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The Accumulation and Structure of Comets

Published online by Cambridge University Press:  12 April 2016

Bertram Donn
Bertram Donn*
Affiliation:
Laboratory for Extraterrestrial Physics NASA/Goddard Space Flight Center

Abstract

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The evidence for the accumulation of the terrestrial planets and comets from solid grains is reviewed briefly. The various proposals for formation of cometary nuclei are described and commented upon. With three exceptions, all hypotheses conclude or imply that a single compact object forms. It has been almost universally assumed that grain accretion produced compact aggregates as is the case with liquid drops, and that this process continued. Several hypotheses start with Goldreich-Ward-type gravitational instabilities. The collapse for this case also occurs at low velocities in the cm·s−1 to m·s−1 range. Experiment and theory show that under these conditions, low-density, filamentary clusters form that are fractal aggregates with a fractal dimension approximately equal to two. Agglomeration of these clusters produces larger, compressible planetesimals or cometesimals, which efficiently combine upon colliding. In order to form cometary nuclei, the initial temperature must be about 50 K and not undergo a significant temperature rise during the accumulation process. The collision process can be analyzed with some simplifying assumptions using the limited experimental data available for the compaction of low-density powders. The calculations show that accumulation will occur at low temperatures. For a more refined analysis, experiments to study impacts on low-density powders are required. Models of cometary nuclei are reviewed, and a simple model of the structure that results from the accumulation of fluffy aggregates is described.

Type
Section III: Comets, Origins, and Evolution
Information
International Astronomical Union Colloquium , Volume 116 , Issue 1: Comets in the Post-Halley Era , 1989 , pp. 335 - 359
Copyright
Copyright © Kluwer 1991

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