Tiny Saturn moons suggest smashup made the rings, study finds
and World Science staff
Four tiny, newfound moons of Saturn are evidence that a breakup of a celestial object furnished the material for Saturn’s rings, astronomers say. The same research, they add, is yielding evidence that the ring formation process is much like planet formation on a small scale.
The team of researchers said disturbances in one of the rings betray the presence of the “moonlets,” and there are probably millions more of them. This would fit neatly into the smashup theory.
The moonlets bridge the gap in size between the larger ring moons Pan and Daphnis, each several miles or kilometers wide, and the much smaller ice particles that comprise the bulk of the rings, according to the researchers.
About the size of an American football field, the moonlets were too small to see directly but large enough to exert a detectable gravitational pull on surrounding particles, the astronomers explained.
The researchers said the moonlets show that ring particles come in a continuous range of sizes. That’s a key prediction of a theory that Saturn’s rings formed when another object fragmented close to the planet, breaking into pieces that were then captured by Saturn’s gravitational pull.
A competing theory holds that the rings formed directly from primordial material, such as dust, that existed since the time the Solar System formed. The discovery doesn’t rule this out, but “it’s a step in that direction,” said Cornell’s Matthew Tiscareno, lead author of a paper describing the discovery in the March 30 issue of the research journal Nature. “It’s hard for direct accretion,” or accumulation of primordial particles, “to produce particles this large. It’s much easier if you start with a solid icy core, like a shard from a breakup.”
The four disturbances appear as pairs of slightly offset bright horizontal streaks in an otherwise bland region of Saturn’s “A ring,” the astronomers reported. Shown in images taken in 2004 by NASA’s Cassini spacecraft, the streaks indicate moons about 100 meters (328 feet) wide, they added.
“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.
Saturn’s larger moons are believed to create the fully cleared openings between the rings, such as the so-called Encke and Keeler gaps. Larger moons do this while smaller ones don’t, the researchers explained, because large ones have a stronger gravitational pull that can affect things all the way around the rings. The smaller ones start to create the effect, but their influence is to weak to keep particles from falling into the rings ahead of and behind them.
Like a motorboat’s wake on a smooth lake, the four observed disturbances were noticeable because the surrounding area is smooth, the astronomers added—but the fact that four appeared in two images covering a tiny fraction of the ring suggests millions exist.
By studying them further, researchers hope to better understand how Saturn’s rings formed, and even how planets form around stars.
The structures found with Cassini are strikingly similar to those seen in many computer models of early-stage planet formation, “even though the scales are vastly different,” said Carl Murray of the University of London, a co-author of the paper. Thus, “Cassini is giving us unique insight into the origin of planets.”
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