[meteorite-list] Cassini Finds Possible Origin of One of Saturn's Rings

[Ron Baalke]

Aug. 2, 2007

Tabatha Thompson
Headquarters, Washington 
202-358-3895
tabatha.thompson-1 at nasa.gov

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

RELEASE: 07-168

CASSINI FINDS POSSIBLE ORIGIN OF ONE OF SATURN'S RINGS

WASHINGTON - Cassini scientists may have identified the source of one 
of Saturn's more mysterious rings. Saturn's G ring likely is produced 
by relatively large, icy particles that reside within a bright arc on 
the ring's inner edge.

The particles are confined within the arc by gravitational effects 
from Saturn's moon Mimas. Micrometeoroids collide with the particles, 
releasing smaller, dust-sized particles that brighten the arc. The 
plasma in the giant planet's magnetic field sweeps through this arc 
continually, dragging out the fine particles, which create the G 
ring.

The finding is evidence of the complex interaction between Saturn's 
moons, rings and magnetosphere. Studying this interaction is one of 
Cassini's objectives. The study is in the Aug. 2 issue of the journal 
Science and was based on observations made by multiple Cassini 
instruments in 2004 and 2005. 

"Distant pictures from the cameras tell us where the arc is and how it 
moves, while plasma and dust measurements taken near the G ring tell 
us how much material is there," said Matthew Hedman, a Cassini 
imaging team associate at Cornell University in Ithaca, N.Y., and 
lead author on the Science paper. 

Saturn's rings are an enormous, complex structure, and their origin is 
a mystery. The rings are labeled in the order they were discovered. 
>From the planet outward, they are D, C, B, A, F, G and E. The main 
rings -- A, B and C-from edge-to-edge, would fit neatly in the 
distance between Earth and the moon. The most transparent rings are D 
-- interior to C -- and F, E and G, outside the main rings. 

Unlike Saturn's other dusty rings, such as the E and F rings, the G 
ring is not associated closely with moons that either could supply 
material directly to it -- as Enceladus does for the E ring -- or 
sculpt and perturb its ring particles -- as Prometheus and Pandora do 
for the F ring. The location of the G ring continued to defy 
explanation, until now.

Cassini images show that the bright arc within the G ring extends 
one-sixth of the way around Saturn and is about 155 miles wide, much 
narrower than the full 3,700-mile width of the G ring. The arc has 
been observed several times since Cassini's 2004 arrival at the 
ringed planet and thus appears to be a long-lived feature. A 
gravitational disturbance caused by the moon Mimas exists near the 
arc.

As part of their study, Hedman and colleagues conducted computer 
simulations that showed the gravitational disturbance of Mimas could 
indeed produce such a structure in Saturn's G ring. The only other 
places in the solar system where such disturbances are known to exist 
are in the ring arcs of Neptune.

Cassini's magnetospheric imaging instrument detected depletions in 
charged particles near the arc in 2005. According to the scientists, 
unseen mass in the arc must be absorbing the particles. "The small 
dust grains that the Cassini camera sees are not enough to absorb 
energetic electrons," said Elias Roussos of the Max-Planck-Institute 
for Solar System Research, Germany, and member of the magnetospheric 
imaging team. "This tells us that a lot more mass is distributed 
within the arc."

The researchers concluded that there is a population of larger, 
as-yet-unseen bodies hiding in the arc, ranging in size from that of 
peas to small boulders. The total mass of all these bodies is 
equivalent to that of a 328-foot-wide, ice-rich small moon.

Joe Burns, a co-author of the paper from Cornell University and a 
member of the imaging team, said, "We'll have a super opportunity to 
spot the G ring's source bodies when Cassini flies about 600 miles 
from the arc 18 months from now."

The Cassini-Huygens mission is a cooperative project of NASA, the 
European Space Agency and the Italian Space Agency. The Jet 
Propulsion Laboratory, Pasadena, Calif., 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 the laboratory. The imaging team is based at the 
Space Science Institute, Boulder, Colo. The magnetospheric imaging 
instrument team is based at Johns Hopkins University, Laurel, Md.

G ring movies and images are available on the Internet at:

http://www.nasa.gov/cassini

	
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