Monday, November 14, 2011

Saturn’s Moons Spin Out Moonlets

SATURN’S rings are far from the serene place you might imagine. Instead, they resemble a space beltway where high-speed crashes between icy particles create and destroy snowballs the size of mountains. This constant thrum of activity could explain how moonlets emerge from the rings, and perhaps even show how planets materialise around a star.
“We used to think of the rings as a perfect, placid society where nothing ever happens,” says Larry Esposito of the University of Colorado in Boulder. “Not only is that not the case — there are collisions happening all the time — but the collisions are actually throwing the rings out of equilibrium.”
In 2009, Saturn's rings were aligned nearly edge-on with the sun, and the low angle of sunlight revealed that the famously flat rings are actually bumpy.
Clumps at the edges of the prominent B ring cast dramatic shadows across the icy plane. “People were totally surprised by those images,” Esposito says. “Nobody expected this.” Now Esposito and colleagues think they can explain the clumps, which come together and break up again over periods of hours to months.
Observations made since 2004 using the Cassini spacecraft provided the team with clues. In the B ring, the zoo to 2ooo-metre- — wide clumps appear most often ahead of and behind the moon Mimas, whose passage carves out the ring’s edge. In the more distant F ring, the clumps precede and lag behind the moon it Prometheus.
This suggests that the gravity of the moons triggers the formation of the clumps. Indeed, the team's simulations show that when a moon sweeps by the edge of a ring, its gravity gives the particles in the ring a kick, driving them into one another.
Some stick together, and their gravity attracts more particles towards them, so the clumps keep growing. As a clump grows, its gravity accelerates the surrounding ring particles. And when a clump gets to be about 1 kilometre across, the particles around it are moving so fast their impacts can break up the clump.
The comings and goings of such clumps follow the mathematical rules that describe predator-prey ecology, the team has found (Icarus, DOI: 10.1o16/ j.icarus.2o11.o9.o29). Think of the clumps as a group of rabbits whose population is held in check by the foxes that eat them, says Esposito. When the rabbit population booms, the fox population grows too. But then the foxes overhunt the rabbits, and both populations fall.
Occasionally, a clump could get big enough to retain the bulk of its mass, even after collisions, and become a moon. “Even if only once in a billion collisions do the clumps stick together and survive, those moons could persist for millions of years,” Esposito says.
Saturn's rings are considered to be a laboratory for studying how planets form within the dusty discs around stars. In the same way that Mimas might seed moonlets in Saturn's B ring, a planet like Jupiter could help form smaller planets like Mars, Esposito speculates.



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