We often view the solar system as constant and unchanging, at least over human time scales. This, of course, is not entirely accurate and astronomers have detected a surprisingly rapid phenomenon inside one of Saturn’s rings: moonlets the size of mountains are created and destroyed over a matter of days or even hours.
This discovery centers around the gas giant’s F-ring where, over the course of 30 years, has dramatically changed its morphology.
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“The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C,” said Robert French of the SETI Institute, at Mountain View, Calif., in a news release “It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps.”
French and co-investigator Mark Showalter (also from the SETI Institute) studied photographs of the F-ring taken by NASA’s twin Voyager spacecraft when they encountered the ringed planet in the early 1980s. On comparison with photographs from NASA’s Cassini spacecraft that is currently in orbit around Saturn, the F-ring has changed appearance extensively.
Further investigations revealed that bright lumps in the ring come and go over periods of only hours or days — features that the researchers believe are small moons.
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“We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring,” said French. “These moons are small enough to coalesce and then break apart in short order.”
The researchers believe the F-ring’s surprising dynamic behavior is down to its special location. This particular ring occupies a region of tidal chaos called the “Roche limit” — a distance from a gravitational body (a planet) below which the tidal forces are too strong for moons to form. Beyond the Roche limit, moons can readily coalesce, overcoming powerful tidal forces. The F-ring is located right at this boundary, so moonlets (no bigger than 3 miles wide) that coalesce are inherently unstable and may succumb to the rough tides.
Adding to the complexity of this region is Saturn’s innermost moon Prometheus, a 60 mile-wide moon that patrols the outer edge of the F-ring once every 17 years. Prometheus’ weak gravitational field aligns with the ring in such a way that it can stabilize the region, catalyzing moonlet formation.
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“These newborn moonlets will repeatedly crash through the F ring, like bumper cars, producing bright clumps as they careen through lanes of material,” said Showalter. “But this is self-destructive behavior, and the moons — being just at the Roche limit — are barely stable and quickly fragmented.”
Now that we’ve had an insight to the rapid processes that could be occurring int he F-ring, further observations by Cassini are needed to confirm that these small brightenings are small moons (or moonlets) that clump together very quickly from ring ices only to be ripped apart by Saturn’s tides. It’s almost as if the F-ring is a scaled down Petri dish of planetary formation, a process that planetary scientists are trying to fully understand.
“The sort of processes going on around Saturn are very similar to those that took place here 4.6 billion years ago, when the Earth and the other large planets were formed,” said French. “It’s an important process to understand.”