Cassini testing for water on one of Saturn's moons

LOS ANGELES, California (AP) -- Three years after gigantic geysers were spied on an icy Saturn moon, the international Cassini spacecraft is poised to plunge through the fringes of the mysterious plumes to learn how they formed.

Wednesday's flyby will take Cassini within 30 miles of the surface of Enceladus at closest approach.

The unmanned probe will be about 120 miles above the moon as it sweeps through the edge of the geysers and measures their chemical makeup.

The carefully orchestrated event will take Cassini "deeper than we've been before," mission scientist Carolyn Porco of the Space Science Institute said in an e-mail.

Scientists long believed Enceladus, the shiniest object in the solar system, was cold and still because it resides hundreds of millions of miles from the sun. But recent evidence shows the Arizona-size satellite is geologically active, with a significant atmosphere and a relatively warm south pole.

In 2005, Cassini surprised scientists when it snapped images of geyser-like eruptions of ice particles and water vapor spewing from the south pole. The dramatic images effectively put Enceladus (en-SELL'-uh-duhs) on the short list of places within the solar system most likely to have conditions suitable for extraterrestrial life.

Scientists generally agree the presence of water, organic compounds and a stable heat source are needed to support primitive life.

Previous measurements by Cassini showed the eruptions were frequent, with gases and particles venting from the surface at about 800 mph and forming plumes hundreds of miles high.

The source of the geysers is a mystery, but some theorize reservoirs of liquid water below the surface are likely supplying the ice and vapor seen in the plumes.

Until now, scientists have not been able to measure the plumes' makeup in detail. Using its particle analyzers, Cassini will calculate the density, size and speed of the various gases and particles. The spacecraft's cameras will also image the moon during the flyby.

Of particular interest is whether the plumes contain ammonia, which can keep water in liquid form and would bolster the theory that liquid water lies beneath.

"There's not much for us ... to do regarding the upcoming flyby except to hold our breaths and cross our fingers," John Spencer of the Southwest Research Institute in Boulder, Colorado, wrote on the Cassini blog.

The close encounter poses little danger to Cassini because the plume particles are small compared with the dust-size debris the spacecraft is used to flying through while orbiting Saturn, scientists said.

The Cassini mission is a collaboration between NASA and the European and Italian space agencies. E-mail to a friend

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