Iapetus

Moon of Saturn, Iapetus shows a great contrast in brightness of its surface. Half of its surface (the polar areas) is 10 times brighter than the other, which gives the moon a suiting name (Iapetus means "two-faced"). One possible reason to the dark side of Iapetus may be that the material from volcanic eruptions made it dark. Another possibility is that the moon passed through a cloud of organic matter, which colored it. Many impact craters can be seen on the bright sides.

Equatorial Seam

As if this dramatic feature isn't enough, Iapetus has a seam, which runs halfway through the equator. Some believe that this seem may be of volcanic origin. Others believe that Iapetus passed through a ring and captured some of the particles as its orbit changed. Evidence points to the fact that something changed Iapetus' orbit before colliding with ring material.

The ridge is unique in the Sol System and its formation could be related to whatever process "painted" the moon's surface with dark material. It is not yet clear whether the ridge around Iapetus arose through compression, like a mountain belt that has folded upward, or through material from inside Iapetus erupting on to the surface and locally accumulating, forming the ridge.

History

Billions of years ago, shortly after its formation, Iapetus spun quickly, taking just 5 hours to complete a rotation. This fast spin gave it the oblate shape it has today. Over time, its rotation slowed down to about 16 hours. It also cooled down enough that its surface froze solid. It couldn't absorb the excess surface material. Instead, this rubble built up the chain of mountains around its equator. At this point, its formation completely halted in its tracks. The moon now orbits at a relatively slow rate, turning only once every 80 days.

Scientists were able to confirm these predictions for Iapetus, using observations of its rocks containing short-lived isotopes aluminum-26 and iron-60. These decay at a rate that allowed scientists to carbon date the moon at roughly 4.564 billion years old.