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Saturn's Moon Iapetus Has A 'Belt'
Believe writes "In another unexpected find by Cassini-Huygens, Saturn's moon Iapetus shows a bulging waistline. According to the story, the dark side of the moon is almost perfectly bisected by a tall, narrow ridge that runs for 1300 km (808 mi) and rises up to 20 km (12 mi) high. This height is amazing in such a small moon; it rivals Olympus Mons on Mars which is a body 5 times its size."
...but you don't see it on the front page of Slashdot.
Sheesh, evil *and* a jerk. -- Jade
For several reasons:
On Earth mountains are caused by plate tectonics, i.e. disconnected area of crust floating on magma that run into each other, but such mechanism are impossible on small bodies because they cool too fast, i.e their crust quickly become too thick and form a single fused objet.
Of course mountains can also be volcanoes, but similarly this implies magma that can rise to the surface, i.e a crust that is not too thick.
The exception are moons close enough to their parent body so that internal heat can be sustained by tidal effects. This is the case on Io, for example.
However there can only be tidal effects if the moon is rotating around itself at a different rate as it revolves around its parent body. For Iapetus, just like our moon, the two rates are the same and they always present the same face to their parent. This implies only minimal tidal effects due to the eccentricity of the orbit.
Of course the mountain/volcano may have been formed a very long time ago when the moon wasn't as cool as it is now, probably this is the case for mount Olympus on Mars, however there is erosion on most planetary bodies even without atmosphere or low gravity, caused by the myriad of asteroid impact they sustain.
One remaining option is impact by a large asteroid. We now have to come up with a reasonable impact scenario that can produce a feature similar to the one seen on Iapetus, which is indeed very strange.
Article submitter didn't take Astronomy 101 apparently. Small planetoids tend to have more prominent geological features than larger planets because stronger gravity pulls everything together harder and flattens things out. For instance, Olympus Mons on Mars is much higher than any mountain on Earth precisely because Earth has stronger gravity.