Saturn has small moon hidden in ring
(麻豆淫院Org.com) -- NASA's Cassini spacecraft has found within Saturn's G ring an embedded moonlet that appears as a faint, moving pinprick of light. Scientists believe it is a main source of the G ring and its single ring arc.
Cassini imaging scientists analyzing images acquired over the course of about 600 days found the tiny moonlet, half a kilometer (about a third of a mile) across, embedded within a partial ring, or ring arc, previously found by Cassini in Saturn鈥檚 tenuous G ring.
The finding is being announced today in an International Astronomical Union circular.
鈥淏efore Cassini, the G ring was the only dusty ring that was not clearly associated with a known moon, which made it odd,鈥� said Matthew Hedman, a Cassini imaging team associate at Cornell University in Ithaca, N.Y. 鈥淭he discovery of this moonlet, together with other Cassini data, should help us make sense of this previously mysterious ring.鈥�
Saturn鈥檚 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, 250-kilometer-wide (150-miles) arc of ring material, which extends 150,000 kilometers (90,000 miles), or one-sixth of the way around the ring鈥檚 circumference. The moonlet moves within this ring arc. 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鈥檚 cameras, so its size cannot be measured directly. However, Cassini scientists estimated the moonlet鈥檚 size by comparing its brightness to another small Saturnian moon, Pallene.
Hedman and his collaborators also have found that the moonlet鈥檚 orbit is being disturbed by the larger, nearby moon Mimas, which is responsible for keeping the ring arc together.
This brings the number of Saturnian ring arcs with embedded moonlets found by Cassini to three. The new moonlet may not be alone in the G ring arc. Previous measurements with other Cassini instruments implied the existence of a population of particles, possibly ranging in size from 1 to 100 meters (about three to several hundred feet) across. 鈥淢eteoroid impacts into, and collisions among, these bodies and the moonlet could liberate dust to form the arc,鈥� said Hedman.
Carl Murray, a Cassini imaging team member and professor at Queen Mary, University of London, said, "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."
Early next year, Cassini鈥檚 camera will take a closer look at the arc and the moonlet. The Cassini Equinox mission, an extension of the original four-year mission, is expected to continue until fall of 2010.
Provided by NASA
