A faint pinprick of moving light spotted by NASA's Cassini spacecraft in Saturn's G ring appears to be a moonlet that could be the main source of the ring, astronomers announced today.

Cassini scientists analyzing images acquired over the course of about 600 days found the tiny moonlet, which measures about a third of a mile (half a kilometer) across, embedded within a partial ring, or ring arc, previously found by Cassini in Saturn's tenuous G ring.

Before the discovery, announced today in an International Astronomical Union circular, astronomers a single particular moon that could explain the formation of the G ring.

"Before Cassini, the G ring was the only dusty ring that was not clearly associated with a known moon, which made it odd," said Matthew Hedman, a Cassini imaging team associate at Cornell University in Ithaca, N.Y. "The discovery of this moonlet, together with other Cassini data, should help us make sense of this previously mysterious ring."

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Saturn's rings were named in the order they were discovered. Working outward they are: D, C, B, A, F, G and E. The G ring is one of the outer diffuse rings.

Within the faint G ring there is a relatively bright and narrow, 150-mile- (250-kilometer-) wide arc of ring material, which extends 90,000 miles (150,000 kilometers), or one-sixth of the way around the ring's circumference. The moonlet moves within this ring arc.

This brings the number of Saturnian ring arcs with embedded moonlets found by Cassini to three.

[Saturn has at least 60 moons, though the precise figure is hard to determine.]

Previous Cassini plasma and dust measurements indicated that this partial ring may be produced from relatively large, icy particles embedded within the arc, such as this moonlet.

Scientists imaged the moonlet on Aug. 15, 2008, and then they confirmed its presence by finding it in two earlier images. They have since seen the moonlet on multiple occasions, most recently on Feb. 20, 2009.

The moonlet is too small to be resolved by Cassini's cameras, so its size cannot be measured directly. However, Cassini scientists estimated the moonlet's size by comparing its brightness to another small Saturnian moon, Pallene.

Hedman and his collaborators also have found that the moonlet's orbit is being disturbed by the larger, nearby moon Mimas, which is responsible for keeping the ring arc together.

The new moonlet may not be alone in the G ring arc, as previous Cassini measurements indicated many other bodies ranging in size from a few to several hundred feet could be lurking there.

"Meteoroid impacts into, and collisions among, these bodies and the moonlet could liberate dust to form the arc," Hedman said.

Early next year, Cassini's camera will take a closer look at the arc and the moonlet.

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