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October 10, 2007
A lthough Saturn's rings are so thin they disappear when turned edge-on to our line of sight, they apparently contain more material than meets the eye. "We'll need to rewrite the textbooks on how much mass is in the rings," claims University of Colorado planetary scientist Glen Stewart. Stewart presented his team's findings Monday at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Orlando, Florida.
Scientists probe ring structure by monitoring starlight passing through the rings. During such a stellar occultation, ring material dims the star — the light dims most when it traverses the thickest regions. The problem: When the Cassini spacecraft observes occultations, the results vary depending on viewing geometry. The only way this can happen is if the ring particles clump together.
Stewart's computer simulations show the gravitational attraction between ring particles causes them to bunch into clusters organized into connected strands, resembling a giant spider web. As the particles orbit Saturn, gravity slowly shears apart some strands while building new ones.
When Cassini observes starlight passing through dense ring material, it records the amount of area between the opaque strands rather than the density of web material itself. Stewart estimates the densest part of the rings — in the core of the bright B ring — contains more than three times the mass scientists previously estimated. This means the entire ring system possesses at least three times the mass of the mid-sized saturnian moon Mimas.
Stewart thinks the rings' large mass also provides a clue to their origin. Scientists have three theories describing how the beautiful system formed: It's the remains of a moon shattered by an impact inside Saturn's Roche lobe (where tidal forces would keep the debris from reforming); the break-up of a large comet in the fairly recent past; or the remnants of a proto-satellite accretion disk. Most scientists lean toward the comet breakup theory because the outer A ring shows features thought to be only 100 million years old or so.
Stewart's new mass estimate leads him to reject this idea simply because no known comet comes close to being three times Mimas' size. He also eliminates the leftover accretion disk idea because theory now suggests moons likely would migrate through and disrupt such a disk.
That leaves debris from a catastrophic impact into a massive moon as the most likely scenario, says Stewart. Such a collision almost certainly would have occurred early in the solar system's history, during the so-called late heavy bombardment, when huge collisions happened much more frequently. Stewart believes the massive B ring — which contains more than 95 percent of the overall rings' mass — can be ancient and evolve slowly while the A and C rings remain relatively young.
A lthough Saturn's rings are so thin they disappear when turned edge-on to our line of sight, they apparently contain more material than meets the eye. "We'll need to rewrite the textbooks on how much mass is in the rings," claims University of Colorado planetary scientist Glen Stewart. Stewart presented his team's findings Monday at the annual meeting of the American Astronomical Society's Division for Planetary Sciences in Orlando, Florida.
Scientists probe ring structure by monitoring starlight passing through the rings. During such a stellar occultation, ring material dims the star — the light dims most when it traverses the thickest regions. The problem: When the Cassini spacecraft observes occultations, the results vary depending on viewing geometry. The only way this can happen is if the ring particles clump together.
Stewart's computer simulations show the gravitational attraction between ring particles causes them to bunch into clusters organized into connected strands, resembling a giant spider web. As the particles orbit Saturn, gravity slowly shears apart some strands while building new ones.
When Cassini observes starlight passing through dense ring material, it records the amount of area between the opaque strands rather than the density of web material itself. Stewart estimates the densest part of the rings — in the core of the bright B ring — contains more than three times the mass scientists previously estimated. This means the entire ring system possesses at least three times the mass of the mid-sized saturnian moon Mimas.
Stewart thinks the rings' large mass also provides a clue to their origin. Scientists have three theories describing how the beautiful system formed: It's the remains of a moon shattered by an impact inside Saturn's Roche lobe (where tidal forces would keep the debris from reforming); the break-up of a large comet in the fairly recent past; or the remnants of a proto-satellite accretion disk. Most scientists lean toward the comet breakup theory because the outer A ring shows features thought to be only 100 million years old or so.
Stewart's new mass estimate leads him to reject this idea simply because no known comet comes close to being three times Mimas' size. He also eliminates the leftover accretion disk idea because theory now suggests moons likely would migrate through and disrupt such a disk.
That leaves debris from a catastrophic impact into a massive moon as the most likely scenario, says Stewart. Such a collision almost certainly would have occurred early in the solar system's history, during the so-called late heavy bombardment, when huge collisions happened much more frequently. Stewart believes the massive B ring — which contains more than 95 percent of the overall rings' mass — can be ancient and evolve slowly while the A and C rings remain relatively young.
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