[meteorite-list] Saturn's Rings Carry Extra Weight

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 12 Oct 2007 10:50:50 -0700 (PDT)
Message-ID: <200710121750.KAA01618_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/asy/default.aspx?c=a&id=6104

Saturn's rings carry extra weight

Recent computer simulations show Saturn's rings may be more massive and
much older than astronomers thought.

Richard Talcott
Astronomy Magazine
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.
Received on Fri 12 Oct 2007 01:50:50 PM PDT