Largest Known Planetary Ring Discovered

This band can’t be beat: It circles Saturn and stretches more than 24 million kilometers across.

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By Ron Cowen, Science News

FAJARDO, Puerto Rico — A newly discovered planetary ring can run circles around all the others. The gossamer band of dust encircles Saturn and has a measured diameter of about 24 million kilometers, or 200 times the diameter of the planet.

This finding makes the band the largest known planetary ring in the solar system, researchers reported October 6 at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences. A billion Earths could fit inside the ring.

Calculations indicate the tenuous ring is probably even more extensive and is likely to have a diameter reaching 36 million kilometers.

Too faint to be seen from Earth, the ring extends beyond Saturn’s outer moon Phoebe. Anne Verbiscer and Michael Skrutskie of the University of Virginia in Charlottesville, along with Douglas Hamilton of the University of Maryland in College Park, reported evidence suggesting that the ring is supplied by material that is continually knocked loose from the dark moon’s surface.

“It’s an exciting discovery,” says planetary scientist John Spencer of the Southwest Research Institute in Boulder.

The existence of the ring may explain the long-standing puzzle of why the Saturnian moon Iapetus, which lies inside Phoebe’s orbit, is two-toned (SN: 8/18/07, p. 104), the researchers said. Like Phoebe, the debris in the newly discovered ring orbits Saturn in the opposite direction of Iapetus and the planet’s other inner moons. The ring has the same orbital tilt as Phoebe’s orbit, a 27-degree incline to Saturn’s equatorial plane.

As a result, dark debris from the ring could coat the front, or leading, hemisphere of Iapetus, explaining why this half of the moon is much darker than the other half, the researchers suggested. Throughout the solar system’s history, material from the ring could have generated a coating on Iapetus 20 centimeters thick, Verbiscer and her colleagues calculate.

“This discovery is most welcome and should settle lots of arguments,” planetary scientist Joseph Burns of Cornell University said at the meeting. Burns proposed in the 1970s that a then-unknown ring generated by Phoebe could account for Iapetus’ colors. “The cause of Iapetus' bizarre yin-yang [color] distribution … has been a nagging puzzle that now seems solved.”

Spencer and other researchers note that the colors of Phoebe and the dark side of Iapetus, while both dim, are not identical. The surface of Phoebe is gray while the leading hemisphere of Iapetus is more reddish. That mismatch suggests that, though it is in the right place to deposit material on Iapetus, the newly found ring may not suffice to explain the moon’s coloration, Spencer says.  But Burns disagrees.

“The color mismatch is a red herring,” he asserted. Because the Phoebe particles would strike Iapetus at a speed of 6 kilometers per second, organic bonds in the particles would break and the exact color of the particles may not be retained, Burns says.

“There are color differences, but they can be understood,” Burns says. “The Phoebe ring is the smoking gun that identifies the culprit in coating Iapetus.”

Verbiscer and her colleagues used NASA’s infrared Spitzer Space Telescope to find the ring in February. Its dust particles absorb sunlight and re-emit the radiation at infrared wavelengths of around 30 micrometers. Distant stars shine through the tenuous ring, which the researchers estimate has a minimum mass of a trillion kilograms, roughly the heft of a kilometer-wide chunk gouged from Phoebe.

Phoebe’s pockmarked surface bears the battle scars of pummelling by small moons and space debris over billions of years, Verbiscer noted. Small grains of material knocked loose from Phoebe could be easily pushed around and spread out by the feeble pressure exerted by sunlight, models indicate. Particles about 10 micrometers in diameter might take a million years to settle into a ring, the researchers estimate. Models suggest that continual pummelling of Phoebe would provide a fresh supply of ring material.