[meteorite-list] Cassini Spacecraft Finds Evidence of Football-Sized Moonlets in Saturn's A Ring

[Ron Baalke]

http://www.news.cornell.edu/stories/March06/Tiscareno.Saturn.lg.html

Cassini spacecraft finds evidence of football-field sized moonlets in 
Saturn's A ring

By Lauren Gold (lg34 at cornell.edu)
Cornell University
March 29, 2006

New observations of propeller-shaped disturbances in Saturn's A ring 
indicate the presence of four small, embedded moons -- and most 
likely millions more, Cornell University astronomers report.

This is the first evidence of the existence of moonlets bridging the 
gap in size between the larger ring moons Pan and Daphnis (several 
miles each in diameter) and the much smaller ice particles that 
comprise the bulk of the rings. The discovery could lead to a better 
understanding of the origin and formation of Saturn's rings and the 
solar system as a whole.

Matthew Tiscareno, a Cornell research associate, is lead author of a 
paper describing the discovery in the March 30 issue of the journal 
Nature.

The four disturbances, which appear as pairs of slightly offset 
bright horizontal streaks in an otherwise bland region of the ring, 
were captured in two images taken in 2004 by NASA's Cassini 
spacecraft. Astronomers say the streaks are indicators of orbiting 
moons about 100 meters (328 feet) in diameter: about the length of a 
football field, but still too small for even Cassini's highly 
sensitive Imaging Science Subsystem (ISS) to see directly, but large 
enough to exert an observable gravitational pull on the particles 
around them.

"The discovery of these intermediate-sized particles tells us that 
Pan and Daphnis are probably just the largest members of the ring 
population, rather than interlopers from somewhere else," said 
Tiscareno.

A continuum of particle sizes lends strong support to the theory that 
Saturn's rings were formed when another object fragmented close to 
the planet, breaking into pieces which were then captured by Saturn's 
gravitational pull.

"There has always been the question about whether the rings were 
primordial material that was unable to grow into a moon or debris 
left over from a breakup event," said Joseph Burns, Cornell professor 
of astronomy and of theoretical and applied mechanics and paper 
co-author, along with Cornell research associate Matthew Hedman and 
researchers at other institutions. The discovery doesn't rule out the 
accretion model, but "it's a step in that direction," said Tiscareno. 
"It's hard for direct accretion to produce particles this large. It's 
much easier if you start with a solid icy core, like a shard from a 
breakup."

The discovery also helps explain fully cleared openings such as the 
Encke and Keeler gaps within the rings. The gravitational influence 
of a larger moon like Pan or Daphnis wraps around the circumference 
of the rings, creating a gap. The smaller moonlets begin to create 
this effect, the researchers say, but their influence is not strong 
enough to prevent particles from falling into the rings ahead of and 
behind them.

Like a motorboat's wake on a smooth lake, the four observed 
disturbances are particularly visible since the area they inhabit is 
otherwise smooth. But the fact that four were found in just two 
images covering only a tiny fraction of the ring makes it likely that 
millions more exist. By studying them further, researchers hope to 
gain a better understanding of how Saturn's rings formed -- and even 
about how solar systems form around stars.

"The structures we observe with Cassini are strikingly similar to 
those seen in many numerical models of the early stages of planetary 
formation, even though the scales are vastly different," said Carl 
Murray, a co-author and astronomer at Queen Mary College, University 
of London. "In this way, Cassini is giving us unique insight into the 
origin of planets."

The Cassini-Huygens mission is a cooperative project of NASA, the 
European Space Agency and the Italian Space Agency. The Jet 
Propulsion Laboratory, a division of the California Institute of 
Technology, manages the mission for NASA.

During its Saturn tour, as currently planned, Cassini will complete 
74 orbits of the ringed planet, 44 close flybys of the moon Titan and 
numerous flybys of Saturn's other icy moons.

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