Chemistry Of Comets Research Articles

Examination of Fragment Species in the Comae of Several Comets Using an Integral Field Unit Spectrograph

Abstract Spectra of the comae of four comets were obtained with an integral field unit spectrograph on the Harlan J. Smith Telescope at McDonald Observatory. The passband of the spectrograph permitted the observation of C2, C3, CH, CN, and NH2 transitions for these comets. The classical Haser model was used to derive production rates for each observed species. The production rates obtained for the comets were also used to obtain mixing ratios relative to CN. The relative abundances with respect to CN obtained for these comets vary greatly, but are largely consistent with ranges established from prior comet chemistry surveys. The notable exception is 168P/Hergenrother, which the results suggest is extremely depleted in volatiles, even with respect to many other comets designated as volatile depleted in prior surveys. The results for comet Hergenrother add to a small, but growing, body of data suggesting that there may be a subgroup of carbon-chain-depleted Jupiter-family comets that are also depleted in ammonia.

Data-Driven Astrochemistry: One Step Further within the Origin of Life Puzzle.

Astrochemistry, meteoritics and chemical analytics represent a manifold scientific field, including various disciplines. In this review, clarifications on astrochemistry, comet chemistry, laboratory astrophysics and meteoritic research with respect to organic and metalorganic chemistry will be given. The seemingly large number of observed astrochemical molecules necessarily requires explanations on molecular complexity and chemical evolution, which will be discussed. Special emphasis should be placed on data-driven analytical methods including ultrahigh-resolving instruments and their interplay with quantum chemical computations. These methods enable remarkable insights into the complex chemical spaces that exist in meteorites and maximize the level of information on the huge astrochemical molecular diversity. In addition, they allow one to study even yet undescribed chemistry as the one involving organomagnesium compounds in meteorites. Both targeted and non-targeted analytical strategies will be explained and may touch upon epistemological problems. In addition, implications of (metal)organic matter toward prebiotic chemistry leading to the emergence of life will be discussed. The precise description of astrochemical organic and metalorganic matter as seeds for life and their interactions within various astrophysical environments may appear essential to further study questions regarding the emergence of life on a most fundamental level that is within the molecular world and its self-organization properties.

AN ANALYSIS OF CO PRODUCTION IN COMETARY COMAE: CONTRIBUTIONS FROM GAS-PHASE PHENOMENA

Understanding the sources of CO in cometary comae is important for understanding comet chemistry and the roles comets have played in the development of the solar system. Among comets sampled to date, the CO abundances vary widely and no direct correlation of CO abundance with other known comet properties has been identified. The picture is complicated further by the discovery of CO production in the comae of some comets, most notably comets Halley and Hale-Bopp. In this study, we investigate the conditions under which CO can be produced in the coma via gas-phase phenomena. We include photochemistry of several parent molecules, as well as two-body chemical reactions that involve the parents and their photodissociative daughter and granddaughter products. We also consider the level of excitation of 'hot' hydrogen (H*) and O({sup 1}D) in the network, because the level of excitation of these reactants strongly influences reaction rates. Our results suggest that the dominant gas-phase contributor to CO formation is the photodissociation of H{sub 2}CO. Even though typical abundances of H{sub 2}CO are at {approx}1% relative to water in the coma, it produces more CO than other processes due to its relatively short photodissociation lifetime. Because other studies have shown H{submore » 2}CO to have a distributed source as well, it suggests that at least some CO formation in the coma is connected to the H{sub 2}CO distributed source. We take the time to examine the CO{sub 2}/CO ratio and note that while the CO{sub 2}/CO ratio in comets Halley, Hale-Bopp, and Hyakutake are noticeably different when only native CO is considered, the CO{sub 2}/CO ratios show greater similarity when total CO is considered. Although this sample is relatively small, should the relatively similar CO{sub 2}/CO{sub Total} ratio of {approx}0.25 indeed be constant for comets with distributed CO sources, it suggests that the extended CO source of these comets is tied directly to the overall C, H, O chemistry of comets, as is likely to happen if hydrogenation of CO occurred on icy grains.« less

Astrochemistry: from astronomy to astrobiology

Preface. 1. The molecular universe. 1.1 The Standard Model Big Bang Theory. 1.2 Galaxies, stars and planets. 1.3 Origins of life. 1.4 Other intelligent life. 1.5 Theories of the origin of life. Concepts and calculations. 2. Starlight, galaxies and clusters. 2.1 Simple stellar models-black body radiation. 2.2 2.726 K-cosmic microwave background radiation. 2.3 Stellar classification. 2.4 Constellations. 2.5 Galaxies. 2.6 Cosmology. Concepts and calculations. Problems. 3. Atomic and molecular astronomy. 3.1 Spectroscopy and the structure of matter. 3.2 Line shape. 3.3 Telescopes. 3.4 Atomic spectroscopy. 3.5 Molecular astronomy. 3.6 Molecular masers. 3.7 Detection of hydrogen. 3.8 Diffuse interstellar bands. 3.9 Spectral mapping. Concepts and calculations. Problems. 4. Stellar chemistry. 4.1 Classes of stars. 4.2 Herzprung-Russell diagram. 4.3 Stellar evolution. 4.4 Stellar spectra. 4.5 Exotic stars. 4.6 Cycle of star formation. Concepts and calculations Problems. 5. The interstellar medium. 5.1 Mapping clouds of molecules. 5.2 Molecules in the interstellar and circumstellar medium. 5.3 Physical conditions in the interstellar medium. 5.4 Rates of chemical reactions. 5.5 Chemical reactions in the interstellar medium. 5.6 Photochemistry. 5.7 Charged particle chemistry. 5.8 Polycyclic aromatic hydrocarbons. 5.9 Dust grains. 5.10 Kinetic models of molecular clouds. 5.11 Prebiotic molecules in the interstellar medium. Concepts and calculations. Problems. 6. Meteorite and comet chemistry. 6.1 Formation of the solar system. 6.2 Classification of meteorites. 6.3 Meteorite mineralogy. 6.4 Geological time. 6.5 Chemical analysis of meteorites by L2MS. 6.6 The Murchison meteorite-kerogen. 6.7 Meteorite ALH84001. 6.8 Comet chemistry. 6.9 Structure of a comet. 6.10 Physicochemical conditions in a cometary coma. 6.11 Chemical composition of comets. 6.12 Cometary collisions. 6.13 The Rosetta mission-origin of the solar system. Concepts and calculations. Problems. 7. Planetary chemistry. 7.1 Structure of a star-planet system. 7.2 Surface gravity. 7.3 Formation of the Earth. 7.4 Earth-Moon system. 7.5 Geological time. 7.6 Radiative heating. 7.7 The habitable zone. 7.8 Extrasolar planets. 7.9 Planetary atmospheres. 7.10 Atmospheric photochemistry. 7.11 Biomarkers in the atmosphere. Concepts and calculations. Problems. 8. Prebiotic chemistry. 8.1 Carbon- and water-based life forms. 8.2 Spontaneous chemical reactions. 8.3 Rates of chemical reactions. 8.4 Endogenous production of organic molecules. 8.5 Exogenous delivery of organic molecules. 8.6 Homochirality. 8.7 Surface metabolism-'clay organisms'. 8.8 Geothermal vents-'black smokers'. 8.9 RNA World hypothesis. Concepts and calculations. Problems. 9. Primitive life forms. 9.1 Self-assembly and encapsulation. 9.2 Protocells. 9.3 Universal tree of life. 9.4 Astrobiology. 9.5 Microbial Mars. Concepts and calculations. Problems. 10. Titan. 10.1 Physical properties. 10.2 The atmosphere. 10.3 Temperature-dependent chemistry. 10.4 Energy balance and the greenhouse effect. 10.5 Atmospheric chemistry. 10.6 Astrobiology on Titan. Concepts and calculations. Problems. Glossary of terms and abbreviations. Appendix A: constants and units. Appendix B: astronomical data. Appendix C: thermodynamic properties of selected compounds. Answers to problems. Bibliography. Index.

Physics and chemistry of comets: recent results from comets Hyakutake and Hale–Bopp Answers to old questions and new enigmas

1996 and 1997 years were very prolific years for cometary studies. In March 1996, C/1996 B2 (Hyakutake), a medium-bright comet, passed at only 0.10 astronomical units (AU) from the Earth. On 1st April, 1997, C/1995 O1 (Hale–Bopp) passed at perihelion. This comet was 10 to 100 times brighter than comet Halley when compared at the same distance. It could be studied and monitored from its discovery in July 1995 as far as 7 AU from the Sun until its perihelion at 0.9 AU, where it was the object of extensive observations. It was thus possible to study these two exceptional comets with modern instrumentation covering the electromagnetic spectrum from radio to X-ray domains. Our knowledge of the composition and nature of comets has been transformed. New cometary molecules were observed, as well as isotopic species. New physical processes could be studied, owing to the large amounts of matter escaping from these objects, to the proximity of Earth allowing a detailed investigation of the region close to the nucleus (comet Hyakutake), or to the unusual physical conditions far from the Sun (comet Hale–Bopp). Selected results of these observations, relevant to the chemistry and physics of molecules and grains, are discussed. It is shown that whereas some old problems are now solved, new ones have emerged. The needs for laboratory data and theoretical work are pointed out.

Radiation Chemistry Approach to Cometary Chemistry

AbstractThe chemistry of comets is analyzed in the light of radiolytic processes occurring in the life-time of the comet due to its interaction with radiation. A simple model of a cometary nucleus is carried out at the laboratory which simulates as a first approximation the behavior of the organics in the comet. Most of the results obtained confirm that the parent molecules of the observed ones can be deduced from these simple simulated experiments. A variety of molecules were obtained (amino acids, carboxylic acids, gaseous products, etc.), even in frozen solutions the formation of oligomers was observed. The simplified mixture consisted of five compounds in a proportion in which they appear in a dense interstellar cloud (HCN/CH3OH/CH3CN/C2H5CN/HCOOH = 1:0.6:0.2:0.1:0.05). The total amount was adjusted to correspond to a C/N ratio 1.8. At this end, radiation chemistry can be a very precise and useful tool to simulate the evolution of organic molecules exposed to high energy radiation, as cosmic rays, during the life time of a comet.

Physics and chemistry of comets

Physics and chemistry of comets

Telling tails

Physics and Chemistry of Comets. Edited by W. F. Heubner. Springer: 1990. Pp.392. á£35, $59.

Structure of the Coma: Chemistry and Solar Wind Interaction

Much has been learned about the physics and chemistry of comets from the successful spacecraft encounters and intensive remote observing programs of Comets Halley and Giacobini-Zinner. Instead of being the panacea for our comet curiosity, these tantalizing “snapshots” have raised new questions, and many fundamental problems remain unsolved. To reap fuller benefits, extensive modeling is necessary to characterize the physical structure and chemical properties of the coma and to infer the composition and structure of the nucleus.

Physics and chemistry of comets

Physics and chemistry of comets

The chemistry of comets

In comets, most elements seem to be present in their cosmic abundances. This includes the metals whose abundances are the same as in chondrites, but also the light elements C, N, O, S that are the same as in the Sun; only hydrogen (and presumably helium and neon) is depleted by a factor close to 1000. In the bright comets of the 1970s, three-quarters of the cosmic abundance of carbon was found to be missing from the gaseous fraction. The missing carbon has now been found in Comet Halley: it was in the large organic fraction representing 33 % of the cometary dust. A part of this fraction vaporizes slowly out of the dust grains: it is the origin of an extended source of gas discovered around the nucleus of Comet Halley. Water remains the major constituent being 80% of the volatile ices. Formic acid, formaldehyde, carbon dioxide and carbon monoxide explain together more than 13% of the rest of the volatiles. The last 7% include the parent molecules of the radicals excited by fluorescence and observed in the traditional spectra, like hydrogen cyanide HCN (for CN), probably acetylene C2C2(for C2) and cyclopropadiene C3H 2 (for C3). The inorganic fraction of the dust contains mainly silicates and some iron sulphide, whereas the organic fraction also contains unsaturated hydrocarbons and probably hydrogen cyanide, acetonitrile, aminoethylene, pyrrole, pyridine, pyrimidine and possibly purines including adenine. Some prebiotic precursors of the nucleic bases are present, but no traces of any amino acids have been found.