This artist’s conception shows a nearly invisible ring around Saturn — the largest of the giant planet’s many rings. It was discovered by NASA’s Spitzer Space Telescope. The artist’s conception simulates an infrared view of the giant ring. Saturn appears as just a small dot from outside the band of ice and dust. The bulk of the ring material starts about six million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers (7.4 million miles). The ring’s diameter is equivalent to roughly 300 Saturns lined up side to side.The inset shows an enlarged image of Saturn, as seen by the W.M. Keck Observatory at Mauna Kea, Hawaii, in infrared light. The ring, stars and wispy clouds are an artist’s representation. PASADENA, Calif. — NASA’s Spitzer Space Telescope has discovered an enormous ring around Saturn — by far the largest of the giant planet’s many rings. The new belt lies at the far reaches of the Saturnian system, with an orbit tilted 27 degrees from the main ring plane. The bulk of its material starts about six million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers (7.4 million miles). One of Saturn’s farthest moons, Phoebe, circles within the newfound ring, and is likely the source of its material. Saturn’s newest halo is thick, too — its vertical height is about 20 times the diameter of the planet. It would take about one billion Earths stacked together to fill the ring. “This is one supersized ring,” said Anne Verbiscer, an astronomer at the University of Virginia, Charlottesville. “If you could see the ring, it would span the width of two full moons’ worth of sky, one on either side of Saturn.” Verbiscer; Douglas Hamilton of the University of Maryland, College Park; and Michael Skrutskie, of the University of Virginia, Charlottesville, are authors of a paper about the discovery to be published online tomorrow by the journal Nature.
The ring itself is tenuous, made up of a thin array of ice and dust particles. Spitzer’s infrared eyes were able to spot the glow of the band’s cool dust. The telescope, launched in 2003, is currently 107 million kilometers (66 million miles) from Earth in orbit around the sun. The discovery may help solve an age-old riddle of one of Saturn’s moons. Iapetus has a strange appearance — one side is bright and the other is really dark, in a pattern that resembles the yin-yang symbol. The astronomer Giovanni Cassini first spotted the moon in 1671, and years later figured out it has a dark side, now named Cassini Regio in his honor. Saturn’s newest addition could explain how Cassini Regio came to be. The ring is circling in the same direction as Phoebe, while Iapetus, the other rings and most of Saturn’s moons are all going the opposite way. According to the scientists, some of the dark and dusty material from the outer ring moves inward toward Iapetus, slamming the icy moon like bugs on a windshield. “Astronomers have long suspected that there is a connection between Saturn’s outer moon Phoebe and the dark material on Iapetus,” said Hamilton. “This new ring provides convincing evidence of that relationship.”Verbiscer and her colleagues used Spitzer’s longer-wavelength infrared camera, called the multiband imaging photometer, to scan through a patch of sky far from Saturn and a bit inside Phoebe’s orbit. The astronomers had a hunch that Phoebe might be circling around in a belt of dust kicked up from its minor collisions with comets — a process similar to that around stars with dusty disks of planetary debris. Sure enough, when the scientists took a first look at their Spitzer data, a band of dust jumped out. The ring would be difficult to see with visible-light telescopes. Its particles are diffuse and may even extend beyond the bulk of the ring material all the way in to Saturn and all the way out to interplanetary space.
The relatively small numbers of particles in the ring wouldn’t reflect much visible light, especially out at Saturn where sunlight is weak. “The particles are so far apart that if you were to stand in the ring, you wouldn’t even know it,” said Verbiscer. Spitzer was able to sense the glow of the cool dust, which is only about 80 Kelvin (minus 316 degrees Fahrenheit). Cool objects shine with infrared, or thermal radiation; for example, even a cup of ice cream is blazing with infrared light. “By focusing on the glow of the ring’s cool dust, Spitzer made it easy to find,” said Verbiscer. These observations were made before Spitzer ran out of coolant in May and began its “warm” mission. NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. The multiband imaging photometer for Spitzer was built by Ball Aerospace Corporation, Boulder, Colo., and the University of Arizona, Tucson. Its principal investigator is George Rieke of the University of Arizona. This diagram illustrates the extent of the largest ring around Saturn, discovered by NASA’s Spitzer Space Telescope. The ring is huge, and far from the gas planet and the rest of its majestic rings. The bulk of the ring material starts about six million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers (7.4 million miles). The diameter of the ring is equivalent to 300 Saturns lined up side to side. The ring is thick too — it’s about 20 times as thick as the diameter of the planet. In fact, the entire volume of the ring is big enough to hold one billion Earths! Saturn’s newest halo is tilted at about 27 degrees from the main ring plane and encompasses the orbit of the moon Phoebe.
Both the ring and Phoebe orbit in the opposite direction of Saturn’s other rings and most of its moons, including Titan and Iapetus. Why did it take so long to find something so big? The answer is that the ring is very tenuous, made up of a sparse collection of ice and dust particles. If you could transport yourself to the ring, you wouldn’t even know you were there because the particles are so far apart. There’s not a lot of sunlight out at Saturn, so this small density of particles doesn’t reflect much visible light. Spitzer was able to spot the band because it sees infrared light, or heat radiation, from objects. Even though the ring material is very cold, it still gives off heat that can Spitzer can see. The discovery offers a possible solution to the mystery of the moon Iapetus. Years after Giovanni Cassini discovered Iapetus in 1671, he correctly deduced that one side of the moon is white and the other dark in a pattern that some say resembles the yin-yang symbol or a tennis ball. Astronomers think it is possible that the newfound ring, which orbits in the opposite direction of Iapetus, is the cause of the two-faced coloring. As the ring circles around, particles could be drifting inward and splattering the icy moon on the face like bugs on a windshield. The size of Phoebe relative to Iapetus has been enlarged to better show Phoebe. Phoebe is about 200 kilometers (124 miles) in diameter, while Iapetus is about 1,500 kilometers (932 miles) across. This diagram highlights a slice of Saturn’s largest ring. The ring (red band in inset photo) was discovered by NASA’s Spitzer Space Telescope, which detected infrared light, or heat, from the dusty ring material. Spitzer viewed the ring edge-on from its Earth-trailing orbit around the sun. The ring has a diameter equivalent to 300 Saturns lined up side to side. And it’s thick too — about 20 Saturns could fit into its vertical height. The ring is tilted about 27 degrees from Saturn’s main ring plane. The Spitzer data were taken by its multiband imaging photometer and show infrared light with a wavelength of 24 microns. The picture of Saturn was taken by NASA’s Hubble Space Telescope. news from nasa.gov
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