EVOLUTION OF COMETARY NUCLEI AS INFLUENCED BY A DUST COMPONENT

If a cometary nucleus is "dirty iceball"--a solid mix of dust, pebbles, and volatiles trapped in a hydrate clathrate matrix--then any theory of the time development of comets must account for the disposition of the nonvolatile grains that are constantly being exposed by the retreating, sublimating surface of the nucleus. A Loose Lattice model describing a balance between the outward thrust of escaping gases and the force of gravity upon particles on the nucleus surface is developed. The effect is shown to be dependent on the ratio of dust to volatiles in the original mix.

The concept of "negative feedback", or the moderating influence of dust on the thermal history of the nucleus is explained.

A second, Heavy Mantle, model is developed, in which gas velocity and its attendant effects are seen to be dependent on level within the dust layer. A technique for predicting the number and size distribution of grains entrained is proposed, and the resulting feed function, into the comet's dust tail, is used to modify the model for "shadowing" of the nucleus by escaped dust.

Both models are used to explain a major type of cometary "outburst" as well as secular changes observed in many periodic comets.

A third type of dust layer, an agitated maelstrom of incompletely entrained particles, is probably present at some stages of a comet's life, particularly near perihelion passage. This type of layer can be described as a Fluidized Bed. The basic concepts of fluidization are presented, and their implications for heat transfer and elutriation are discussed. Heuristic arguments are given to demonstrate compatibility with observation and with other prominent theories.

The results of both the Loose Lattice and the Heavy Mantle models depend upon assumptions regarding momentum transfer in a dusty gas. These assumptions are now checked in a detailed hydrodynamic analysis of several test cases.

The concept of a mobile dust bed is developed further, showing that grains once lifted may still fall and participate in the "seasonal effect" of Sekanina and Whipple, explaining still another variety of cometary outbursts. The likelihood that there is a population of grains that, once lifted, can still fall back, is developed as a theory of "sedimentation" upon the nucleus.