Why are vast sections of Iapetus as dark as
No one knows for sure.
third largest moon of
was inspected again as the Saturn-orbiting robot
Cassini spacecraft swooped past the
enigmatic world again late last year.
The dark material covers most of the surface visible in the
while the small portion near the top that appears almost white is of a
color and reflectance more typical of Saturn's other moons.
The unknown material covers about half of the 1,500 kilometer
The material is
that it reflects less than five percent of incident sunlight,
yet overlays craters indicating that it was spread after the
craters were formed.
has other unexplained features.
The bright part of
is covered with unexplained long thin streaks.
The orbit of
Iapetus is also unusual, being tilted to the plane of Saturn's
orbit by an unusually high fifteen degrees.
A strange ridge
about 13 kilometers high crosses much of Iapetus near the
equator and is visible
Oddly, this ridge is almost exactly parallel with Iapetus' equator.
The exact shape of Iapetus remains undetermined,
but images indicate that it is quite strange --
something like a
Research into the formation and history of
is active and ongoing.