What's hiding in Saturn's G ring?  The answer, NASA recently discovered, is a new moonlet.  NASA's Cassini probe, currently in orbit around Saturn, snapped pictures of the small Saturnian satellite. Scientists think that debris from the moonlet may be creating Saturn's outer G ring and its ring arc.

The new moonlet is a mere half a kilometer (~0.333 mi) across and was discovered only by comparing images from 600 days of orbit.  Saturn's rings starting from the innermost are the D, C, B, A, F, G and E rings.  The G ring, the second to outermost, is a rather diffuse ring.  However, it contains a bright arc, 250 km (~150 mi) wide.  The noticeable arc only extends a sixth of the way around Saturn, or approximately 150,000 km (~90,000 mi).

Cassini has measured plasma and dust in the arc, and scientists believe it is the product of large, icy particles, coming from small moonlets.  With the new discovery, they believe that the new moonlet is the primary source of this bright ice.

The new moonlet is small enough that its size is impossible to determine exactly.  However, approximates were made using the moon's brightness in comparison to another small Saturnian moon, Pallene.  The researchers also discovered that Mimas, a larger moon that shepherds the G ring's arc and keeps it from floating away is disrupting the moonlet's orbit.

Cassini has now found 3 moonlets within Saturn's ring arcs.  Scientists believe that numerous bodies from 1 to 100 meters (~3 to 300 ft) exist within Saturn's rings.  However, moonlets are still something rather unusual.  Both types of satellites are predicted to give rise to the moon. 

States Matthew Hedman, "Before Cassini, the G ring was the only dusty ring that was not clearly associated with a known moon, which made it odd.  The discovery of this moonlet, together with other Cassini data, should help us make sense of this previously mysterious ring.  Meteoroid impacts into, and collisions among, these bodies and the moonlet could liberate dust to form the arc."

Carl Murray, a researcher on the project and professor at the Queen Mary, University of London adds, "The moon's discovery and the disturbance of its trajectory by the neighboring moon Mimas highlight the close association between moons and rings that we see throughout the Saturn system. Hopefully, we will learn in the future more about how such arcs form and interact with their parent bodies."

Saturn has 61 known moons.  In addition it has hundreds of moonlets in orbit as well.  While some of these moonlets seem inconsequential, some are thought to be very important in the formation and maintenance of Saturn's rings.

Cassini is a joint project between NASA and European Space Agency and the Italian Space Agency.  It's has two onboard cameras, which were designed by the Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, and controlled by the Space Science Institute, Boulder, CO.

It is currently in its Cassini Equinox mission, an extension of its original four year mission.  The new mission will end in the fall of 2010.  Scientists are still deciding whether at that point to retire the probe or to keep it active.  Either way, the probe has been a shining success for NASA