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Anonymous
no-one really knows for sure, but the best theory so far is that it's an organic residue (organic in the chemical sense of the word, meaning it's comprised of compounds of mainly hydrogen, carbon and nitrogen, as opposed to organic meaning 'from an organism') left over after water ice was sublimated away.
http://saturn.jpl.nasa.gov/multimedia/products/pdfs/CHARM_20071030_Iapetus.pdf
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