[meteorite-list] Images of Saturn's Small Moons Tell the Story of Their Origins

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 7 Dec 2007 19:23:15 -0800 (PST)
Message-ID: <200712080323.TAA18367_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2007-142

Images of Saturn's Small Moons Tell the Story of Their Origins
Jet Propulsion Laboratory
December 06, 2007

Imaging scientists on NASA's Cassini mission are telling a tale of how
the small moons orbiting near the outer rings of Saturn came to be. The
moons began as leftover shards from larger bodies that broke apart and
filled out their "figures" with the debris that made the rings.

It has long been suspected that Saturn's rings formed in the
disintegration of one or several large icy bodies, perhaps pre-existing
moons, by giant impacts. The resulting debris quickly spread and settled
into the equatorial plane to form a thin disk surrounding the planet.
And the small, irregularly shaped ring-region moons were believed to be
the leftover pieces from this breakup.

Now, several years' worth of cosmic images of Saturn's 14 known small
moons have been used to derive the sizes and shapes of most of them, and
in about half the cases, even masses and densities. This information,
published in the Dec. 7 issue of the journal Science, has led to new
insights into how some of these moons may have formed.

The tip-off was the very low density of the inner moons, about half that
of pure water ice, and sizes and shapes that suggested they have grown
by the accumulation of ring material. The trouble was, these moons are
within and near the rings, where it is not possible for small particles
to fuse together gravitationally. So how did they do it? They got a jump
start.

"We think the only way these moons could have reached the sizes they are
now, in the ring environment as we now know it to be, was to start off
with a massive core to which the smaller, more porous ring particles
could easily become bound," said Carolyn Porco, Cassini imaging team
leader from the Space Science Institute in Boulder, Colo. Porco is the
lead author of the first of two related articles published in this
week's issue of Science.

Simple calculations and more complicated computer simulations have shown
that ring particles will readily become bound to a larger seed having
the density of water ice. By this process, a moon will grow even if it
is relatively close to Saturn. The result is a ring-region moon about
two to three times the size of its dense ice core, covered with a thick
shell of porous, icy ring material. To make a 30-kilometer moon (19
miles) requires a seed of about 10 kilometers (6 miles).

Where did such large cores come from? And when did this all take place?

"The core may in fact be one of the remnants from the original
ring-forming event," said co-author Derek Richardson, professor of
astronomy at the University of Maryland, College Park, "which might have
been left intact all this time and protected from additional collisional
breakup by the mantle of ring particles around it."

Just exactly when the rings formed is not known. "But it is not out of
the question that the moons date back to the time of ring formation,"
said Porco.

The researchers show that the cores of Pan and Daphnis, which orbit
within gaps in the outer A ring, were large enough to open narrow gaps.
Accretion, or accumulation of material, they say, probably occurred
quickly. The moons grew and their gaps widened, achieving their present
sizes before the gaps were completely emptied of material, and probably
before the local rings reached their present thickness.

So how did Pan in the main rings, and Atlas, which orbits just beyond
the outer edge of the main rings, get the prominent equatorial ridges
that make them look like flying saucers? The second paper reports
evidence for a secondary stage of accretion that occurred after the
moons' growth was completed and after the rings flattened to their
present 20-meter (66 feet) thickness.

"Our computer simulations show that the ridges must have accreted
rapidly when Saturn's rings were thin, forming small accretion disks
around the equators of Pan and Atlas," said Sebastien Charnoz, lead
author and an associate of imaging team member Andre Brahic at the
University Paris-Diderot and CEA Saclay, in France. "The ridges might be
the remains of 'fossilized' accretion disks, fundamental structures seen
at all scales in the universe, from planetary rings to galaxies."

Images of Saturn's small moons are available at:
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini and
http://ciclops.org.

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
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 team is based at the Space Science
Institute, Boulder, Colo.

------------------------------------------------------------------------

Media Contact: Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
carolina.martinez at jpl.nasa.gov

Preston Dyches 720-974-5859
Space Science Institute, Boulder, Colo.
media at ciclops.org

2007-142
Received on Fri 07 Dec 2007 10:23:15 PM PST