Comet collisions buckle planetary rings
BY DR EMILY BALDWIN
Posted: 05 April 2011
Corrugated patterns seen in rings around Saturn and Jupiter could be imprints of past comet collisions, say astronomers studying data from Cassini, Galileo and New Horizons spacecraft.
The astronomers, based at the SETI Institute and Cornell, reported spiral ripples spanning a 2,000 kilometre-wide swath in Saturn's D-ring – the ring closest to the planet – in 2005. Then, in 2009, similar features were found to dominate the entire 17,000 kilometre wide C-ring.
Saturn's C and D rings are the locations of the corrugated patterns currently under scrutiny. Image: NASA/JPL/Space Science Institute.
By comparing the 2005 and 2009 data, the researchers charted the change in the ripples' characteristics over time, by essentially "unwinding" the spiral features into a flat sheet whose orbit was tilted relative to its normal path around Saturn's equatorial plane.
“You could explain this corrugation as if the entire ring had become tilted at some time in the past,” says Matt Hedman of Cornell, who determines autumn 1983 as the time in which the corrugated features in both rings began to form.
Mosaic of images of Saturn's C ring along with close-ups of selected radial regions showing the periodic bright and dark bands that permeate the entire C ring. These brightness variations are almost certainly caused by the changing slopes in the rippled ring plane, much like the corrugations of a tin roof.
Although there is no known event that coincides with this timing, a similar study conducted by Mark Showalter at SETI revealed similar corrugations in the main ring around Jupiter, which could be pinned to a specific event. In this case the ring was found to be tilted in the summer of 1994 – coinciding with the dramatic impact of comet Shoemaker Levy-9 into the gas giant.
However, one comet would not have the power to single-handedly knock the rings into a tilted orbit – it would plough straight through and slam into the planet. But a fragmented comet – such as Shoemaker Levy-9, which was ripped into 21 pieces – would result in a diffuse cloud of particles raining across a broad region that could tilt the ring plane.
"Based on the analogy with the Shoemaker-Levy 9 ring-tilting event at Jupiter, our favoured model is that cometary debris hit Saturn's rings in 1983," Hedman tells Astronomy Now. "In order to affect a broad region of the rings, the impacting material would need to have been a diffuse cloud instead of a single object (thus a broken-up comet or perhaps an intense meteor shower) – the total mass in debris would be comparable to the mass of a comet a few kilometres wide."
Hedman comments that since Saturn and Earth were opposite sides of the Sun when this event occurred, they have no direct observations of the ring-tilting event at Saturn. "The above scenario is the most likely scenario we have found thus far, but alternatives remain possible," he adds.
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