Explanation:
Why are vast sections of Iapetus as dark as
coal?
No one knows for sure.
Iapetus, the
third largest moon of
Saturn,
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
above image,
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
wide moon.
The material is
so dark
that it reflects less than five percent of incident sunlight,
yet overlays craters indicating that it was spread after the
craters were formed.
Iapetus
has other unexplained features.
The bright part of
Iapetus
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
near
the bottom.
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
walnut.
Research into the formation and history of
mysterious Iapetus
is active and ongoing.
