Miranda: shattering an old image.

Measuring a mere 470 kilometers across, Uranus" moon Miranda has barely enough gravity to maintain its spherical shape. Yet its scarred surface seems more appropriate for a planet 10 times that size. When astronomers get their first close-up view of this icy moon, in images from the Voyager 2 spacecraft in 1986, they were struck by a strange feature--three ovoid regions, now known as coronas, that contain sets of parallel ridges, troughs, and scraps.

Researchers initially proposed that the rough-hewn surface formed because Miranda, after being struck by a comet or other large object early in the history of the solar system, shattered and then influence of Uranus' gravity. That scenario has appeared in several textbooks as well as popular accounts of Mirandahs evolution. But in reanalyzing the Voyager images, Robert T. Pappalardo, Ronald Greeley, and Stephen J. Reynolds of Arizona State University in Tempe say they have found evidence to demolish that notion.

Pappalardo and others note that if the breakup theory were correct, the moon's rocky, denser chunks would have sunk toward the reassembled core if the satellite. Internal currents created by the sinking would have formed the coronas by compressing the surface, and the ridges and troughs would represent compressional folds within the coronas. But when he and his colleagues exmained the Voyager images using a filter that highlighted Miranda's unusual topography, they found no evidence of compression.

Instead, says Pappalardo, the coronas appear to have formed atop giant upwellings of material from the moon's interior. The alternating ridges anf troughs were created when internal forces pulled the surface apart--in much the same way that such features were formed in the American Soutwest and on Jupiter's moon Ganymede, he notes.

Images of Miranda's Arden and Inverness coronas indicate that giant blocks material, some more than 10 kilometers dominoes to create fault scraps, Pappalardo says. He suggests that tidal stretching and distortion by Uranus' gravity could have supplied the heat source necessary to fuel such uprisings.