Giant, gassy Jupiter is the colorful king of the solar system. It's the largest planet and the one with the Great Red Spot--a 16,000-mile-wide storm--on its surface. It's one of only four planets known to have hazy rings around their middle. Yes, its pretty neighbor Saturn has a reputation as the ring-bearing planet, but that's because Saturn's rings are visible through a regular telescope. The rings around Jupiter (and Uranus and Neptune) are made of more delicate stuff that's nearly impossible to detect, even with a very powerful telescope.
Last week scientists announced that they have unlocked the mystery of how Jupiter's rings are formed. While studying 36 new pictures taken by the Galileo spacecraft, which has hovered around Jupiter since December 1995, scientists figured out that Jupiter's rings are delicate layers of moondust in orbit around the planet. They are made from the dust of four of Jupiter's tiniest moons.
Attack Of The Flying Meteoroids!
Bits of asteroids and comets, known as meteoroids, are drawn toward Jupiter by the tremendous force of its gravity. On the way, many of them crash into the four closest of Jupiter's 16
known moons: Metis (Med-is), Adrastea (A-dra-stee-a), Amalthea
(A-mal-the-a) and Thebe (Thee-be). Meteoroids that get close to Jupiter are usually traveling at 25 miles a second--about 100 times the speed of a bullet! When one of them hits a tiny moon, the impact kicks up a huge cloud of dust.
If a meteoroid smacked into the Earth's surface that hard, our planet's gravity would pull any dust and debris back toward Earth. But Jupiter's little moons do not have enough gravity to pull the dust back to their surface. The dust escapes in a big powdery plume and streams into orbit. The rings around Jupiter are made of dust from these collisions.
"We can see the rings only when we're behind Jupiter, looking back at the sun," says Maureen Ockert-Bell, a member of the Galileo-spacecraft scientific team, who studied the new pictures. "It's kind of like when you're in a movie theater, and you turn around to look at the projector and hold up your hand so that the light's not in your eyes. Suddenly, you see a lot of tiny dust particles floating around."
When the scientists got a good look at the dust around Jupiter, they noticed a pattern. The brighter main ring, which is close to Jupiter, seemed to be framed by the orbits of two moons, Metis and Adrastea.
A fainter ring, farther from Jupiter, seemed to really be two rings--one inside the other. The inner ring stops abruptly when it meets the orbit of the moon Amalthea. The outer ring stops at Thebe's orbit. These two rings together form the gossamer (goss-uh-mer) ring, which was first spotted by the Voyager spacecraft in 1979. Gossamer means delicate and wispy.
Color provided another clue that the rings are made of moondust. The reddish-brown dust of the rings matches the moons. "We have a definitive answer to the origin of this ring system," says Michael Belton of the National Optical Astronomy Observatories. "It's a big step forward."
Other Planets, Other Rings
Uranus and Neptune also have dark, dusty rings and multiple moons, and scientists now say it's a pretty safe bet that their rings are formed in the same way.
Saturn's bigger, brighter ring system is a bit different. One of the reasons Saturn's rings are so easy to see is that they're made of bigger pieces. "They're not particles; they're chunks!" says Ockert-Bell. "Saturn's rings have very large, three-foot rocks."
Outside Saturn's main rings are dusty rings that look more like Jupiter's. When the Cassini (Ca-see-nee) spacecraft reaches Saturn in 2003, scientists will get a closer look.
Jupiter's tiny moons continue to take a constant beating from comets and asteroids. So the dainty trail around the planet may slowly grow thicker. Thousands of years from now, those four little moons may finally be bombarded to bits. Their ghostly rings may be all that's left of them.