Images captured by NASA's Cassini spacecraft have shown jets of fine, icy particles streaming from Enceladus, a moon around Saturn, scientists reported on Tuesday.

These images provide unambiguous evidence that Enceladus is geologically active, the Cassini mission scientists said at a American Geophysical Union meeting in San Francisco.

The images, backlit by the sun, clearly show multiple jets of the fine spray of material that towers over the moon's south polar region. The fainter, extended plume stretches at least 500 km above the surface of Enceladus, which is only 500 km wide.

Cassini flew through the plume in July, when it passed a few hundred kilometers above the moon. During that flyby, Cassini's instruments measured the plume's constituent water vapor and icy particles, according to the scientists.

"For planetary explorers like us, there is little that can compare to the sighting of activity on another solar system body," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute.

"This has been a heart-stopper, and surely one of our most thrilling results."

Based on earlier data, scientists strongly suspected these jets arise from warm fractures in the region. The fractures, informally dubbed "tiger stripes," are viewed essentially broadside in the new images.

"I think what we're seeing are ice particles in jets of water vapor that emanate from pressurized vents," said Cassini imaging team member Andrew Ingersoll from the California Institute of Technology.

"To form the particles and carry them aloft, the vapor must have a certain density, and that implies surprisingly warm temperatures for a cold body like Enceladus."

In an effort to demonstrate earlier apparitions of the plumes, mission scientists analyzed images of Enceladus taken earlier this year at similar viewing angles.

Imaging scientists are comparing the new images to earlier Cassini data in hopes of arriving at a more detailed, three-dimensional picture of the plumes and understanding how activity has come about on such a small moon.

But they are not sure about the precise cause of the moon's unexpected geologic vitality.

"In some ways, Enceladus resembles a huge comet," said Torrence Johnson, imaging team member from NASA's Jet Propulsion Laboratory (JPL).

"Only, in the case of Enceladus, the energy source for the geyser-like activity is believed to be due to internal heating by perhaps radioactivity and tides rather than the sunlight which causes cometary jets."

The new data also give yet another indication of how Enceladus keeps supplying material to Saturn's gossamer E ring, scientists said.

Source: Xinhua