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Saturn Moon Mystery Continues




NASA's Hubble Space Telescope may have discovered several orbiting clumps of icy rubble that could be the remnants of recently shattered moonlets orbiting near the outer edge of Saturn's ring system.

Astronomers say this could represent the discovery of a new class of ephemeral, transitional object in the solar system which provides new clues to the origin and evolution of Saturn's spectacular rings.

This startling conclusion is based upon Hubble Space Telescope observation of Saturn made during the ring plane crossing on August 10, which provided a rare opportunity to seek out faint satellites in and near the ring plane.

"Ring plane crossing" refers to those moments when the Earth or Sun crosses the plane of Saturn's rings, allowing them to be seen (or illuminated) edge on. At such times, the usually bright rings are seen only as a faint, thin line, and Saturn's smaller satellites become visible. These events are rare, occurring in groups of two or four at intervals of about 14.5 years. The previous series of crossings occurred in 1980.

The latest Hubble pictures gave astronomers an opportunity to confirm the presence of two new satellite first discovered by the Space Telescope in images taken during the May 22 ring plane crossing. Rather than solving the moon question, however, the August observations confront astronomers with a new mystery: "We realized these moons are too bright to have gone undetected when the Voyager spacecraft flew by Saturn in 1980 and 1981" said Phil Nicholson of Cornell University.

A further complication is that the August pictures seem to show at least three new objects, and in different orbits from the two May objects. "They also appear to be very elongated or arc-like, unlike a satellite should be. One possibility is that they are large clouds of debris from small satellites shattered by impacts with chunks of space debris (possibly comets), sometime during the 14 years since the Voyager 2 flyby." Just as a small handful of chalk dust can make a large dust cloud if tossed in the air, a shattered moonlet would be much brighter and more visible than when all of its mass is compressed into a single solid body.

The discovery of objects in this transitional phase is not totally unexpected, says Nicholson, because one scenario for the origin of Saturn's ring system is that it is made up of countless fragments from several pulverized moons. This idea is reinforced by the fact the new objects orbit Saturn near the narrow F ring, which is a dynamic transition zone between the main rings and the larger satellites. Moonlets in this region can be easily disrupted by Saturn's tidal pull if they are fractured by an impact, forming a cloud of debris. Eventually such a cloud would spread around the moon's orbit to form a new ring.

The dynamics of this "bumper car" zone are also evident in Hubble's observations of the satellite Prometheus. Although a third object seen in the May images was first suspected to be another new satellite because its location did not match the predicted position for any of the known satellites charted by Voyager, it now appears that this body is in fact Prometheus, which has slipped in its orbit by 20 degrees from the predicted position. Nicholson suggests that this may be a consequence of a "collision" of Prometheus with the F ring, which is believed to have occurred in early 1993. The moon may have passed close enough to one of the denser, lumpy regions of the F ring to have its orbit changed.

The researchers plan to obtain further observations of Saturn's moons and rings during the third ring plane crossing on November 21.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

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