Forensic Sleuthing Ties Ring Ripples to Impacts

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Jia-Rui Cook 818-354-0850
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Michael Buckley 240-228-7536
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
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News release: 2011-102 March 31, 2011

Forensic Sleuthing Ties Ring Ripples to Impacts

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-102&cid=release_2011-102

PASADENA, Calif. – 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 ago.

The ripple-producing culprit, in the case of Jupiter, was comet Shoemaker-Levy 9, whose debris cloud
hurtled through the thin Jupiter ring system during a kamikaze course 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. The findings are detailed in a pair of
papers published online today in the journal Science.

"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, a Cassini imaging team associate, lead author of one of the papers, and a research associate
at Cornell University, Ithaca, N.Y. "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, a Cassini co-investigator based at the SETI Institute
in Mountain View, Calif., then realized that the grooves in the D ring appeared to wind together more
tightly over time. Playing the process backward, Hedman then demonstrated the pattern originated
when something tilted the D ring off its axis by about 100 meters (300 feet) 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 19,000 kilometers (12,000 miles) 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, the lead author on the second paper, 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 kilometers
wide.

"Finding these fingerprints still in the rings is amazing and helps us better understand impact
processes in our solar system," said Linda Spilker, Cassini project scientist, based at NASA's Jet
Propulsion Laboratory, Pasadena, Calif. "Cassini's long sojourn around Saturn has helped us tease out
subtle clues that tell us about the history of our origins."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the
Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages
the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini
orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging
team is based at the Space Science Institute in Boulder, Colo. JPL managed the Galileo mission for
NASA, and designed and built the Galileo orbiter. The New Horizons mission is led by Principal
Investigator Alan Stern of Southwest Research Institute, Boulder, Colo., and managed by the Johns
Hopkins Applied Physics Laboratory, Laurel, Md., for NASA's Science Mission Directorate.

More information about Cassini can be found at http://www.nasa.gov/cassini .

Additional contacts: Blaine Friedlander, Cornell University, Ithaca, N.Y., 607-254-6235,
bpf2@cornell.edu; Karen Randall, SETI Institute, Mountain View, Calif., 650-960-4537,
krandall@seti.org; and Joe Mason, Space Science Institute, Boulder, Colo., 720-974-5859,
jmason@ciclops.org.

-end-

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