Forensic sleuthing ties ring ripples to impacts
Space missions have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments.
April 1, 2011
Like forensic scientists examining fingerprints at a cosmic crime scene, scientists working with data from NASA's Cassini, Galileo, and New Horizons missions have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments dating back more than 10 years.
This artist's concept shows comet Shoemaker-Levy 9 heading into Jupiter in July 1994, while its dust cloud creates a rippling wake in Jupiter's ring. The comet, as imaged by NASA's Hubble Space Telescope, appears as a string of reddish fragments falling into Jupiter from the south. A later Hubble image shows the dark blotches where pieces of the comet had already collided with the planet. The faint ring, based on images obtained by NASA's Galileo mission, is normally very faint, but has been enhanced for this illustration. The streaks show the tracks of the comet's dust cloud. Impacts from these dust particles tilted the ring off its axis. copyright M. Showalter
The ripple-producing culprit, in the case of Jupiter, was comet Shoemaker-Levy 9, whose debris cloud hurtled through the thin Jupiter ring system into the planet in July 1994. Scientists attribute Saturn's ripples to a similar object, likely another cloud of comet debris plunging through the inner rings in the second half of 1983.
"What's cool is we're finding evidence that a planet's rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago," said Matthew Hedman from Cornell University in Ithaca, New York. "The solar system is a much more dynamic place than we gave it credit for."
From Galileo's visit to Jupiter, scientists have known since the late 1990s about patchy patterns in the jovian ring. But the Galileo images were a little fuzzy, and scientists didn't understand why such patterns would occur. The trail was cold until Cassini entered orbit around Saturn in 2004 and started sending back thousands of images. A 2007 paper by Hedman and colleagues first noted corrugations in Saturn's innermost ring, dubbed the D ring.
A group, including Hedman and Mark Showalter from the SETI Institute in Mountain View, California, then realized that the grooves in the D ring appeared to wind together more tightly over time. Playing the process backward, Hedman then demonstrated that the pattern originated when something tilted the D ring off its axis by about 300 feet (100 meters) in late 1983. The scientists found the influence of Saturn's gravity on the tilted area warped the ring into a tightening spiral.
Cassini imaging scientists got another clue when the Sun shone directly along Saturn's equator and lit the rings edge-on in August 2009. The unique lighting conditions highlighted ripples not previously seen in another part of the ring system. Whatever happened in 1983 was not a small, localized event — it was big. The collision had tilted a region more than 12,000 miles (19,000 kilometers) wide, covering part of the D ring and the next outermost ring called the C ring. Unfortunately, spacecraft were not visiting Saturn at that time, and the planet was on the far side of the Sun, hidden from telescopes on or orbiting Earth, so whatever happened in 1983 passed unnoticed by astronomers.
Hedman and Showalter began to wonder whether the long-forgotten pattern in Jupiter's ring system might illuminate the mystery. Using Galileo images from 1996 and 2000, Showalter confirmed a similar winding spiral pattern. They applied the same math they had applied to Saturn, but now with Jupiter's gravitational influence factored in. Unwinding the spiral pinpointed the date when Jupiter's ring was tilted off its axis — between June and September 1994. Shoemaker-Levy plunged into the jovian atmosphere during late July 1994. The estimated size of the nucleus was also consistent with the amount of material needed to disturb Jupiter's ring.
The Galileo images also revealed a second spiral, which was calculated to have originated in 1990. Images taken by New Horizons in 2007, when the spacecraft flew by Jupiter on its way to Pluto, showed two newer ripple patterns in addition to the fading echo of the Shoemaker-Levy impact.
"We now know that collisions into the rings are very common — a few times per decade for Jupiter and a few times per century for Saturn," Showalter said. "Now scientists know that the rings record these impacts like grooves in a vinyl record, and we can play back their history later."
The ripples also give scientists clues to the size of the clouds of cometary debris that hit the rings. In each of these cases, the nuclei of the comets, before they likely broke apart, were a few miles wide.
"Finding these fingerprints still in the rings is amazing and helps us better understand impact processes in our solar system," said Linda Spilker from NASA's Jet Propulsion Laboratory in Pasadena, California. "Cassini's long sojourn around Saturn has helped us tease out subtle clues that tell us about the history of our origins."
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