The evolution of cometary activity

Abstract

This dissertation is a two part investigation into the manner in which cometary activity originates. The first part is a simulation of the formation of a refractory residue layer on a surface subliming dirty ice. The second part traces to morphology of the sublimed ice (now gas) as it flows out through the inner coma. The formation of filamentary sublimate (FSR) residues from subliming dirty ice has been discussed by Saunders et al, (1984). The first part of this dissertation expands on the study of Saunders et al, by testing all major silicate mineral classes, and investigating the effects of organic materials on the formation of these residues. FSR is a light, strong material, an excellent thermal insulator, and its presence on the surface of cometary nuclei (and, potentially, on Martian polar layered terrain and icy satellites) would strongly affect the evolution of gas from the volatile ices it covers. FSR is formed when hydrated phyllosilicate grains are released from an icy matrix. These mineral grains attract a coating of several monolayers of semifluid water around themselves, even though the surrounding water is frozen. This semifluid layer allows the grains to coordinate and rebond into larger . units. These units are chemically identical to the original material, while retaining the physical morphology of the dirty ice. Organic tars can also bind mineral grains together. The structure formed is stronger and less porous than pure mineral FSR. Organic FSR resembles the structure observed to cover most of the nucleus of Halley's comet (Keller et al. 1986). The second part of this dissertation investigates the morphology of the outflow of the OH radical in the inner coma of Halley's comet. When a ratio is made of images of the comet in the OH (1-1) and (0-0) bands (around 3100 A), the resulting image maps the projection along the line of sight of the sunward velocity of the gas in the coma Qualitative interpretation of these images shows that, although constant velocity, spherically symmetric outflow is usually observed, gas jets and other asymmetries are not uncommon. Poor quantitative agreement with the theory (Schleicher and A'Hearn 1982) suggests either gas outflows on the order of 4 km/sec, or fine structure in the ratio which was not modeled by Schleicher and A'Hearn.