Cassini images rule out rings around Rhea

Jul 30, 2010 By Lauren Gold
Saturn's Moon Rhea from Cassini (2005). Image: Cassini Imaging Team, SSI, JPL, ESA, NASA

(PhysOrg.com) -- Something unknown is causing a strange, symmetrical structure in the charged-particle environment around Rhea, Saturn's second-largest moon. But contrary to 2008 reports, it's not a system of rings.

Using NASA's orbiting , a team of astronomers led by Cornell research associate Matthew Tiscareno searched for narrow rings, broad rings and any material from dust to giant boulders that might be orbiting the 1,500 km- (950 mile)-wide .

They report their non-detection in the July 29 issue of .

The research contradicts earlier suggestions that Rhea has a system of narrow rings embedded in a broad circumsatellite disk or cloud. That 2008 announcement was based on a sharp, symmetrical drop in electrons detected around the moon by Cassini during a 2005 flyby.

If the hypothesis had been true, it would have been the first report of rings orbiting a moon. "It would be surprising," Tiscareno said.

Putting the hypothesis to the test, they positioned Cassini to view the moon at what would be edge-on to the rings, where the greatest amount of material would be within its line of sight. The researchers took 65 images between 2008 and 2009, some at high phase angles (toward the unlit side of the moon) to detect micron-sized particles that diffract light forward; and others at low phase angles (toward the lit side of the moon) to see larger objects that absorb and reflect light back toward the sun.

Looking for larger objects was more difficult, because the light reflected back from such objects would be faint compared to the light reflected back from nearby Rhea. The researchers used a series of short exposures added together to bring out any potential faint objects -- but still found none.

Using the charged-particle data from the 2008 report, they also refined previous calculations of the size and abundance of ring particles that would be necessary to explain the odd observations. "We were able to say [that for] the amount of dust that you need to account for [the earlier] observations, if it were there, we would have seen it," Tiscareno said.

By disproving the ring hypothesis, Tiscareno added, the findings reopen the mystery about the cause of the 2008 observations.

"There are very strong and interesting and unexplained electromagnetic effects going on around Rhea," he said. "But we're making a pretty strong case that it's not because of solid material orbiting the moon."

Explore further: Total lunar eclipse before dawn on April 4th

Related Stories

Saturn has small moon hidden in ring

Mar 03, 2009

(PhysOrg.com) -- NASA's Cassini spacecraft has found within Saturn's G ring an embedded moonlet that appears as a faint, moving pinprick of light. Scientists believe it is a main source of the G ring and its ...

Cassini Finds Possible Origin of One of Saturn's Rings

Aug 02, 2007

Cassini scientists may have identified the source of one of Saturn's more mysterious rings. Saturn's G ring likely is produced by relatively large, icy particles that reside within a bright arc on the ring's ...

Cassini Radio Signals Decipher Saturn Ring Structure

May 23, 2005

The Cassini spacecraft has obtained the most detailed look ever at Saturn's rings, including the B ring, which has eluded previous robotic explorers. Its structure seems remarkably different from its two neighbors, ...

Saturn Propellers Reflect Solar System Origins

Jul 08, 2010

(PhysOrg.com) -- Scientists using NASA's Cassini spacecraft at Saturn have stalked a new class of moons in the rings of Saturn that create distinctive propeller-shaped gaps in ring material. It marks the first ...

The Vanishing Rings of Saturn

Mar 18, 2008

Saturn: jewel of the solar system, taker of breaths, ringed beauty. Even veteran astronomers can't help but gasp when they see her through a small telescope. Red Alert: Saturn's rings are vanishing.

Recommended for you

Total lunar eclipse before dawn on April 4th

10 hours ago

An unusually brief total eclipse of the Moon will be visible before dawn this Saturday, April 4th, from western North America. The eclipse happens on Saturday evening for Australia and East Asia.

Cassini: Return to Rhea

23 hours ago

After a couple of years in high-inclination orbits that limited its ability to encounter Saturn's moons, NASA's Cassini spacecraft returned to Saturn's equatorial plane in March 2015.

Comet dust—planet Mercury's 'invisible paint'

Mar 30, 2015

A team of scientists has a new explanation for the planet Mercury's dark, barely reflective surface. In a paper published in Nature Geoscience, the researchers suggest that a steady dusting of carbon from p ...

It's 'full spin ahead' for NASA soil moisture mapper

Mar 30, 2015

The 20-foot (6-meter) "golden lasso" reflector antenna atop NASA's new Soil Moisture Active Passive (SMAP) observatory is now ready to wrangle up high-resolution global soil moisture data, following the successful ...

What drives the solar cycle?

Mar 30, 2015

You can be thankful that we bask in the glow of a relatively placid star. Currently about halfway along its 10 billion year career on the Main Sequence, our sun fuses hydrogen into helium in a battle against ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Caliban
1 / 5 (1) Jul 30, 2010
Perhaps evidence of tidal compression-induced static discharge, like cracking an ice tray?

http://solarsyste...Sat_Rhea

Tidally locked in a nearly circular orbit, and thought to be composed of water ice(principally).

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.