“NASA scientists are on the trail of Iapetus’ mysterious dark side, which seems to be home to a bizarre “runaway” process that is transporting vaporized water ice from the dark areas to the white areas of the Saturnian moon.

This “thermal segregation” model may explain many details of the moon’s strange and dramatically two-toned appearance, which have been revealed exquisitely in images collected during a recent close flyby of Iapetus by NASA’s Cassini spacecraft.

Infrared observations from the flyby confirm that the dark material is warm enough (approximately minus 230 degrees Fahrenheit or 127 Kelvin) for very slow release of water vapor from water ice, and this process is probably a major factor in determining the distinct brightness boundaries.

“The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process,” said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, Boulder, Colo. ”

continues at:-

http://www.jpl.nasa.gov/news/news.cfm?release=2007-113

I find it difficult to discern between ice on a rocky surface and “soot”* on an icy surface. I could be persueded either way from these images, which does not provide conclusive evidence. Now the comments about fresh craters and the exposed white does seem more suggestive.

*I don’t think it is soot, but just provoking the imagery of soot on ice.

Would the dark area absorb sufficient energy to keep itself ice-free?

Does the dark side of Iapetus face toward or away from Saturn? (Effect of more incident sunlight?)

Could Iapetus have enough heat generated internally (Saturn squeezing it must cause some) to keep the surface partially ice-free? Perhaps the internal structure is non-uniform, so the energy output is also non-uniform. (Would mapping Iapetus for gravity distribution at surface confirm the internal structure?)

Quite a neat wee moon.

I am more interested in what further surprises Europa might have in store, though. A water-ice covered moon with an oxygen atmosphere (well, OK, tenuous atmosphere). Hmm.

I’ve studied the blown up image of Iapetus in detail and I can’t find anything that convinces me that the icy material isn’t overlying the dark matt material. If someone can convince me otherwise, I’d be happy to accept their arguments.

To me, the lighter areas, look very much like what happens when snow begins to melt. ie. patches of hardened icy-stuff persist and gradually pock-marks and holes develop.

If there is some dynamic melting process going on, couldn’t it be solved rather simply by observing the moon in detail over a few weeks?

Since there is evidence that it’s been something like this for centuries, I like the idea that Saturn’s causing it. Perhaps something to do with magnetic fields, or maybe it just radiates enough e-m? Isn’t it like Jupiter, a planet which emits more than it receive?

See the third panel in this photo series:
http://www.planetary.org/image/iapetus_voyagermountain_adjust-levels.jpg

By making the black part of Iapetus correctly exposed, and overexposing the light part, it can be seen that there are dark colored craters partly covered by white. It wouldn’t make sense to have a white base that was partly covered by a half crater.

I say the dark area of Iapetus is where the “snow” (or whatever it is) has been removed (or didn’t accumulate); perhaps by sublimation or some such process. If the dark side of Iapetus faces Saturn, maybe the Saturnshine or other radiation from Saturn has caused the removal of the white stuff from that side.