FOR RELEASE OCT 5, 1995
PRESS RELEASE NO. : STScI-PR95-42

SATURN MOON MYSTERY CONTINUES:
COULD HUBBLE HAVE DISCOVERED SHATTERED SATELLITES?

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.
 
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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).