Moonopolies: the solar system's outer planets host a multitude of irregular satellites

Science News, Nov 22, 2003 by Ron Cowen

The collision theory will soon be put to the test. When the Cassini spacecraft enters orbit around Saturn this summer, it will pass near the planet's largest irregular satellite, Phoebe. The spacecraft's images should reveal whether Phoebe has the large gouge that would be expected if it had suffered an ancient collision that gave rise to a host of smaller irregulars orbiting Saturn.

"In the last 10 years or so ... we have made observations that give a new and different view of our solar system," says Jewitt. "Where this will lead is still unclear, but few people would doubt that the end result will be a much improved understanding of the complex process of planet formation."

Puzzling Pair The split personality of Mars' moons

While scientists analyze the spate of irregular moons recently found around the outer planets, they continue to puzzle over two more familiar objects--Phobos and Deimos, the two moons that orbit Mars. "These two satellites are still a mystery," says Scott S. Sheppard of the University of Hawaii in Honolulu. "Their orbits are not like the irregular satellites of the giant planets," since they're close to Mars and have nearly circular orbits with nearly zero inclination, he notes. "It is hard to obtain orbits like the current Martian satellites have from captured bodies."

On the other hand, he notes, their color resembles that of C asteroids, a common type of asteroid found in the outer part of the asteroid belt. So "based on their physical characteristics, they may be captured asteroids and should be classified as irregulars," says Sheppard. "It is still a debate if they were captured or not." R.C.