Figuring out the fragments of Hyperion.

Figuring out the fragments of Hyperion

Some scientists believe Saturn's moon Hyperion, photographed in 1981 by Voyager 2, consists of fragments that recombined after a collision with another large object shattered it. Rather than rotating with the same side always facing Saturn as moons normally do, Hyperion moves "chaotically" on its axis. It also has an irregular shape and a shortage of large craters, both cited as possible evidence of such a catastrophic event.

But did the fragments indeed recombine into a satellite after such a collision? And if not, where did they go?

Hyperion's present shape suggests its fragments could not have reunited, say Paolo Farinella of the University of Pisa, Italy, and colleagues from there, the University of Arizona in Tucson and Italy's Torino Astronomical Observatory. They note that the fragments would have separated with escape velocities of several hundred meters per second -- higher than the gravitational attraction of any surviving large chunk could overcome. Instead, the scientists suggest, the fragments might have crashed into the planet's big satellite Titan or else spread through the inner portion of the Saturnian system.

"Our models predict that in fact the vast majority of fragments were swept up by Titan," with the total energy of fragments (the product of their masses and velocities) declining rapidly as they moved inward, the researchers write in the January ICARUS.

Farinella's group calculates that if such a Hyperion debris storm took place, it should have created more impact craters on the inner moons' rear portions than on their front-facing hemispheres. Yet Voyager 2 photos of the satellites show no consistent difference. The researchers conclude that "only a tiny fraction of fragments escaped a collision with Titan and could hit the other satellites."

The one-exception, they say, is the moon Rhea, Saturn's only satellite for which there is evidence of a leading/trailing asymmetry." Photos sharp enough for detailed crater studies do not show the whole of any of the satellites' surfaces, but the researchers acknowledge a more fundamental problem: the difficulty of distinguishing craters produced by Hyperion shrapnel from those due to other chunks that were presumably hurtled around in the solar system's early days.