Abstract
The bulk and microstructure of comet nuclei are derived from the morphological structure and chemical composition of submicron sized interstellar dust grains which have undergone cold aggregation in the pre-solar nebula. The density, size distribution and chemical composition of comet dust deduced from observations of masses, mass spectra, and infrared emission in the 3.4 μm, 10 μm and the continuum are compared with models of fluffy aggregates of interstellar dust. It is shown that the 10 μm emission of comet Halley is produced by predominantly interstellar amorphous silicates with a small (−5% admixture) of crystalline silicates. The organic refractory mantles on interstellar silicates are absolutely required to raise the emitting grain temperature high enough to make the 10 μm peak observable. Pure silicates would be too poor emitters even at submicron sizes without this extra heating. Furthermore it is shown that even silicate particles as small as 0.5 μm radius are still too large even with organic mantles to provide efficient 10 μm emission and that the most probable silicate core size is ∼0.05 μm with a mantle thickness > 0.02 μm. Finally, it is shown that the number of small cometary particles with masses ≲ 10 −g g must be substantially larger than has generally been assumed, and that they must be fluffy.
Published Version
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