News Service
Cornell University
Media Contact:
Lauren Gold, (607) 255-9736
March 29, 2006
Cassini spacecraft finds evidence of football-field sized moonlets in
Saturn’s A ring
By Lauren Gold
New observations of propeller-shaped disturbances in Saturn’s A ring
indicate the presence of four small, embedded moons – and most likely
millions more, Cornell University astronomers report.
This is the first evidence of the existence of moonlets bridging the gap
in size between the larger ring moons Pan and Daphnis (several miles each
in diameter) and the much smaller ice particles that comprise the bulk of
the rings. The discovery could lead to a better understanding of the
origin and formation of Saturn’s rings and the solar system as a whole.
Matthew Tiscareno, a Cornell research associate, is lead author of a paper
describing the discovery in the March 30 issue of the journal Nature.
The four disturbances, which appear as pairs of slightly offset bright
horizontal streaks in an otherwise bland region of the ring, were captured
in two images taken in 2004 by NASA’s Cassini spacecraft. Astronomers say
the streaks are indicators of orbiting moons about 100 meters (328 feet)
in diameter: about the length of a football field, but still too small for
even Cassini’s highly sensitive Imaging Science Subsystem (ISS) to see
directly, but large enough to exert an observable gravitational pull on
the particles around them.
“The discovery of these intermediate-sized particles tells us that Pan and
Daphnis are probably just the largest members of the ring population,
rather than interlopers from somewhere else,” said Tiscareno.
A continuum of particle sizes lends strong support to the theory that
Saturn’s rings were formed when another object fragmented close to the
planet, breaking into pieces which were then captured by Saturn’s
gravitational pull.
“There has always been the question about whether the rings were
primordial material that was unable to grow into a moon or debris left
over from a breakup event,” said Joseph Burns, Cornell professor of
astronomy and of theoretical and applied mechanics and paper co-author,
along with Cornell research associate Matthew Hedman and researchers at
other institutions. The discovery doesn’t rule out the accretion model,
but “it’s a step in that direction,” said Tiscareno. “It’s hard for direct
accretion to produce particles this large. It’s much easier if you start
with a solid icy core, like a shard from a breakup.”
The discovery also helps explain fully cleared openings such as the Encke
and Keeler gaps within the rings. The gravitational influence of a larger
moon like Pan or Daphnis wraps around the circumference of the rings,
creating a gap. The smaller moonlets begin to create this effect, the
researchers say, but their influence is not strong enough to prevent
particles from falling into the rings ahead of and behind them.
Like a motorboat’s wake on a smooth lake, the four observed disturbances
are particularly visible since the area they inhabit is otherwise smooth.
But the fact that four were found in just two images covering only a tiny
fraction of the ring makes it likely that millions more exist. By studying
them further, researchers hope to gain a better understanding of how
Saturn’s rings formed – and even about how solar systems form around
stars.
“The structures we observe with Cassini are strikingly similar to those
seen in many numerical models of the early stages of planetary formation,
even though the scales are vastly different,” said Carl Murray, a
co-author and astronomer at Queen Mary College, University of London. “In
this way, Cassini is giving us unique insight into the origin of planets.”
The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory,
a division of the California Institute of Technology, manages the mission
for NASA.
During its Saturn tour, as currently planned, Cassini will complete 74
orbits of the ringed planet, 44 close flybys of the moon Titan and
numerous flybys of Saturn’s other icy moons.
Related Information:
- NASA’s Cassini-Huygens Mission site
http://saturn.jpl.nasa.gov/home/index.cfm
IMAGE CAPTION:
[http://ciclops.org/view_media.php?id=9965 (462KB)]
The left image shows the B ring, Cassini Division, A ring and F ring, with
the location of the propeller-shaped disturbances indicated. The center
image is a closer view of the A ring, showing the radial locations where
propeller features were spotted. In the right-hand image, the propellers
appear as double dashes in the two close-up images. The unseen moonlets,
each roughly the size of a football field, lie in the center of each
structure. (The horizontal lines in the image represent electronic noise
and do not correspond to ring features.)