I don't think anyone correctly predicted what would happen when Deep Impact struck Tempel 1. I'm not sure anyone knows even now! We do know there was ice, but ice represented a small percent of the dust. There were also clays - not expected to exist in Kuiper belt objects. We don't know if the few comets we have observed are oddballs, or if the Kuiper belt is full of clay. We have composition records now of about 4 comets, and they all appear to contain an unexpectely high percentage of rather ordinary dirt.

I will never understand why many insist that comet tails are hard evidence of electromagical zapping. Black object has frozen water ice on or near the surface...black object is heated by the sun. Water volatizes and is blown away by the solar wind. The basic physics is, well basic.

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jwj

It's ok not to know. We should try harder to find out.