Hubble Reveals New Moons, Rings Around Uranus

Ben Harder
for National Geographic News
December 22, 2005
California-based astronomers have found two previously undiscovered rings and two new moons in orbit around Uranus.

The discovery shows that the celestial neighborhood surrounding the seventh planet is both more populous and more physically volatile than had previously been realized.

As the newfound moons and the 16 other known satellites jockey for position around Uranus, "they're pushing and tugging on each other gravitationally in an unpredictable and apparently chaotic manner," said Mark R. Showalter of the SETI Institute in Mountain View, California.

The constant buffeting could ultimately erase the new rings and destroy at least some of the moons, Showalter said.

He co-discovered the new entities with Jack J. Lissauer of NASA Ames Research Center in Moffett Field, California.

Their study, which relied on images from the Hubble Space Telescope, was published online today in the journal Science.

New Moons

Stargazers have known for decades that Uranus is girded by narrow rings and orbited by several groups of moons, including five large, relatively distant ones that astronomers discovered long ago.

Passing by Uranus in January 1986, the Voyager 2 spacecraft discovered ten new satellites, all of them much smaller and much closer to the planet.

Scrutiny of the craft's images later led to the identification of an eleventh inner moon.

One of the newly discovered moons, dubbed Cupid, lies in the midst of that inner group. The other, called Mab, circles slightly farther out.

Uranus inhabits a region of the solar system that's a "shooting gallery of comets," said Joseph A. Burns, a planetary scientist at Cornell University in Ithaca, New York.

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."

Star-Crossed Lovers

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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