The moon with the split personality, Iapetus, presents a
perplexing appearance in the latest images snapped by the Cassini
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 (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
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
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.