[meteorite-list] Forensic Sleuthing Ties Ring Ripples to Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 31 Mar 2011 11:49:30 -0700 (PDT)
Message-ID: <201103311849.p2VInUEE020718_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2011-102

Forensic Sleuthing Ties Ring Ripples to Impacts
Jet Propulsion Laboratory
March 31, 2011

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 at cornell.edu; Karen Randall, SETI Institute,
Mountain View, Calif., 650-960-4537, krandall at seti.org; and Joe Mason,
Space Science Institute, Boulder, Colo., 720-974-5859, jmason at ciclops.org.

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook at jpl.nasa.gov

Michael Buckley 240-228-7536
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
michael.buckley at jhuapl.edu

2011-102
Received on Thu 31 Mar 2011 02:49:30 PM PDT