NASA's Cassini spacecraft skimmed 46 miles above the icy surface of Saturn's moon Enceladus on Wednesday, taking radio measurements to deduce what might feed powerful geysers of water vapor streaming into space from the enigmatic moon's south pole.

File photo of Enceladus from a previous Cassini encounter. Credit: NASA/JPL/Space Science Institute
 
Wednesday's flyby also included a pass 5,000 miles from Dione, another of Saturn's moons believed to be two-thirds ice.

Cassini's flyby of Enceladus on Wednesday was the third visit to the moon to focus on studying its gravity field. Scientists can detect variations the moon's internal mass by measuring how its gravity warps radio signals traveling from Cassini back to Earth.

The frequency fluctuations caused by Enceladus are indicative of its physical properties.

Researchers used the flyby to search for what lies underneath plates of ice at the south pole of Enceladus, where fissures in the ice sheets discharge plumes of water dozens of miles into space.

The fissures resemble tiger stripes, and the moon's ice sheets behave like tectonic plates on Earth. But instead of volcanoes of lava and rock, eruptions of water emanate from the plate boundaries on Enceladus.

Scientists believe the tectonic activity is driven by relatively warm water or ice under the south pole of Enceladus.

"The radio science team is particularly interested in learning how mass is distributed under [the] south polar region, which features jets of water ice, water vapor, and organic compounds spraying out of long fractures," said a flyby summary posted on a NASA website. "A concentration of mass in that region could indicate subsurface liquid water or an intrusion of warmer-than-average ice that might explain the unusual plume activity."

Other flybys of Enceladus have used instruments aboard Cassini to taste and sniff the material inside the geysers as the probe flew through the jets.

Wednesday's sweep of Cassini through the vicinity of Enceladus and Dione is the last time the spacecraft will pass near the icy moons until 2015.

Cassini will use the gravity of Titan, Saturn's largest moon, to swing the probe into an inclined orbit around the gas giant, allowing it to observe the northern and southern reaches of the planet, according to NASA.

The three-year inclined orbit phase will include a series of close flybys of Titan to study the hazy moon in more detail than ever before. Titan's surface is covered with lakes of liquid ethane, and the moon is shrouded in thick noxious smog and clouds which rain hydrocarbon precipitation.

Cassini is in an extended mission running through 2017. The spacecraft arrived in orbit around Saturn in 2004.