Saturn's rings show evidence of a modern-day collision
Thu Oct 12, 2006 at 08:39 UTC
Scientists with Cassini mission have spied a new, continuously changing feature that provides circumstantial evidence that a comet or asteroid recently collided with Saturn's innermost ring, the faint D ring.
Imaging scientists see a structure in the outer part of the D ring that looks like a series of bright ringlets with a regularly spaced interval of about 30 kilometers (19 miles). An observation made by Hubble Space Telescope in 1995 also saw a periodic structure in the outer D ring, but its interval was then 60 kilometers (37 miles). Unlike many features in the ring system that have not changed over the last few decades, the interval of this pattern has been decreasing over time.
These findings are being presented at the Division for Planetary Sciences Meeting of the American Astronomical Society held in Pasadena, Calif.
"This structure in the D ring reminds us that Saturn's rings are not eternal, but instead are active, dynamical systems, which can change and evolve," said Dr. Matt Hedman, Cassini imaging team associate at Cornell University, Ithaca, N.Y.
When Cassini researchers viewed the D ring along a line of sight nearly parallel to the ringplane, they observed a pattern of brightness reversals: a part of the ring that appears bright on the far side of the rings appeared dark on the near side of the rings, and vice versa.
This phenomenon would occur if the region contains a sheet of fine material that is vertically corrugated, like a tin roof. In this case, variations in brightness would correspond to changing slopes in the rippled ring material.
Image Credit: NASA/JPL/Space Science Institute
High resolution image
Saturn's D ring--the innermost of the planet's rings -- sports an intriguing structure that appears to be a wavy, or "vertically corrugated," spiral. This continuously changing ring structure provides circumstantial evidence for a possible recent collision event in the rings.
Support for this idea comes from the appearance of a structure in the outer D-ring that looks, upon close examination, like a series of bright ringlets with a regularly spaced interval of about 30 kilometers (19 miles). When viewed along a line of sight nearly in the ringplane, a pattern of brightness reversals is observed: a part of the ring that appears bright on the far side of the rings appears dark on the near side of the rings, and vice versa.
This phenomenon would occur if the region contains a sheet of fine material that is vertically corrugated, like a tin roof. In this case, variations in brightness would correspond to changing slopes in the rippled ring material.
An observation made with NASA's Hubble Space Telescope in 1995 also saw a periodic structure in the outer D ring, but its wavelength was then 60 kilometers (37 miles). There were insufficient observations to discern the spiral nature of the feature. Thus, it appears the wavelength of the wavy structure has been decreasing: that is, this feature has been winding up like a spring over time.
The rate at which the pattern appears to be winding up is quite close to the rate scientists would expect for a vertically corrugated spiraling sheet of material at this location in the rings that is responding to gravitational forcing from Saturn.
As Cassini imaging scientists extrapolated the spiraling trend backward in time, they found that it completely unwound in 1984, leaving only an inclined, or tilted, sheet of material. The researchers speculate such an inclined sheet may have been produced around that time by the impact of a comet or meteoroid into the D ring which kicked out a cloud of fine particles that ultimately inherited some of the tilt of the impactor's trajectory as it slammed into the rings. Another possibility is that the impactor struck an already inclined moonlet, shattered it to bits and the debris remained in an inclined orbit.
Image Credit: NASA/JPL/Space Science Institute
High resolution image
A vertical corrugation in Saturn's almost transparent D ring can give rise to brightness variations.
In this graphic representation, the diagonal lines (or arrows) correspond to lines of sight through the corrugated ring. The grayscale plot along the bottom shows the ring's total optical depth (a measure of opacity) and brightness as seen by the observer.
Both the changes over time and the "corrugated" structure of this region could be explained by a collision of a comet or meteoroid into the D ring, which then kicked out a cloud of fine particles. This cloud might have inherited some of the tilt of the colliding object's path as it slammed into the rings. An alternate explanation could be that the object struck an already inclined moonlet, shattering it to bits and leaving its debris in an inclined orbit.
In either case, the researchers speculate the aftermath of such a collision would be a ring slightly tilted relative to Saturn's equatorial plane. Over a period of time, as the inclined orbits of the ring particles evolve, this flat sheet of material would become a corrugated spiral that appears to wind up like a spring over time, which is what was observed.
Based on observations between 1995 and 2006, scientists reconstructed a timeline and estimated that the collision occurred in 1984.
Jet Propulsion Laboratory News Release
