In August of 2009, the Cassini orbiter was in place as Saturn reached a point where its rings were illuminated by the Sun edge-on. The images it captured showed a regular, repeating pattern of bright and dark stripes within the planet’s C-ring. These had previously been seen in the D-ring, but there was something notable about this: previous images hadn’t shown them. Now, researchers have proposed an explanation for these features, one that involves an extraplanetary body striking the rings and twisting them out of their normal axis of rotation, and have found that a similar thing occurred as the comet Shoemaker-Levy slammed into Jupiter.
In today’s issue of Science, there’s one paper each for the C-ring and Jupiter’s ring. The C-ring case is quite a bit simpler (we’ll explain why in a moment), so we’ll start with that. The authors describe the features as a “corrugation,” one with an amplitude of about 2-20 meters, and a wavelength of 30-80km. They show a regular pattern throughout the ring, indicating that they’re part of a single structure.
A paper back in 2007 that examined the D-ring had indicated that this sort of pattern could be caused by twisting the ring off-axis slightly. Over time, the initial large disturbance would spread out and dissipate, creating a more dense pattern of smaller disturbances (see the image above). The nice thing about this is that the time-dependent nature of this process allowed the researchers to calculate back to when the initial disturbance occurred: 263 days into 1983, give or take 40. To make the implications of that clear, we’re looking at echoes of an event that occurred over 20 years ago.
So, what could twist the rings in the first place? Since it had to be a single event to get the sort of evolution they saw, an impact from a comet or meteor would be an obvious choice, although an intact body would simply punch through the rings with little else in the way of effect. They conclude that it had to have been a “dispersed cloud of debris,” probably from a comet that had been gravitationally disrupted by the planet. The magnitude of the disruption suggests the cloud of material weighed over 100 billion kg.
Support for this model comes from Jupiter’s ring (yes, it has one). Here, the changes were a bit more complex, and lacked the regular pattern seen at Saturn. The authors of that paper provide a rather neat explanation: the ring has an interference pattern derived from two separate impacts. The two events could be separated out, and the authors managed to trace the larger of the two back to between July and October of 1994. That timing is rather suspicious given that we know the comet Shoemaker-Levy 9 slammed into the planet in July of that year, and that it had been gravitationally disrupted before impact. That impact had apparently twisted the ring by about 2km.
Again, these echoes are durable, since Galileo was able to image them both in 1996 and 2000, and the details of the Shoemaker-Levy impact were still apparent in images taken by New Horizons as it passed the planet 13 years after. By that point, the smaller of the two disturbances visible in the Galileo images was no longer apparent. Instead, two new patterns seemed to be present, suggesting that Jupiter’s ring had received a kilometer-scale bump in 2001 and 2003.
As the papers point out, Jupiter would be expected to receive a lot more interplanetary bodies than Saturn due to its larger gravitational pull. But these figures suggest it’s getting smacked by something significant once or twice a decade, indicating that the solar system is probably a bit more active than many suspected just a couple of decades ago, when Shoemaker-Levy seemed like a once-in-a-lifetime opportunity.
The authors of the Jupiter paper, however, chose to end it on a nearly poetic note, saying of the ripples, “Within these subtle patterns, planetary rings chronicle their own battered histories.”
Science, 2011. DOI : John Trimmer – thanks