by Robert Sanders
The first infrared images of the faint E ring of Saturn show it is even dimmer than expected at longer and redder wavelengths, bolstering claims that it is the bluest object in the solar system.
The measurements were obtained with the near infrared camera mounted on the world's largest telescope, the Keck Telescope in Hawaii, and were reported Oct. 12 in Hawaii at the 27th annual meeting of the Division for Planetary Sciences of the American Astronomical Society.
Coauthors of the paper are Berkeleyans Imke de Pater, professor of astronomy who presented the findings, and James Graham, an associate professor of astronomy; Mark Showalter, an astronomer from Stanford's Center for Radar Astronomy; and Jack J. Lissauer of the State University of New York at Stony Brook.
The observations were made earlier this year as astronomers worldwide trained their telescopes on Saturn to catch the planet's rings edge-on for the first time in 15 years. During this period the Keck Telescope also scored the first ground-based observations of the faint G ring, which had been thought too faint to see from Earth.
What makes the E ring bluer than anything else in the solar system is probably its unusually uniform composition, de Pater says. Nearly all the particles in the ring appear to be the same size, about two microns in diameter. Showalter calculates an average radius of 1 0.3 microns, which means that laid side by side there would be 12,500 particles to the inch.
Particles of such uniform size, unheard of in any other ring structures, reflect light of only a narrow band of wavelengths, in this case in the blue, giving the E ring a unique blue cast. Saturn's other rings contain a broad range of particle sizes that reflect many wavelengths, making them look white.
The uniform size distribution suggests that the E ring, which is coincident with the orbit of the moon Enceladus, has a very different origin than the other ring systems.
The new near infrared data on the E ring are important additions to earlier ground-based observational data obtained in 1966 and 1980 when the rings also were visible edge-on from Earth. The research team found that the E ring is much fainter in the infrared than at visible wavelengths, and fainter than indicated by infrared measurements obtained in 1980.
Since 1980, when astronomers obtained visible light images of the ring, theorists have debated whether the uniform size of particles in the E ring is a result of the way in which the particles formed or a consequence of a survival advantage for bodies in this narrow size range.
One theory put forward is that these particles are thrown into orbit by geysers on Enceladus, yielding the narrow size distribution observed. The team doubts, however, that the E ring they observe could be produced by geyser activity alone.
They favor an alternative explanation: that particles of many sizes are thrown into orbit--either by geysers or eroded from the surface of Enceladus by impacting dust--but only those of a certain size avoid being swept up by the moon as it circles Saturn. That is because light from the sun, combined with Saturn's magnetic field, changes the orbit of micron-sized particles, allowing them to avoid Enceladus.
Only micron-sized particles would then remain in the ring, following highly eccentric orbits around the planet. The broad extent of the ring seems to favor the latter explanation, de Pater says. She and her colleagues see the E ring in the infrared out to six Saturnian radii (230,000 miles), which fits with previous visible observations that indicate the E ring is not a narrow band at four Saturnian radii but a broad highway stretching from three to eight radii, a width of 190,000 miles.
No matter how they originate, the particles would have to be replenished constantly because ultimately they are lost from the ring system as they are absorbed by moons, large pieces of rock or the planet itself.
The other rings probably are full of debris from moons that were torn apart by tidal forces generated as they got too close to Saturn's surface.
The G ring is even fainter than the E ring, and was judged too faint to see from Earth in the glare of Saturn's brighter rings. It was first discovered in 1980 by the Pioneer 11 spacecraft, and subsequently photographed by Voyager in 1981.
"It's much brighter than predicted--probably half as bright as the E ring," de Pater says. "Based on the Voyager data, it was estimated to be about one tenth the brightness of the E ring."
The G ring appears narrow and confined to the planet's ring plane, with no evidence yet of an embedded moon, the researchers report.
The team's success in collecting data from the E and G rings owes a lot to advances in technology since the Earth last passed through the plane of Saturn's rings, de Pater says.
"We really have made leaps in detector technology in infrared wavelengths since that time," she says.