Galileo catches Io in a slump. (Astronomy).

Giant mountains and lava-flooded craters, towering volcanic plumes and layers of sulfurous snow: When it comes to diversity of terrain, Jupiter's moon Io is no slouch. But some features on this tiny, volcanically active satellite do slump.

Flying within 190 kilometers of the moon on Oct. 16, the Galileo spacecraft took close-up images that show for the first time material that has slid downward along a cliff on Io. NASA released the images on Nov. 27.

Unlike most erosion on Earth, the slumping can't be due to wind or water since airless Io has neither. Rather, the erosion suggests that the cliff, perched on a mountain called Telegonus Mensa, is succumbing to the tug of Jupiter's immense gravity, says Galileo researcher Elizabeth P. Turtle of the University of Arizona in Tucson.

If researchers can determine how tall the cliff was originally, the amount of slumping could indicate the strength and perhaps the composition of the cliff, she notes. Turtle and her colleagues may be able to determine the height by analyzing shadows cast by the cliff, which lies in Io's southern hemisphere.

The new Galileo images reveal features on Telegonus Mensa as small as 10 meters across. Earlier Galileo images, which show a larger area because they were taken from a greater distance, show that the cliff has scalloped edges. That pattern suggested a very different explanation for the slumping--liquid sulfur dioxide seeping from the base of the cliff.

Volcanic plumes on Io spew sulfur dioxide vapor, which turns to snow at high altitudes and falls onto the moon as frost. If further volcanic activity buries the frost, the enormous pressure can melt the sulfurous snow. Although the newest images reveal that gravity alone is eroding the cliff, sulfur dioxide might still play a role in the erosion of other mountains on Io, Turtle notes. --R.C.