Photogenic precipice. This picture of a cliff on Io (stripes, upper right) offers new clues to the moon's geological processes.

Io Says Cheese

With its kilometer-high cliffs and lakes of molten lava, Jupiter's moon Io is, geologically speaking, the most exciting place in the solar system. Planetary scientists have long debated what's shaping the craggy surface. Now, a rare high-resolution image taken by the Galileo spacecraft is offering a fresh look at the geological processes molding this moon.

Io has baffled planetary scientists for decades. The moon is spectacularly rugged, yet it lacks the forces that shape Earth, such as wind erosion and plate tectonics. There are exotic theories--like subterranean glaciers of sulfur dioxide that could be undercutting the terrain--but getting a good look at what's happening on Io is difficult because of its proximity to Jupiter. That giant's gravitational pull makes it nearly impossible to place an orbiter around the tiny moon, and its magnetosphere frequently disrupts satellites' instruments.

Thanks to good timing--and a little luck--NASA's Galileo spacecraft has now taken a high-resolution snapshot of an Ionian cliff. The image, taken in October, reveals piles of debris at the cliff's base, suggesting that a combination of gravity and earthquakes caused landslides that created the cliff. Further analysis of the collapse might also yield valuable information about the composition of the mountains, which randomly dot the surface. "Understanding how the mountains fall apart helps us to understand what they're made of," explains Elizabeth Turtle, a member of the Galileo imaging team at the University of Arizona, Tucson. "It all fits into how Io works as a planet."

The image does not support the more spectacular theories about Io's erosion, but it doesn't discount them either, says Jeff Moore at NASA's Ames Research Center in Moffet Field, California. "I think that there is evidence for exotic processes," he says--but it may take more snapshots to close the case.

Related sites

The Galileo Spacecraft's Web site
A Web site about Io