NASA Spacecraft Reveal Mysteries Of Jupiter And Saturn Rings

PASADENA, Calif. -- In a celestial forensic exercise, scientists
analyzing data from NASA's Cassini, Galileo and New Horizons missions
have traced telltale ripples in Saturn and Jupiter's rings to
specific collisions with cometary fragments that occurred decades,
not millions of years, ago.

Jupiter's ripple-producing culprit was comet Shoemaker-Levy 9. The
comet's debris cloud hurtled through the thin Jupiter ring system on
a collision 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 1983. The findings are
detailed in two papers published Thursday in the journal Science.

"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 on one of the papers, and a
research associate at Cornell University in Ithaca, N.Y. "The solar
system is a much more dynamic place than we gave it credit for."

Scientists learned about the patchy patterns in Jupiter's rings in the
late 1990s from Galileo's visit to Jupiter. Unfortunately, the images
from that mission were fuzzy, and scientists didn't understand why
such patterns would occur. Not until Cassini entered orbit around
Saturn in 2004 and started sending back thousands of images did
scientists have a better picture of the activity. A 2007 science
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., saw that the
grooves in the D ring appeared to wind together more tightly over
time. Playing the process backward, Hedman demonstrated the pattern
originated when something tilted the D ring off its axis by about 300
feet (100 meters) in late 1983. The scientists found Saturn's gravity
on the tilted area warped the ring into a tightening spiral.

Cassini imaging scientists received another clue around August 2009
when the sun shone directly along Saturn's equator and lit the rings
edge-on. The unique lighting conditions highlighted ripples not
previously seen in another part of the ring system. Whatever happened
in 1983 was big - not a small, localized event.

The collision 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
out of sight from ground or space-based telescopes.

Hedman and Showalter, the lead author on the second paper, wondered
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 by applying the
same math they had applied to Saturn and factoring in Jupiter's
gravitational influence. Galileo was launched on a space shuttle in
1989 and studied Jupiter until 2003.

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 in late July. 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."

Launched in Oct. 15, 1997, Cassini began orbiting Saturn in 2004 and
sends back data daily.

"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 in 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. The mission is
managed by JPL for NASA's Science Mission Directorate in Washington.
The imaging team is based at the Space Science Institute in Boulder,
Colo. For more information about Cassini, visit:

http://www.nasa.gov/cassini

Pluto New Horizons launched in 2006 on the first mission to study
Pluto and the Kuiper Belt. The mission is managed by the Johns
Hopkins Applied Physics Laboratory in Laurel, Md., for NASA. The
mission is part of the New Frontiers program managed at the agency's
Marshall Space Flight Center in Huntsville, Ala. For more information
about Pluto New Horizons, visit:

http://www.nasa.gov/newhorizons

Source: NASA

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