascha Kempf, a physicist at the Max Planck Institut in Heidelberg, said in a telephone interview.

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"This is the kind of thing planetary scientists hope for."

The scientists published their findings in the journal Nature.

Mars, Jupiter's moon Europa and Enceladus are the only places in the solar system with direct evidence of water. Finding organisms different from those on Earth may provide scientists with answers to questions ranging from where diseases come from to how our sun and planets formed more than 4.5 billion years ago.

Scientists have taken an especially close look at Enceladus because it seems to have a smooth surface -- suggesting recent geological activity that, in turn, could mean liquid water.

They are also intrigued by the plume itself, a gigantic geyser of water vapor and tiny ice particles. One mystery was how the dust particles slowed down to keep the plume restrained by the gravity of the moon, said Kempf, who worked on the study.

Their model showed most of the dust particles collide with the walls of the surface crack as they are ejected and constrain the gas flow to keep the plume close to the surface rather than shooting into the atmosphere, Kempf said.

The team used images of the plume and properties of the escaping gas and dust particles to run their model. They found it only reproduced the plume when the temperature was at zero degrees at the bottom of the cracks, implying water exists there in liquid form.

"The density of the gas jet inside the cracks is so high that the small dust particles should have the same speeds to escape from the gravity of the moon," he said. "If this was true you wouldn't see plumes. You would see a long jets expanding into the system."

Saturn has at least 47 moons and at least seven rings. The joint U.S.-European Space Agency Cassini mission, launched in 1997, is spending four years examining Saturn.

Cassini is scheduled to fly 50 kilometers (31 miles) over the moon's surface in March, which will provide more information on the precise chemical composition of the particles and water vapor as scientists try to better understand the plume, Kempf said.

(Editing by Maggie Fox and Matthew Jones)

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