Amalthea, the largest of Jupiter's four inner moons, measures only 106 miles in diameter; Adrastea, the smallest at 10 miles, is even smaller than Manhattan. But astronomers say the four tiny moons are responsible for an outsize Jovian feature--they're the source of Jupiter's rings.
Images from the Galileo space probe show that ring particles mimic the orbit of their mother moon. "Basically what you've got is a satellite out there with an orbit just like all these ring particles, or vice versa," says Joseph Burns, a Cornell astronomer on the team that made the discovery.
Galileo's instruments found that minuscule fragments of asteroids and comets, most smaller than a pea, bombard the inner moons at speeds of 90,000 miles an hour or more. Gravity is so weak on the small moons, says Burns, that dust from the blasts billows off the lunar surfaces into orbit around Jupiter. Tiny Adrastea, with the weakest gravity, is the best ring builder.
Jupiter's main ring, 4,000 miles wide, was produced by the moons Metis and Adrastea, which share nearly the same orbit, some 80,000 miles from Jupiter's center. Thebe and Amalthea helped form the Gossamer rings. Jupiter's innermost ring, the halo ring, consists of particles from the main ring interacting with the planet's magnetic field.
Astronomers now believe that new particles are continually produced while existing ring particles are pulled onto Jupiter, which means that Jupiter's rings are a permanent feature. Some astronomers had theorized that the rings were the result of an ancient collision and would gradually disappear as their dust settled onto Jupiter.
Burns says he expects that after the Cassini spacecraft reaches Saturn in 2004, similar rings will be seen associated with even smaller moons. "The reason we're interested in rings to start with," he says, "is that the dynamics going on in that sort of a system are much like the dynamics, the evolution, that went on back when the solar system itself was put together."