The deep impacts that killed the dinosaurs or excavated our moon's vast craters count among the most spectacular examples of collisions in the solar system. Even little crashes, however, can make a big difference.
Images taken by the Galileo spacecraft reveal that the dust kicked up by scraps of interplanetary debris plowing into four of Jupiter's tiniest moons are the source of the giant planet's dust rings. Mars' moon Phobos also has been pummeled and its surface pulverized into powder perhaps a meter deep.
Faint rings encircling Jupiter's equator between the planet and its large moon Io were discovered by the two Voyager craft in the late 1970s. The craft revealed a flattened main ring, along with a puffier, inner ring called the halo. The observations also hinted at a third, wispy, outer ring. Galileo images, taken in 1996 and 1997 and released this week, show that the outer ring is in fact two rings, dubbed gossamer rings.
Jupiter's tiny moon Adrastea, only 20 kilometers across, skims the main ring's outer edge, while another small satellite, Metis, lies within the ring. The Galileo pictures confirm that the ring's densest part is the outer edge, adding weight to earlier suspicions that Adrastea feeds the ring. As one of the smallest Jovian moons, Adrastea has weak gravity and stands to lose great amounts of dust during any impacts.
The new pictures show that two other moons, Thebe and Amalthea, each orbit the outer edge of a different gossamer ring and provide the material for them. The halo appears to be made of charged dust particles that are lifted out of the main ring by electromagnetic forces, says Joseph Veverka of Cornell University.
"For the first time we understand why Jupiter has rings and how the rings actually work," he says. Veverka and his colleagues unveiled the Galileo images at a Cornell press briefing.
All four moons appear dark, red, and heavily cratered, indicating that they have been bombarded by meteoroids, which are fragments of asteroids and comets. Correspondingly, the rings contain tiny, reddish particles that resemble dark soot. Galileo viewed the rings almost edge-on, lit from behind by the sun, an arrangement that made micrometer-size particles highly visible.
The angles at which the satellites orbit Jupiter, relative to the planet's equatorial plane, correlate with the vertical extent, or height, of the rings. …