Comets structure

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. 

The era of space travel has provided astronomers with a valuable new tool for the study of comet structure, composition, and behavior. During the two-year period 1985-86, no fewer than six spacecraft made flybys of comets. The first of these was NASA s ISEE-3 (International Sun-Earth. Explorer 3), which after completing its primary mission of studying the Sun was targeted to pass through the tail of Comet Giacobini-Zinner in September of 1985. At that point, it was renamed the International Comet E q)lorer (ICE). ICE also observed Comet Halley from a distance of 17 million miles (28 million km) in March 1986. 

Mission Deep Impact In July 2005, NASA steered a projectile, about 370 kg in weight, at the comet 9F/Tempel (dimensions 4x4x14km), in order to obtain more exact information on its structure and composition. The impact was visible from Earth the Rosetta spacecraft discussed above also sent pictures to Earth. The dust/ice ratio determined after the impact is very probably greater than unity, so that comets are probably icy dustballs rather than (as had previously been surmised) dirty snowballs . The density of the cometary nucleus, which seems to consist of porous material, is roughly equal to that of ice. The impact set free around 19 GJ of... 

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

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. 

Figure 4.1 The comet assay. A single-cell suspension is embedded in agarose on a slide. Cells are then subject to lysis followed by electrophoresis. If present, damaged DNA migrates out of the nucleoid structure during electrophoresis to producing a characteristic comet shape. Double-strand breaks are revealed under neutral conditions, whereas alkali conditions additionally show single-strand breaks and alkali labile sites. Image analysis of stained DNA is used to quantitate the amount of damaged DNA in the comet tail.

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

Gilmour, I. (2004) Structural and isotopic analysis of organic matter in carbonaceous chondrites. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 269-290. 

Fine structure on the silicate feature provides more specific mineralogical information. A small bump at 11.2 pm on the 10 pm feature (Fig. 12.4a), observed for several long-period comets, is generally interpreted as indicating crystalline olivine. Another shoulder at... 

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). 

To continue the thoughts that were interrupted by the comets as to why this unpredictability. Examining the nucleic acids from different places, similarity seemed extensive even between samples that came from widely separate spots. There are only four main components in addition to odd occasional nucleotides and that did not make for much variation, and the idea offered itself that life would eventually be read from any nucleic acid by cut and paste processes in line with energy minima. That might be the origin of exon/intron structures in eukaryotes.7... 

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 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)... 

Observations show that protoplanetary disks are dynamic, evolving objects, whose density and temperature structures change with time due to the transport of mass and angular momentum. Chondritic meteorites and comets record a dynamic history of our own solar nebula as they contain materials that formed in a wide variety... 

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).

---