Crystallization and Thermal Models of Icy Bodies

The energy release based on experiments is about 90,000 J/kg. This energy release, under certain circumstances, might be enough to significantly alter the temperature structure in an (initially) icy body. The release is invoked in complicated numerical models by Dina Prialnik and colleagues to explain many aspects of cometary outgassing. She finds that runaway transformation is possible: energy released from one element of amorphous ice is sufficient to trigger transformation of adjacent elements, and so on. A wave of crystallization is the result, in her models.

There are two concerns about the crystallization models, one probably valid and one probably not.

Is the ice initially amorphous?

The models assume that the ice is initially amorphous, but perhaps it is not.

This is very hard to test: the main validation of the presence of amorphous in the interiors of comets and KBOs is the success of the models. But the models involve many free and unknown parameters, leaving the sneaking suspicion that they might "fit the data" no matter what. Observations (of KBOs, at least) show that the surface ice is mainly crystalline, but the surface is not representative of the bulk.

Is the transition really exothermic in nature?

Akira Kouchi has argued that the reaction is NOT exothermic for an impure (i.e. realistic) ice sample, because volatiles trapped within the amorphous ice sublimate upon transition and carry away the energy from the crystallization, leaving none for heating.

This argument is correct for the ice within a few mean-free paths of the surface of the comet or KBO. For the vast bulk of the ice beneath this surface skin, though, Kouchi's effect is unimportant, because the sublimated molecules re-impact ice nearby and freeze there, so returning their energy of vaporization. The energy remains available for heating, as needed in the runaway models. At least, that's the way it looks to me.

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e-mail: David Jewitt