On the Evolution and Activity of Cometary Nuclei

Abstract

The thermal evolution of a spherical cometary nucleus (initial radius of 2.5 km), composed initially of very cold amorphous ice and moving in comet Halley's orbit, is simulated numerically for 280 revolutions. It is found that the phase transition from amorphous to crystalline ice constitutes a major internal heat source. This transition does not occur continuously, but in distinct rounds. Due to the (slow) heating of the amorphous ice between crystallization rounds, the phase transition front advances into the nucleus to progressively greater depths. At the time of crystallization, the temperature of the transformed ice rises to 180K. A large fraction of the gas trapped in the ice at low temperatures is thereby released. Whereas some of the released gas may find its way out through cracks in the crystalline ice layer, the rest is expected to accumulate in gas pockets that may eventually explode, forming "volcanic calderas".