Scientists think they have the reason for the walnut-like ridge straddling the equator of Iapetus, moon of Saturn. I have to say I’ve been looking forward to this explanation.
Scientists calculate Iapetus originally rotated much faster–at least five hours, but less than 16 hours per revolution. The fast spin gave the moon an oblate shape that increased the surface area (in the same way the surface area of a round balloon stretches when the balloon is pressed into an oblate shape). By the time the rotation slowed down to a period of 16 hours, the outer shell of the moon had frozen. Furthermore, the surface area of the cold moon was now smaller. The excess surface material was too rigid to go back smoothly into the moon. Instead, it piled up in a chain of mountains at the equator.
The next (and last ) Cassini close flyby is on September 10th. It will be interesting to see why one hemisphere of this moon is dark and the other snow white.