Most of Saturn's 15 moons appear to consist of the same ghostly materials that comets are made of, scientists directing the Voyager planetary probe said today.
"The main objects that have been striking these moons have been the nuclei of comets," said Dr. Eugene Shoemaker of the California Institute of Technology. "In fact, some comets may still be striking Saturn's moons as they move in and out of the outer solar system."
Shoemaker, speaking here at the Jet Propulsion Laboratory where the Voyager mission is being directed, said that the appearance, size and composition of most of Saturn's moons suggest they formed near the outer edge of the solar system when the sun and planets were being formed. He said the measurements of the moons, sent back by Voyager, place most of them in the same size range (300 to 1,000 miles across). He added that their composition seems to indicate that they were formed from the same type of ice and dust particles that make up comets.
"They're half rock and half ice," Shoemaker said. "That's the typical [comet] nucleus."
The pictures of these icy moons taken by Voyager also show surfaces covered with craters that Shoemaker said were left by the impact of bigger space debris that he called "planetesimals." He said these asteroid-size objects formed near Uranus and Neptune, then wandered into the orbit of Saturn, where they struck the planet's moons with enough force and frequency to pock them with craters. Some of these collisions were so violent that they came very close to destroying the moons.
"Most of these moons are cracked," Shoemaker said. "We see cracks on each side of Mimas, Tethys, Dione and Rhea. These cracks are in the same place on each side of the moons, suggesting they were made by the same impact."
A crack on Mimas, the smallest of the icy moons, is so large that the moon could not have survived a more forceful impact.
"The crater that collision made," Shoemaker said, "is as close in size to a crater you can make on a body that size without knocking it to pieces."
The two moons that do not have cracks are Titan and Enceladus. Titan is so big and has such a dense atmosphere that planetesimals burn up before they hit its surface. Enceladus is so smooth that it may have a heat source that melts its ice and floods over the craters.
Shoemaker said that Enceladus is so close to the large moon Dione that Dione may force it up and down when it passes by, forcing tidal motions on the smaller moon that heat it by friction.