Revelations in Saturn's Rings Continue as Equinox Approaches

http://www.jpl.nasa.gov/news/features.cfm?feature=2261

Revelations in Saturn's Rings Continue as Equinox Approaches
Jet Propulsion Laboratory
August 07, 2009

Thanks to a special play of sunlight and shadow as Saturn continues
its
march towards its August 11 equinox, recent images captured by NASAâs
Cassini spacecraft are revealing new three-dimensional objects and
structures in the planetâs otherwise flat rings. The Cassini
spacecraft
captured this image of a small object in the outer portion of Saturn's
B
ring casting a shadow on the rings as Saturn approaches its August
2009
equinox.

This new moonlet, situated about 300 miles (480 kilometers), inward
from
the outer edge of the B ring, was found by detection of its shadow
which
stretches 25 miles, or 41 kilometers, across the rings. The shadow
length implies the moonlet is protruding about 660 feet, or 200
meters,
above the ring plane. If the moonlet is orbiting in the same plane as
the ring material surrounding it, which is likely, it must be about
1,300 feet, or 400 meters, across. This object is not attended by a
propeller feature, unlike the band of moonlets discovered in Saturn's
A
ring earlier by Cassini. The A ring moonlets, which have not been
directly imaged, were found because of the propeller-like narrow gaps
on
either side of them that they create as they orbit within the rings.
The
lack of a propeller feature surrounding the new moonlet is likely
because the B ring is dense, and the ring material in a dense ring
would
be expected to fill in any gaps around the moonlet more quickly than
in
a less dense region like the mid-A ring. Also, it may simply be harder
in the first place for a moonlet to create propeller-like gaps in a
dense ring.

The search for three-dimensional structures in Saturn's rings has been
a
major goal of the imaging team during Cassini's "Equinox Mission," the
two-year period containing exact equinox -- that moment when the sun
is
seen directly overhead at noon at the planet's equator. This novel
illumination geometry, which occurs every half-Saturn-year, or about
15
Earth years, lowers the sun's angle to the ring plane and causes
out-of-plane structures to cast long shadows across the rings' broad
expanse, making them easy to detect.

The new images can be found at http://saturn.jpl.nasa.gov and
http://www.nasa.gov/cassini