Sustaining the Uranian rings.

Sustaining the Uranian rings

A number of researchers have suggested that the rings of Uranus -- compared with the solar system, which has been around for some 4.6 billion years--look relatively young, perhaps less than a billion years old. But scientists are traditionally suspicious of explanations whose validity depends on observations made exactly at some key moment. How likely is it that the Voyager 2 spacecraft just happened to be around to photograph the rings during the short span when they existed?

An alternative theory from Larry Esposito of the University of Colorado at Boulder suggests the rings may not be young at all. Instead, he proposes, their chunks keep grinding against one another in a way that continually creates new particles. Without some such regenerative process to sustain them, Esposito notes, the Uranian rings as Voyager 2 saw them would be gone in a billion years or less.

Indeed, many of the particles are so small that they would be dragged down to destruction in that time by the planet's extended hydrogen atmosphere. The motions and present locations of two small Uranian moons known as "shepherd" satellites -- believed to be keeping one of the rings as sharp-edged and narrow as it is -- suggest they have been doing so for only about that long. Additional shepherds have been proposed for many of the other nine known Uranian rings, though no one has yet identified any in existing photos.

Besides the main rings, Voyager 2's data also showed 50 to 100 tenuous "dust bands" containing smaller particles than the rings. But Esposito says their different positions mean the bands cannot have come from the rings. In a paper submitted to NATURE, he instead proposes that the dust in the bands may have been generated by micrometeoroids colliding with "unseen moonlets," each about 200 meters across, in belts some tens of kilometers wide, as well as by collisions between the moonlets themselves.

"Both the moonlet belt objects and the main rings of Uranus have been created by the breakup of larger objects," he says. Any dust or rings that existed billions of years ago would long since have been removed by atmospheric drag, he says, so some mechanism must be creating new dust. In the solar system's early history, according to Esposito, the material that later became the Uranian ring system may have consisted simply of perhaps 10 to 12 moons, each about 200 kilometers across. Since then, the ring system has been in a continuing state of gradual evolution.