The images from NASA's Galileo spacecraft include the highest-resolution view yet of Io, one of Jupiter's large moons. A bumpy plain in that image has dark and light patches interspersed like dark rocks reappearing through a shrinking layer of springtime snow. Elsewhere on Io, plains appear fully blanketed by the snow-like material.

It's not frozen water like Earth's snow, but a sulfur-rich material that looks like white snow, said Dr. Alfred McEwen, a planetary scientist at the University of Arizona, Tucson. In some of Io's active volcanic plumes, the volatile material apparently even falls to the ground as frozen particles or crystals, like snowflakes.

"We see this volatile material everywhere on Io where we've had a close-up look," McEwen said. It includes sulfur dioxide, and probably other sulfur-rich substances, he said. On Earth, sulfur dioxide is a gas. On Io, it can be either a solid or a gas at the surface, or a subsurface liquid.

The textures on Io's plains suggest that some material that had been solid has dissipated by sublimating from a solid to a gas.

"It looks like this volatile material is sublimating or eroding away by some means, yet it's still there," McEwen said. "We'd like to know where it is coming from, how the surface layer is being resupplied."

New images also add evidence that many bowl-shaped depressions in volcanic areas may be linked to fracturing and shifting of Io's crust, not just to collapses above underground magma chambers, like the large volcanic craters called calderas on Earth.

Galileo took the images during a flyby that passed within 199 kilometers (124 miles) of Io in February, and transmitted them to Earth during the following eight months. To approach Io, the spacecraft had to enter a region of hazardous radiation around Jupiter.

"It's gratifying to see Galileo continuing to provide us with new discoveries after enduring more than three times the radiation dosage it was designed to handle," said Duane Bindschadler, manager of Galileo's science planning and operations team at JPL.

McEwen and other researchers have been analyzing the new images, and are describing some of their observations during meetings of the American Astronomical Society's Division of Planetary Sciences conference in Pasadena, Calif., this week.

Heat from many of Io's active volcanoes plays a part in redistributing the snow-like volatiles, said University of Arizona researcher Moses Milazzo.

A bluish haze appears at edges of lava spreading from Io's Prometheus volcano. The lava's heat turns the volatile solids into gas, and the haze is likely crystallization of the gas back into solid particles, like snowflakes, above the ground, Milazzo said.

Milazzo said, "We see the bright volatiles being redeposited up to about a kilometer away," (about a half mile away).

Still, researchers are puzzled why, over many years, the volatile stuff has not been lost into space or become concentrated at Io's colder polar regions, instead of being distributed widely across the moon's surface.

"There's still a lot we don't understand about Io," McEwen said.

The new pictures raise questions about large depressions in Io's surface. These features resemble some volcanic calderas on Earth, though they are not generally found on top of mountains and their shapes are less circular. A spectacular mosaic of pictures covering an area with 11 of these holes suggests many are related to cracks in Io's crust. Seven of them form a chain that makes a right-angle bend, and the angular edges of some of the individual depressions seem to follow the alignments of the group.

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