from Jet Propulsion Laboratory:
The moon with the split personality, Iapetus, presents a perplexing
appearance in the latest images snapped by the Cassini spacecraft.

One hemisphere of the moon is very dark, while the other is very
bright. Scientists do not yet know the origin of the dark material or
whether or not it is representative of the interior of Iapetus.

Iapetus (pronounced eye-APP-eh-tuss) is one of Saturn's 31 known
moons. Its diameter is about one third that of our own moon at 1,436
kilometers (892 miles). This image was taken in visible light with
the Cassini spacecraft narrow angle camera on July 3, 2004, from a
distance of 3 million kilometers (1.8 million miles) from Iapetus.
The brightness variations in this image are not due to shadowing, they
are real.

During Cassini's four-year tour, the spacecraft will continue to image
Iapetus and conduct two close encounters. One of those encounters,
several years from now, will be at a mere 1,000 kilometers (622
miles).

Iapetus was discovered by the Italian-French astronomer Jean Dominique
Cassini in 1672. He correctly deduced that the trailing hemisphere is
composed of highly reflective material, while the leading hemisphere
is strikingly darker.

This sets Iapetus apart from Saturn's other moons and Jupiter's moons,
which tend to be brighter on their leading hemispheres. Voyager
images show that the bright side of Iapetus, which reflects nearly 50
percent of the light it receives, is fairly typical of a heavily
cratered icy satellite. The leading side consists of much darker,
redder material that has a reflectivity of only about 3 to 4 percent.

One scenario for the outside deposit of material has dark particles
being ejected from Saturn's little moon Phoebe and drifting inward to
coat Iapetus. One observation lending credence to an internal origin
is the concentration of material on crater floors, which is suggestive
of something filling in the craters.

Iapetus is odd in other respects. It is in a moderately inclined
orbit, one that takes it far above and below the plane in which the
rings and most of the moons orbit. It is less dense than many of the
other satellites, which suggests a higher fraction of ice or possibly
methane or ammonia in its interior.

The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA's Office of
Space Science, Washington, D.C. The Cassini orbiter and its two
onboard cameras were designed, developed and assembled at JPL. The
imaging team is based at the Space Science Institute, Boulder, Colo.

For this and other images and information about the Cassini-Huygens
mission, visit
http://saturn.jpl.nasa.gov
and
http://www.nasa.gov/cassini
Ich höre immer gerne zu,wenn ich auch nicht immer belehrt werden möchte ;-)