Iapetus Cracked Like a Nut

Saturn has a moon named for the two-faced Roman god Janus, but the real two-faced moon is the larger Iapetus.  Since Jean Dominique Cassini discovered the moon in 1671 and noticed its varying brightness, scientists have been mystified by its two hemispheres, one as black as coal, the other white as snow.  Investigators were sure they would figure it out when Voyager 2 flew by in 1981.  They didn’t.  Investigators were hopeful they would figure it out when Cassini flew by less than two weeks ago on December 31, 2004.  They didn’t.  In fact, they were thrown another curve: a ridge that wraps around the equator that gives the moon the appearance of a cracked walnut (see imaging team picture and caption).  This is no ordinary crack; the ridgeline forms a mountain range 800 miles long and 13 miles high in places – three times the height of Mt. Everest.  For a global perspective, see this 3D image and this composite of the dark hemisphere.  The landslide at the right of the previous image slumped from a 9-mile scarp of an impact basin, and flowed tens of miles across a crater floor.
    For decades, scientists have tried to prove one of two models for Iapetus’ black-and-white contrast: either the dark material erupted from the inside (endogenic) and spread over the surface, or was splattered onto the moon from the outside (exogenic).  Since the dark material covers the leading hemisphere (the side facing the orbital motion, like a windshield), the exogenic model has been slightly favored, but planetary scientists could not understand a source for the material that would not have also plastered the inner moons, unless it was dust blown off from an impact on Phoebe (see 06/14/2004 headline) – but the spectra didn’t match.  The new hi-resolution images from Cassini, taken seven times closer than Voyager, still favor the exogenic model, because the dark regions have feathery edges and are distributed around the equator, not the poles.  The pictures seem to rule out a liquidy or mushy ooze spreading out from the interior, but scientists cannot eliminate the possibility that dusty debris erupted from cracks or geysers.  Could geological processes related to the equatorial ridge be related to the dark material?  If so, what drove those processes on a frozen moon?
    The ridge is a geological feature unique in the solar system.  It seems to cut right through more ancient craters.  The albedo difference divides the leading and trailing hemispheres, but this ridge divides the northern and southern hemispheres.  Are they related?  Since Iapetus is “far out” (literally and figuratively), Cassini won’t get another chance to observe it at close range till 2007.  That encounter, in September of that year, will be much closer and provide 100 times better resolution.  The JPL press release says scientists are hoping to determine whether Iapetus was volcanically active in the past – odd for an icy-cold moon far from any tidal influences.  Two weeks ago, scientists had a major mystery to solve at Iapetus; now they have two.

Planetary science is the art of building skeet for observations to shoot down.  Next episode in this exciting sport comes this Friday, when the Huygens Probe parachutes to the surface of Titan.