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Uranus may have two tiny undiscovered moons

Uranus is seen in this false-color view from NASA's Hubble Space Telescope from August 2003.

Uranus is seen in this false-color view from NASA’s Hubble Space Telescope from August 2003. The brightness of the planet’s faint rings and dark moons has been enhanced for visibility. (Click to enlarge) Image & Caption Credit: NASA / Erich Karkoschka (Univ. Arizona)

Launched in 1977, NASA’s Voyager 2 spacecraft is still yielding new discoveries. Scientists using data collected by the spacecraft when it flew by Uranus 30 years ago have found evidence that there may be two tiny, previously undiscovered moonlets orbiting near two of the planet’s rings.

University of Idaho doctoral student Rob Chancia discovered unusual patterns in the rings while studying images of Uranus’ icy rings taken by Voyager 2 in 1986. He noticed that the amount of material on the edge of the alpha ring, one of the brightest of Uranus’ rings, varied periodically. A similar pattern occurred in the same part of the neighboring beta ring.

Uranus as seen by Voyager 2 in 1986

Arriving at Uranus in 1986, Voyager 2 observed a bluish orb with extremely subtle features. A haze layer hid most of the planet’s cloud features from view. Image & Caption Credit: NASA/JPL-Caltech

“When you look at this pattern in different places around the ring, the wavelength is different – that points to something changing as you go around the ring. There’s something breaking the symmetry,” said Matt Hedman, an assistant professor of physics at the University of Idaho, who worked with Chancia to investigate the finding.

Chancia and Hedman are very familiar with the physics of planetary rings – they both study Saturn’s rings using data from NASA’s Cassini spacecraft, which is currently orbiting Saturn. Data gathered by Cassini has yielded new insights into the behavior of planetary rings, and a NASA grant allowed Chanicia and Hedman to study Uranus’ data gathered by Voyager 2.

The researchers analyzed radio occultations, made when Voyager 2 sent radio waves through the rings to be detected on Earth, and stellar occultations, made when the spacecraft measured the light of background stars shining through the rings in order to measure the amount of material the rings contain.

The researchers then found a pattern in Uranus’ rings that was similar to structures in Saturn’s rings called moonlet wakes. They estimate that the hypothesized moonlets in Uranus’ rings are 2 to 9 miles (4 to 14 kilometers) in diameter, smaller than any of Uranus’ known moons. Uranian moons are very difficult to observe because their surfaces are covered in dark material.

“We haven’t seen the moons yet, but the idea is the size of the moons needed to make these features is quite small, and they could have easily been missed,” Hedman said. “The Voyager images weren’t sensitive enough to easily see these moons.”

The researchers finding may help explain why Uranus’ rings are strangely narrow compared to Saturn’s. The moonlets may be acting as “shepherd moons”, guiding the rings and preventing them from spreading out. Two of Uranus’ 27 known moons act as shepherds to the planet’s epsilon ring.

“The problem of keeping rings narrow has been around since the discovery of the Uranian ring system in 1977 and has been worked on by many dynamicists over the years,” Chancia said. “I would be very pleased if these proposed moonlets turn out to be real and we can use them to approach a solution.”

Chancia and Hedman are leaving the confirmation of the existence of the moonlets using telescope or spacecraft images to other researchers. They are continuing to examine patterns and structures in Uranus’ rings, in hopes of uncovering more of the planet’s mysteries.

“It’s exciting to see Voyager 2′s historic Uranus exploration still contributing new knowledge about the planets,” said Ed Stone, project scientist for Voyager, based at Caltech, Pasadena, California.

Uranus flyby: Artist's concept of the Voyager spacecraft. Image Credit: NASA

Artist’s concept of the Voyager spacecraft. Image Credit: NASA



Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

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