Astronomy Picture of the Day
2006 January 3
Dark Terrain on Saturn's Iapetus
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.
Credit: Cassini
Imaging Team, SSI, JPL, ESA, NASA
