During its only close flyby of Saturn's grayish moon Dione, NASA's Cassini spacecraft found an icy world without atmosphere, mission scientists said Tuesday.

On Oct. 11, 2005, the spacecraft came within 500 km of Dione surface, and took fine pictures of the moon, said NASA's Jet Propulsion Laboratory (JPL).

Water ice is detected on the surface of Dione, and striking brightness variations are also found across the surface, said the Cassini ultraviolet imaging spectrograph team.

Dione shows a heavily cratered surface, like most of its counterparts in the Saturnian system. It has a signature style that includes streaky terrains dominating one whole side of the moon. The fine latitudinal streaks appear to crosscut everything and to be the youngest feature type in this region, scientists said.

Like the tiger stripes on Saturn's another moon Enceladus, these striking cracks and fractures might be caused by tectonic activity, scientists added.

"Dione seems to be an older sibling of Enceladus," said Bonnie Buratti, scientist on the Cassini visual and infrared mapping spectrometer team. The team is working on compositional maps of the moon's surface.

Multiple generations of fractures are visible on Dione. Numerous fine, roughly parallel grooves run across the terrain and are interrupted by the larger, irregular, and bright fractures. In several places, fractures postdate some deposits at the bottom of craters.

This could be the result of cracks and fractures in the ice. "The ice in the fractures appears to be different from that in the surrounding terrain. This may be due to the grain size variations, " said Amanda Hendrix, Cassini scientist at the JPL.

As on other Saturnian moons, rockslides on Dione reveal cleaner ice, while the darker materials accumulate in areas of lower topography, such as crater floors and the bases of scarps.

However, scientists on the Cassini fields and particles instruments noted that early results do not support the presence of an atmosphere.

Dione orbits Saturn within the broad, tenuous E-ring. Hence, scientists will be looking to see if Dione, like Enceladus, is a source of material in the E-ring.

They also seek to learn whether the E-ring is affecting Dione's surface. Over the coming months, scientists will begin to piece together a more detailed story of Dione, according to the JPL.

Source: Xinhua