[meteorite-list] Cassini Sees Moon Building Giant Snowballs in Saturn Ring

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 20 Jul 2010 09:04:13 -0700 (PDT)
Message-ID: <201007201604.o6KG4DPo012076_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2010-240

Cassini Sees Moon Building Giant Snowballs in Saturn Ring
Jet Propulsion Laboratory
July 20, 2010

While orbiting Saturn for the last six years, NASA's Cassini spacecraft
has kept a close eye on the collisions and disturbances in the gas
giant's rings. They provide the only nearby natural laboratory for
scientists to see the processes that must have occurred in our early
solar system, as planets and moons coalesced out of disks of debris.

New images from Cassini show icy particles in Saturn's F ring clumping
into giant snowballs as the moon Prometheus makes multiple swings by the
ring. The gravitational pull of the moon sloshes ring material around,
creating wake channels that trigger the formation of objects as large as
20 kilometers (12 miles) in diameter.

"Scientists have never seen objects actually form before," said Carl
Murray, a Cassini imaging team member based at Queen Mary, University of
London. "We now have direct evidence of that process and the rowdy dance
between the moons and bits of space debris."

Murray discussed the findings today (July 20, 2010) at the Committee on
Space Research meeting in Bremen, Germany, and they are published online
by the journal Astrophysical Journal Letters on July 14, 2010. A new
animation based on imaging data shows how one of the moons interacts
with the F ring and creates dense, sticky areas of ring material.

Saturn's thin, kinky F ring was discovered by NASA's Pioneer 11
spacecraft in 1979. Prometheus and Pandora, the small "shepherding"
moons on either side of the F ring, were discovered a year later by
NASA's Voyager 1. In the years since, the F ring has rarely looked the
same twice, and scientists have been watching the impish behavior of the
two shepherding moons for clues.

Prometheus, the larger and closer to Saturn of the two moons, appears to
be the primary source of the disturbances. At its longest, the
potato-shaped moon is 148 kilometers (92 miles) across. It cruises
around Saturn at a speed slightly greater than the speed of the much
smaller F ring particles, but in an orbit that is just offset. As a
result of its faster motion, Prometheus laps the F ring particles and
stirs up particles in the same segment once in about every 68 days.

"Some of these objects will get ripped apart the next time Prometheus
whips around," Murray said. "But some escape. Every time they survive an
encounter, they can grow and become more and more stable."

Cassini scientists using the ultraviolet imaging spectrograph previously
detected thickened blobs near the F ring by noting when starlight was
partially blocked. These objects may be related to the clumps seen by
Murray and colleagues.

The newly-found F ring objects appear dense enough to have what
scientists call "self-gravity." That means they can attract more
particles to themselves and snowball in size as ring particles bounce
around in Prometheus's wake, Murray said. The objects could be about as
dense as Prometheus, though only about one-fourteenth as dense as Earth.

What gives the F ring snowballs a particularly good chance of survival
is their special location in the Saturn system. The F ring resides at a
balancing point between the tidal force of Saturn trying to break
objects apart and self-gravity pulling objects together. One current
theory suggests that the F ring may be only a million years old, but
gets replenished every few million years by moonlets drifting outward
from the main rings. However, the giant snowballs that form and break up
probably have lifetimes of only a few months.

The new findings could also help explain the origin of a mysterious
object about 5 to 10 kilometers (3 to 6 miles) in diameter that Cassini
scientists spotted in 2004 and have provisionally dubbed S/2004 S 6.
This object occasionally bumps into the F ring and produces jets of debris.

"The new analysis fills in some blanks in our solar system's history,
giving us clues about how it transformed from floating bits of dust to
dense bodies," said Linda Spilker, Cassini project scientist at NASA's
Jet Propulsion Laboratory in Pasadena, Calif. "The F ring peels back
some of the mystery and continues to surprise us."

The late Kevin Beurle was made the honorary first author on this paper
because of his contributions in developing software and designing
observation sequences for this research. He died in 2009.

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 mission
for NASA's Science Mission Directorate, Washington, D.C. The Cassini
orbiter and its two onboard cameras were designed, developed and
assembled at JPL. The imaging operations center is based at the Space
Science Institute in Boulder, Colo.

For more information, visit: http://www.nasa.gov/cassini and
http://saturn.jpl.nasa.gov

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

Joe Mason 720-974-5859
Space Science Institute, Boulder, Colo.
media at ciclops.org

Simon Levey 011-44-20-7882-7454
Queen Mary, University of London
s.levey at qmul.ac.uk

2010-240
Received on Tue 20 Jul 2010 12:04:13 PM PDT