BY DR EMILY BALDWIN
Posted: 04 March, 2009
Discovered within Saturn’s outer G ring is a faint moonlet, thought to be responsible for maintaining the ring and its single ring arc.
Moving as a pinprick of light, Cassini imaging team scientists tracked the motion of the half kilometre wide moonlet through the ring arc by studying images acquired over a period of 600 days. The partial ring arc had previously been found by Cassini in Saturn’s tenuous 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,” says Matthew Hedman, a Cassini imaging team associate at Cornell University. “The discovery of this moonlet, together with other Cassini data, should help us make sense of this previously mysterious ring.”
This sequence of images, covering a 10 minute period, reveals the path of the newfound moonlet in Saturn’s G ring. Image: NASA/JPL/Space Science Institute.
Saturn’s rings are named in the order they were discovered, and working outward from the planet are D, C, B, A, F, G and E. The G ring is one of the outer diffuse rings, and embedded within the ring is a relatively bright and narrow, 250 kilometre wide arc of ring material that extends 150,000 kilometres, or one-sixth of the way around the ring’s circumference.
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. Other measurements implied the existence of a population of particles, possibly ranging in size from 1 to 100 metres across. “Meteoroid impacts into, and collisions among, these bodies and the moonlet could liberate dust to form the arc,” says Hedman. So far, three of Saturn’s ring arcs have been found to bear moonlets.
The imaging team also discovered that the moonlet’s orbit is being disturbed by nearby large moon Mimas, which is responsible for keeping the ring arc together. “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,” says Carl Murray, of Queen Mary University of London. “Hopefully, we will learn in the future more about how such arcs form and interact with their parent bodies.”
The moonlet was imaged in August last year, and then confirmed by scrutinizing earlier images. It has since been identified on multiple occasions, and most recently on 20 February. Although it is too small to be resolved by Cassini’s cameras, its size was estimated by comparing its brightness to another small Saturnian moon, Pallene.
Cassini will target the moonlet again early next year as part of the Cassini Equinox mission, which is expected to continue through 2010.