Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997arcn.rept...73f&link_type=abstract
Analysis of Returned Comet Nucleus Samples, Proceedings of a Workshop held at Milpitas, California, 16-18 January, 1989. Compil
Astronomy and Astrophysics
Astronomy
1
Nebulae, Solar System, Planetary Nebulae, Comets, Chondrites, Chemical Equilibrium, Interstellar Chemistry, Stellar Evolution, Interplanetary Dust, Chemical Composition, Halley''S Comet, Comet Nuclei, Astronomical Models, Iras-Araki-Alcock Comet, Kohoutek Comet, Photochemical Reactions, Nitrogen Compounds, Water Vapor, Carbon Compounds, Sulfur Compounds
Scientific paper
A growing body of observations demonstrates that comets, like the chondritic meteorites, are disequilibrium assemblages, whose chemistry and molecular composition cannot be explained solely on the basis of models of equilibrium condensation in the solar nebula. These observations include: (1) The coexistence of reduced (e.g., CH4 and organics) and oxidized (e.g., CO, CO2, and H2CO) carbon compounds observed in the gas and dust emitted by comet P/Halley; (2) The coexistence of reduced (e.g., NH3) and oxidized (e.g., N2) nitrogen compounds in the gas emitted by comet P/Halley; (3) The observation of large amounts of formaldehyde in the gas emitted by comet P/Halley (H2CO/H2O approx. 1.5 - 4%) and by comet Machholz (1988j). Formaldehyde would be rapidly destroyed by thermal processing in the solar nebula and must be formed by some disequilibrating process either in the solar nebula or in some presolar environment. (4) The observation of large amounts of the oxidized carbon gases CO and CO2 in comet P/Halley at levels far exceeding those predicted by chemical equilibrium models of solar nebula carbon chemistry. In fact, oxidized carbon gases (CO+ C02 + H2CO) are the most abundant volatile (after water vapor) emitted by comet P/Halley. (5) The observation of HCN, which is not a predicted low temperature condensate in the solar nebula (e.g., Lewis 1972), in comet P/Halley (e.g., Schloerb et al. 1987) and in comet Kohoutek. (6) The observation of S2, which is argued to be a parent molecule vaporized from the nucleus, in comet IRAS-Araki-Alcock (1983d) by A'Hearn et aL (1983) and Feldman et al. (1984). This molecule is not an equilibrium condensate in the solar nebula and must result from disequilibrium chemistry. (7) The deduction that organic grains (C-H-O-N particles) comprise about 30% of the dust emitted by comet P/Halley and that about 75% of the total carbon inventory of Halley is in these grains also implies substantial disequilibrium chemistry. (8) The deductions that polyoxymethylene (polymerized formaldehyde or POM) is a constituent of the C- H-O-N particles emitted from comet P/Halley (e.g., Huebner 1987; Huebner et aL 1987; Mitchell et al. 1987). If actually present in the C-H-O-N particles, POM is also a product of disequilibrating processes which took place in the solar nebula and/or in a presolar environment. Taken together, the observations listed above indicate that a variety of disequilibrating processes such as the kinetic inhibition of thermochemical reactions, grain catalyzed chemistry, lightning induced shock chemistry, and photochemistry played an important role in establishing the chemistry and molecular composition of comet P/Halley in particular and presumably cometary material in general. However, the observational data do not by themselves constrain the timing and/or location of these various processes. This paper reviews the relevant observational data and attempts to quantify as far as possible by using current theoretical models and experimental data the relative importance of equilibrium and disequilibrium processes for the chemistry of comets. "Key" experimental and observational measurements which are important for better constraints on cometary origins are proposed. Finally, important measurements to be made by a comet nucleus sample return mission such as Rosetta are also suggested.
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