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Uranus' second ring-moon system

The Hubble Space Telescope images two additional moons and two faint new outer rings, for a total of 27 moons and two complex ring systems.
December 28, 2005
Saturn isn't the only planet to harbor a complex ring structure. In 1986, NASA's Voyager 2 spacecraft sent back images of a family of 10 moons and a system of rings orbiting Uranus. New images from the Hubble Space Telescope (HST) increase those numbers.
Uranus rings
HST imaged Uranus' two newly discovered rings in 2003 and 2005.
NASA, ESA, and M. Showalter of the SETI Institute
On December 22, planetary astronomer Mark Showalter of the SETI Institute and Jack Lissauer of the NASA Ames Research Center announced the discovery of two additional moons and two large outer rings. HST's Advanced Camera for Surveys (ACS) imaged new moons Cupid and Mab as well as two faint, dusty rings from July 2003 through August 2005.

Newly discovered moon Cupid orbits in the midst of a swarm of inner moons known as the Portia group, so named after the group's largest moon. The Portia group lies just outside Uranus' inner ring system and inside the planet's larger, classical moon group of Belinda, Perdita, Puck, and Miranda. Mab, the smaller of the two newly detected moons, orbits outside the inner moon group and Cupid, but interior to Uranus' four outer moons.

A second ring system was also detected around Uranus and imaged by Hubble. Rings R/2003 U 1 and R/2003 U 2 (R1 and R2, respectively) both lie outside the orbit of the inner ring system. Researchers believe micron-size dust is a main constituent of these rings.
Uranus rings
This diagram depicts features and locations of Uranus' known moons and rings.
NASA, ESA, and A. Feild of STScI
What is not clear is how these rings formed. Meteoroid impacts on Uranus' moons that eject fine dust may feed the rings or collisions among existing rings may produce new ones. Either way, the moons' small sizes and surface areas keep any ejected material from falling back to their surfaces and reaccreting. According to Showalter, "Dust material is coming off of Mab and spreading out to make this [R1] ring." It's not apparent what body provides the material for the inner (R2) ring. Lissauer theorizes that a disrupted moon may have been a source.

Showalter believes Uranus' ring-moon system is unstable and exhibits chaotic evolution. Since the last observations were made, Uranus' moons changed orbit. This has long-term implications for Uranus' ring-moon system. "Long-term changes to the system include collisions and crossing ring systems," adds Lissauer.

One thing is for sure, says Lissauer — "Our solar system is a dynamic place."
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