The Mystery of Jupiter's Rings
In 1979 the Voyager 1 spacecraft saw that Jupiter has rings like Saturn too, albeit a much fainter system primarily made up of dust. The particles in Jupiter's ring are smaller than those in Saturn's rings and do not reflect light as well. In the case of Jupiter, physical models suggest that the particles should not be able to remain in the rings very long, thus the rings should dissolve over time.
Therefore, it is suspected that the ring systems of the giant planets may be relatively young—only a few hundred million years old -- compared to the nearly 5 billion year old age of the planets themselves.
These rings seem to come from material shed when meteors impact four of Jupiter's tiny inner moons: Ejected stuff from Thebe and Amalthea create the two thick, outer "gossamer" rings, while Adrastea and Metis feed the thin, narrow main ring and the faint inner halo.
The last detailed studies of Jupiter's rings took place in the 1990s, when NASA's Galileo spacecraft was orbiting the planet. But now the wealth of highly detailed data imaged from Cassini's ongoing studies of Saturn is paying off for Jupiter: Using pictures of a wave-like pattern detected in Saturn's rings, scientists went back to older Galileo shots of Jupiter's rings and found a wave.
Enter SETI astronomer and Jupiter ring sleuth Mark Showwalter. While everyone knows about Saturn’s spectacular ring system, it’s often forgotten that Jupiter, Uranus, and Neptune are also encircled by fainter and narrower rings. Each of these systems interacts closely with a family of small, inner moons. Showalter works on some of NASA’s highest-profile missions to the outer planets, including Cassini, now orbiting Saturn, and New Horizons, which flew past Jupiter en route to its 2015 encounter with Pluto. He has also searched for the rings of Mars, although so far with no success. Known for his persistence in planetary image analysis, His work on the earlier Voyager mission led to his discovery of Jupiter’s faint, outer “gossamer” rings and Saturn’s tiny ring-moon, Pan.
"Something funny is going on, as if the tip of the ring is flapping in a breeze," says Showalter. For Saturn, the current theory is that "something" disrupts the rings, briefly tilting them out of the equatorial plane, until the planet's gravity snaps the rings back into place. This gravitational action starts a ripple through the rings -- which Cassini sees as a pattern of light and dark that changes over time.
When Showalter and colleagues looked at Galileo shots of Jupiter's rings from 1996, they again saw this odd waving pattern. They were able to calculate the length of each oscillation, allowing them to see that the wave moving through the rings must have been started by two separate events: Something disrupted the rings in 1990 and again in 1994 -- the year the pieces of broken comet Shoemaker-Levy 9 smacked into the giant planet, leaving black scars the size of the Pacific Ocean in its atmosphere.
Showalter and his colleague Joe Burns at Cornell University believe the comet impact may also have started a wave in Jupiter's rings. The scientists concluded that the rings could start waving if a massive thing such as the 1992 Shoemaker-Levy 9 swung so close by Jupiter that the planet's gravity ripped the comet apart. The debris then went into a long orbit around Jupiter and eventually hit the planet in 1994, while a diffuse band of smaller chunks passed through the rings, creating a widespread gravitational disturbance that kicked off the wave.
The comet debris would have had enough mass to get such a ripple going, the researchers say, while not being so huge that the pieces simply punctured the rings.
"The rings are catching impacts and even telling us the frequency and size of impacting objects," Burns, a member of the imaging teams for the Cassini, said.
—Image courtesy NASA/JPL/Cornell University
Provided by The Daily Galaxy - nationalgeographic.com and psu.edu
