Structure of a comet

Figure 6.16 Structure of the comet showing the nucleus, the coma and two tails - an ion tail and a dust tail...

The comet structure model proposed in Figure 6.16 shows clearly that the observation of molecules from Earth must be limited to those molecules present within the coma of the comet, and whilst they originate in part from the structure and composition of the nucleus the molecular observations are of the coma chemistry only. The coma observations will remain until we send a probe to land on the surface of a comet and report back the composition of the core. The Rosetta mission will do just this and we shall see the composition directly from the data it recovers, if successful. 

Figure 8.11 (a) Possible structure of single ice particle and (b) a model of a piece of a comet consisting of an aggregate of ice particles [6, 33, 34], (Reprinted from Greenberg [34], with permission from Elsevier)... 

Meteor showers occur when Earth passes through the tube-like structure of meteoroids left in the wake of a comet. Such meteoroid tubes, or as they are more commonly called meteoroid streams, are formed after a comet has made many repeated passages by the Sun. Meteoroid streams are composed of silicate (i.e. rocky) grains that were once embedded in the surface ices of a parent comet. Grains are released from a cometary nucleus whenever solar heating causes the surface ices to sublimate. New grains are injected into the meteoroid stream each time the comet passes close by the Sun. 

Herzberg contributed to the field of atomic and molecular spectroscopy, where he and his colleagues determined the structures of a large number of diatomic and polyatomic molecules, the structures of free radicals, and the identification of certain molecules in planetary atmospheres, in comets, and in interstellar space. In 1971 he was awarded the Nobel Prize in chemistry for his contributions to the knowledge of electronic structure and geometry of molecules, particularly free radicals. ... 

Kurt Varmuza was bom in 1942 in Vienna, Austria. He studied chemistry at the Vienna University of Technology, Austria, where he wrote his doctoral thesis on mass spectrometry and his habilitation, which was devoted to the field of chemometrics. His research activities include applications of chemometric methods for spectra-structure relationships in mass spectrometry and infrared spectroscopy, for structure-property relationships, and in computer chemistry, archaeometry (especially with the Tyrolean Iceman), chemical engineering, botany, and cosmo chemistry (mission to a comet). Since 1992, he has been working as a professor at the Vienna University of Technology, currently at the Institute of Chemical Engineering. 

Spectra of comet Hale-Bopp, showing features attributable to silicate minerals, (a) Profile of fine structure in the 10 silicate emission feature a peak at 11.2 and a shoulder at 11.9 are due to olivine, and a slope change at 9.2 results from pyroxene, (b) Expanded infrared spectrum exhibiting a number of sharp peaks due to magnesian olivine and pyroxene. The region of (a) is bounded by a small box. Modified from Crovisier et al. (2000) and Hanner and Bradley (2003). 

Brownlee, D. E. (2004) Comets. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 663-688. A comprehensive and thoughtful review of what is known about the composition and structure of comets and the bodies within the belts that supply comets. 

Given that interstellar ices are the building blocks of comets and comets are thought to be an important source of the species that fell on primitive Earth, the structures of molecules in comets may be related to the origin of life. It is possible that organic materials formed in the solid ice phase of interstellar materials provided raw materials used for life originating solely on Earth. If so, the deep freeze of ice in the Oort cloud would have been an excellent place to store these, especially the unstable ones, awaiting delivery to a planet. 

Figure 6.5 Comparison of the 10 pm Si-O stretching bands of a GEMS-rich IDP and astronomical silicates. (A) Chondritic IDP L2008V42A. Profile derived from transmittance spectrum. (B) Comet Halley (Campins Ryan 1989). (C) Comet Hale-Bopp (Hayward et al. 2000). (D) Late-stage Herbig Ae/Be star HD 163296 (Sitko et al. 1999). The structure at 9.5 qm in (B), (C), and (D) is due to telluric O3. Figure from Bradley et al. (1999).

Fine structure on the silicate feamre was first seen in observations of comet Halley. It consisted of a small 11.2 pm bump on the silicate feature detected in high spectral resolution and good signal-to-noise ratio observations. This small bump has now been seen on the silicate feature of several comets and it is widely interpreted as evidence for the presence of olivine because IR studies of powdered olivine samples show fine stmcmre at 11.2 pm. In the astronomical literature this feature is considered to be crystalline olivine as compared to amorphous silicates that cannot produce the pronounced 11.2 pm bump on the overall 10 pm silicate feamre. 

Oort J. H. (1950) The structure of the cloud of comets surrounding the solar system and a hypothesis concerning its origin. Bull. Astron. Inst. Neth. 11, 91-110. 

There is a poor knowledge of the initial structure of cold ice deposited under vacuum, and, with greater reason, after exposure to radiations. Such cold ice is a dominant component in several astrophysical environments (comets, planetary satellites, and interstellar grains) and its properties are important for the understanding of processes such as molecule formation in the interstellar space and comet outgassing. 

The diamond structure (solution to Exercise 20.17(a)) is a very open structure, which is dictated by the tetrahedral bonding of the carbon atoms. As a result many atoms that would be touching each other in a normal fee structure do not in diamond for example, the C atom in the center of a face does not toudi the C atoms at the comets of the face. 

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