The Surprise of Jupiters Rings

Prior to the Voyager 1 mission in 1979, it was thought that only Saturn and Uranus had rings. Saturn was known as the ringed planet, and its gorgeous rings had become a generalized symbol for planets.Then, when Voyager 1 visited Jupiter, the scientific world was shocked to see clearly that Jupiter also had rings. Now, scientists have come to think that rings are the norm rather than the exception: Jupiter, Saturn, Uranus, and Neptune all have rings, and some scientists think that even Mars may have very thin and wispy rings.

Jupiter has three rings: a flattened main ring, an inner cloudlike ring called the halo, and a third ring called the gossamer ring. NASA's two Voyager spacecraft first revealed that Jupiter has a flattened main ring and an inner halo, both composed of small, dark particles. Jupiter's rings are made of dust rather than ice, making them finer and darker than Saturn's rings. The main ring is 4,400 miles (7,000 km) wide, with a radius of 79,000 miles (126,000 km), but its thickness is a mere half mile (1 km).The halo ring extends from the main ring almost all the way to Jupiter itself, and is the only ring in the solar system to do so. The halo is a faint ring, 12,500 miles (20,000 km) wide. The image on page 58 shows the halo (top) and the main ring (bottom) but does not show the gossamer ring.

Jupiter's main and halo rings, discovered in 1979 by Voyager 1, are darker and finer than Saturn's bright rings. (NASA/JPL/Ga/i/eo)

Only one Voyager image showed a hint of the third, faint outer ring. Later Galileo data revealed that this third ring, known as the gossamer ring because of its transparency, consists of two rings (see the figure above). One is embedded within the other, and both are composed of microscopic debris. The Galileo spacecraft acquired this mosaic of Jupiter's ring system (top) when the spacecraft was in Jupiter's shadow looking back toward the Sun, and a figure showing the positions of the rings is shown at bottom.

Joseph Burns, Maureen Ockert-Bell, Joseph Veverka, and Michael Belton, from Cornell University and the National Optical Astronomy Observatories, have developed theories for how Jupiter's unusual ring system formed. Some of the three dozen Galileo images from 1996 and 1997 actually showed the gossamer-bound dust shedding from Amalthea and Thebe, which may be the main sources of the ring material.These images provided one of the most significant discoveries of the entire Galileo imaging experiment, because they showed that the gossamer rings are formed in an entirely unexpected way.

The rings contain very tiny particles resembling dark reddish soot. Unlike Saturn's rings, Jupiter's rings show no signs of ice. Scientists believe that dust is kicked off the small moons when meteoroids strike them, and perhaps by magnetic field interactions. Though interplanetary speeds of six to 50 miles per second (10 to 70 km /sec) are normal for asteroids and meteoroids, Jupiter's huge gravity field accelerates the tiny bodies beyond even these huge velocities.Without any atmospheres to slow the meteoroids, they strike the moons so fast that they instantly become buried in the surface of the moon and vaporize and explode from frictional heating and shock.

The impacts of the meteoroids cause clouds of dust particles to be thrown off the moon and into Jupiter orbit (these inner moons are so small that their escape velocity is negligible). As dust particles are blasted off the moons, they enter orbits that are much like those of their source satellites, both in their distance from Jupiter and in their slight tilt relative to Jupiter's equatorial plane. The range of orbits forms a disk shaped like a washer, with a flat outer edge.This explains why, in Galileo's close-up, edge-on view, the ring-tip profiles are rectangular rather than the familiar elliptical arc seen on Saturn's rings and Jupiter's main ring.

The innermost halo ring appears to contain escaped particles from the main ring. The particles are electrically charged and interact with Jupiter's enormous electromagnetic force, causing the cloud of particles to swell into a vast cloud slowly drawn down into the planet.

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