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Comets
Evidence of amorphous ice in comets is found in the high levels of activity observed in long-period, Centaur, and Jupiter Family comets at heliocentric distances beyond ~6 AU. These objects are too cold for the sublimation of water ice, which drives comet activity closer to the sun, to have much of an effect. Thermodynamic models show that the surface temperatures of those comets are near the amorphous/crystalline ice transition temperature of ~130 K, supporting this as a likely source of the activity. The runaway crystallization of amorphous ice can produce the energy needed to power outbursts such as those observed for Centaur Comet 29P/Schwassmann-Wachmann 1.