A comet strike can shatter a small, fragile moon, transforming it into a ring of scattered dust particles, he said.
But, Burns added, such "rings themselves don't last very long. The particles can reaccumulate into a little moon, albeit one that's very weakly bound."
Such moons are often less dense than water, he said.
It is not yet certain how Mab and Cupid formed as moons, the researchers said.
Of Rings and Wrecks
The newfound rings, called R/2003 U 1 and U 2, are made mainly of dust, but they don't seem to be disappearing quickly, Showalter said.
"The dust has to have some continual source," he said.
Uranus's previously known rings consist of larger rocks and boulders, so their stability is less surprising.
One of the new dust rings lies exactly in Mab's orbit, which suggests that the ring is made of material ejected from the moon by meteorite impacts, Showalter said.
"Mab is continually replenishing this ring," he said.
The other dust ring contains no moon large enough to maintain the ring by itself, he noted.
"Its source is probably not a single moon but a belt of moons too small for us to see," Showalter said.
A collision between two larger moons might have created that swarm of dust-emitting bodies, he added.
Showalter predicts future collisions, too. He and Lissauer compared various moons' positions on recent images and older ones. The orbits of most of the satellites have shifted in the past 20 years, they determined.
Those observations, along with computer simulations, "suggest the system may be very unstable over longer periods," Showalter said.
"Tiny Cupid appears to be in the most tenuous position of all," he said.
"Sometime in the next million years or so, it's likely to bang into Belinda"a significantly larger moon"and that will be the end of Cupid."
Showalter said the issue of what to call the new moons weighed heavily on his mind.
"Not many people get to name celestial objects. So you take it pretty seriously," he said.
By convention, astronomers label moons that they discover according to international rules.
In the case of Uranus each moon must bear the name of a character from classical literature.
Previously discovered moons had been dubbed Juliet, Miranda, and Portia, for instance, after Shakespearean heroines.
By 2003, when the astronomers spotted the first new satellite in Hubble images, Showalter had settled on using the name of Queen Mab. In Romeo and Juliet, Mercutio blames that sprite for tampering with people's dreams.
The name Cupid seemed appropriate for the smaller of the newfound moons, which is just 11 miles (about 18 kilometers) across, Showalter said.
"Jack and I like the image of little Cupid orbiting among the great lovers of Shakespeare's literature," he said.
Showalter knows that the satellites he has given such attention to naming may eventually vanish. But for a while, at least, "they'll still be there for people to talk about."
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