Dump Truck Around Jupiter

Sunday, September 01, 1996
While Saturn’s rings are obvious to anyone who picks up a telescope, few scientists even suspected Jupiter had a ring until images from the Voyager spacecraft in 1979 showed a faint, backlit arc on either side of the planet. Unlike Saturn’s rings, which are thin disks made of boulder-size chunks of frozen gas, Jupiter’s ring is thought to consist mostly of dust knocked off the tiny moons Metis and Adrastea by micrometeorites, and ash from volcanoes on the moon Io. Jupiter’s ring also has an odd shape that has puzzled astronomers: the inner edge is about 6,000 miles thick but tapers to a depth of just 200 miles at the outer edge. Two astrophysicists recently suggested an explanation for that distortion.

Mihály Horányi of the University of Colorado and his colleague Thomas Cravens of the University of Kansas have created a computer model in which Jupiter’s powerful magnetic field indirectly distorts the ring. In their model, ultraviolet radiation from the sun strips electrons from dust particles in the ring, leaving the dust with a slight positive charge.

Below the ring, in Jupiter’s upper atmosphere, the model assumes, ultraviolet light also tears electrons (which have their own tiny magnetic fields) away from atmospheric gases. Those electrons circulate around the planet, accelerated by its magnetic field. The number of electrons is highest near the planet and falls off farther out.

As positively charged dust in the ring, pulled by Jupiter’s gravity, sinks toward the planet, it is attracted to the negatively charged electrons circulating in the planet’s magnetic field above and below the plane of the ring. Because the ring particles encounter more electrons nearer the planet, the ring flares out on its inner edge, its particles pulled by an increasingly dense population of electrons.

Not only does the model explain the shape of Jupiter’s ring, it also predicts that a typical particle survives only 14 weeks before plunging into Jupiter. Does this mean the ring itself may one day disappear? The mass loss rate is puny even though the particle lifetimes are short, Horányi says. If you look at a dump truck speeding down the highway, you’ll see a dust cloud trailing behind it. Every single dust particle lives for just a few minutes, but you always see the cloud. Horányi says that, just like the dump truck, Jupiter’s ring is losing dust, but the loss is insignificant compared with the overall mass of the ring, which is replenished by debris from Metis, Adrastea, and Io.
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