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Exploring Organic Environments in the Solar System II —The Formation, Modification, and Preservation of Organic Compounds in the Solar System The search for understanding of how organic environments originated on the early Earth and throughout the universe and their association with life processes is the ultimate interdisciplinary field. Subdisciplines of the Earth sciences, biology, chemistry, astronomy, and the space sciences are all needed to contribute to the contemporary understanding of this complex problem. The search for organics in the solar system involves a series of interrelated questions. These include where organic reservoirs are located in the solar system,1 what specific organic compounds are present in these reservoirs, and what the location and identification of these organic compounds can tell us about both the evolution of the solar system and the possible presence of life at locations other than on Earth. The discussion in Part II serves as an overview of all known inventories of organic compounds in the solar system, the possible means by which they were formed, and also—based on current observations—locations, not yet examined in detail, where organic compounds may be present. Recommendations for further exploration take into consideration the likelihood that significant organics will be found, the ease with which they can be found, and the anticipated amount and significance of data that can be accumulated from a particular research activity or spacecraft mission. All the carbon in the universe is made by fusion reactions in stars. Carbon-12 (12C) is created by the fusion of three helium-4 (4He) nuclei. Carbon-13 (13C) is made late in the lives of red giant stars, where formation of helium, catalyzed by 12C, results in the formation of 13C, 14N, and 15O via the CNO cycle outlined below: 1 For the purposes of this report, the interstellar medium is included in this search, both because Earth-based observations can be used to identify organics present there, and because it is thought that the interstellar medium was a source for organics present in the solar system’s protoplanetary disk.
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Exploring Organic Environments in the Solar System where e+, γ, and ν are a positron, a gamma ray, and a neutrino, respectively. The 12C/13C ratio resulting from the CNO cycle is in the range of 15 to 20. The CNO cycle proceeds in the shells of the red giant stars that contain high levels of carbon, where the conversion of 12C to 13C, and to 14N and 15O, proceeds after the bulk of the hydrogen has been converted to He. The pulsation of the red giant during this energetic process disperses the elements formed into the interstellar medium, where they serve as the starting materials for the formation of a new star. The first organic compounds were formed from the carbon injected into the interstellar medium under the influence of cosmic rays and ultraviolet light. Simple hydrocarbons and other compounds that contain nitrogen, oxygen, and sulfur were formed in this cloud of dust and molecules. This process proceeded for about 107 years, producing additional organics before the dust cloud collapsed to form stars and their associated planetary systems. In the solar system the evolution of carbon compounds proceeded during planetary system formation. The existing compounds were subjected to the shock waves resulting from the collapse of the dust cloud to stars and protoplanetary disks. The intense ultraviolet and x-rays emitted by the new star effected changes in some of the organics. Carbon compounds ultimately derived from the interstellar medium were accreted onto planetesimals in the early solar system, where considerable thermal and aqueous modification may have occurred. These planetesimals then aggregated to form planets, a process that further modified some of their organic constituents. The organics present in the atmospheres of the newly formed planets were subjected to solar ultraviolet radiation as well. Organics on and below the surface of planets were further changed by energy sources including heat from volcanoes, heating by transport into planetary interiors where they were subjected to heat and pressure, contact with hydrothermal systems that initiated reactions with water at high temperatures and pressure, and reduction by minerals. Volcanoes also injected volatile organics into the atmosphere where solar ultraviolet radiation and x-rays changed them.
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Exploring Organic Environments in the Solar System 2 Interstellar Chemistry THE INTERSTELLAR MEDIUM Inventory of Organic Compounds in the Interstellar Medium Some 20 to 30 percent of the mass of our galaxy is in the form of the interstellar medium (ISM), i.e., the material between the stars. The ISM consists primarily of gas, with atomic or molecular hydrogen and helium contributing approximately two-thirds and one-third of the total mass, respectively. The next most abundant atoms, oxygen, carbon, and nitrogen, collectively account for about 1 percent of the ISM’s mass. The remaining elements are present only in trace amounts. Approximately 1 percent of the mass of the ISM is in the form of micron-size dust particles. Astronomical observations, combined with studies of interstellar grains preserved in meteorites, suggest that the dust might consist variously of amorphous carbon, complex fullerenes, polycyclic aromatic hydrocarbons, diamond, silicon carbide, silicates, carbonates, and a host of other candidates, all with or without mantles of ices and/or organic compounds.1 Important components of the ISM are molecular clouds, which are dense, massive objects found throughout the Milky Way and in many external galaxies. In these molecular clouds—also known as dense clouds—the gas density is 103 to 106 particles/cm3, which is very high by interstellar standards, and their masses can be as large as a million times the mass of the Sun. They are also usually very cold objects, with temperatures typically in the range from 10 to 100 K. Because of their high masses, these objects are the sites of star and planet formation, and also where complex gas-phase chemistry occurs. Astronomical observations of the ISM have revealed the presence of numerous organic compounds. More than 125 different chemical species have been identified in interstellar and circumstellar regions, some containing 10 or more carbon atoms (Table 2.1). Assuming that the carbon in the ISM is present in cosmic abundance, then only 0.04 percent (by number) of the material there is carbon, even though approximately 80 percent of the observed species in the ISM are organic, including almost all of the larger molecules, many of which are relatively complex.2 Organic compounds are, however, only a trace constituent of the ISM and account for less than 1 percent of its total mass. Inorganic compounds abound, with CO, for example, accounting for some 20 percent of the carbon in dense interstellar clouds. CO is, itself, outnumbered by the most common molecular species, H2, by a factor of approximately 10,000. The majority of the molecular species identified in the ISM have been discovered using high-resolution (1 part in 106 to 108) spectroscopic techniques of radio and millimeter astronomy. This
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Exploring Organic Environments in the Solar System TABLE 2.1 Known Interstellar and Circumstellar Molecules Number of Atomsa 2 3 4 5 6 7 8 9 10 11 12 13 H2 C3 c-C3H C5 C5H C6H CH3C3N CH3C4H CH3C5N? HC9N CH3OC2H5 HC11N AlF C2H l-C3H C4H l-H2C4 CH2CHCN HCOOCH3 CH3CH2CN (CH3)2CO AlCl C2O C3N C4Si C2H4 CH3C2H CH3COOH? (CH3)2O NH2CH2COOH? C2 C2S C3O l-C3H2 CH3CN HC5N C7H CH3CH2OH CH3CH2CHO CH CH2 C3S c-C3H2 CH3NC HCOCH3 H2C6 HC7N CH+ HCN C2H2 CH2CN CH3OH NH2CH3 CH2OHCHO C8H CN HCO CH2D+? CH4 CH3SH c-C2H4O CH2CHCHO CO HCO+ HCCN HC3N HC3NH+ CH2CHOH CO+ HCS+ HCNH+ HC2NC HC2CHO CP HOC+ HNCO HCOOH NH2CHO CSi H2O HNCS H2CHN C5N HCl H2S HOCO+ H2C2O HC4N KCl HNC H2CO H2NCN NH HNO H2CN HNC3 NO MgCN H2CS SiH4 NS MgNC H3O+ H2COH+ NaCl N2H+ NH3 OH N2O SiC3 PN NaCN C4 SO OCS SO+ SO2 SiN c-SiC2 SiO CO2 SiS NH2 CS H3+ HF SiCN SH AlNC FeO(?) SiNC NOTE: The observations are of molecular emission by high-resolution spectroscopy with lines having aquality factor of 1 part in 106 to 108. The identification of these molecules has been made on the basis of their pure rotational, rovibrational, or electronic spectra, which occur in the radio/millimeter, infrared, and optical/ultraviolet regions of the electromagnetic spectrum, respectively. The species are a composite of data obtained from avariety of astronomical sources, including comets, several dense interstellar clouds, and circumstellar envelopes. aAlower-case “c” indicates a cyclic structure; alower-case “1,” a linear structure; and a “?,” a tentative identification. SOURCE: Courtesy of H. Alwyn Wootten, National Radio Astronomy Observatory, available at http://www.cv.nrao.edu/~awootten/allmols.html, last accessed January 22, 2007.
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Exploring Organic Environments in the Solar System triumph of radio astronomy has changed our perception of the universe as being predominantly a rarified atomic environment to one containing a large (organic) molecular component. The Synthesis of Interstellar Molecules The observed interstellar species listed in Table 2.1 are not those expected based on conditions of thermodynamic equilibrium. One indication of large deviations from equilibrium are the relatively large abundances of high-energy isomeric forms of species. For example, both HCN and its high-energy isomer HNC are observed with relative abundances such that [HNC]/[HCN] approaches or exceeds unity in some molecular clouds. At a canonical temperature of 20 K, the equilibrium abundance ratio is expected to be many orders of magnitude less than 1. The presence of many reactive free radicals and molecular ions in interstellar gas also indicates non-equilibrium conditions. Further evidence that interstellar molecular abundances are not controlled by thermodynamics is found by consideration of the well-known reaction At 20 K, the equilibrium constant for this process is calculated to be 10500 molecules−2 cm6. Given a typical interstellar gas density of 105 particles cm−3, a complete conversion of CO to CH4 should occur in molecular clouds if chemical equilibrium prevailed. In contrast, CO is the second most abundant interstellar gas-phase molecule, so abundant, in fact, that it is used to map the distribution of molecular clouds in our galaxy and in external galaxies. Closely related to the high abundance of interstellar CO is the presence of polycarbon molecular species in unsaturated forms, in particular the long polyacetylene chains. These phenomena can all be explained by considering a chemical environment that is kinetically controlled, as opposed to thermodynamically controlled. The low-temperature, low-density conditions present in molecular clouds in fact favor a chemistry governed by kinetic effects. The types of chemical reactions that can occur in interstellar clouds are limited by the physical environment in these objects. Although these regions are dense by interstellar standards, they are extremely rarified in comparison with conditions that can be obtained in terrestrial laboratories. This low density limits any chemical reaction, restricting it to a two-body process, whereas most reactions in the laboratory involve three bodies. These reactions have negligible activation energies because of the strong attraction between the positively charged ion and the neutral molecule. The energy released in the association of these two molecules drives the reaction at the low temperatures of the ISM. Processes with activation barriers generally will not occur within the lifetime of a molecular cloud (typically a million years). One type of chemical reaction that fulfills interstellar criteria (i.e., two-body process, low activation energy barrier) are positive ion-molecule reactions of the general form Because of the attractive force between a positive ion and a neutral species, these processes generally lack significant activation energies and have relatively fast rates, despite the fact that a third body is not participating to stabilize the products. The ion-molecule rate is also usually independent of temperature. For ion-molecule reactions to occur, however, positive molecular ions must be present initially. The bulk volume of the dense clouds is not penetrated by starlight; thus, the energy source for generating such ions is high-energy cosmic rays. Because the bulk composition of any given molecular cloud will be molecular hydrogen and atomic helium, cosmic-ray (100-MeV)-induced ionization produces primarily H2+ and He+ cations. The secondary reactions then proceed via ion-molecule processes. One important process is the reaction of H2+ with H2, which occurs at the typical ion-molecule rate of k = 2 × 10−9 cm3s−1 molecule−1:
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Exploring Organic Environments in the Solar System The reaction of He+ with H2, in contrast, is too slow to be significant. If it were faster, it would immediately destroy the highly reactive He+ and therefore essentially quench the formation of organic molecules. CO, which is formed by ion-molecule reactions in molecular clouds, is present in relation to molecular hydrogen at a ratio of about 10–4. Reactions with CO are also important in the ion-molecule scheme. CO reacts rapidly with H3+ and He+: The product of the first reaction, HCO+, is extremely stable and is a major ion in dense molecular clouds. It has been used as a tracer of ionization and of course ion-molecule chemistry, given that, until very recently, H3+ had not been observed. (The detection of H3+ was eventually accomplished by infrared absorption spectroscopy, and the observed abundance in dense and diffuse clouds is in excellent agreement with theoretical calculations.) The second reaction is the primary basis for the very rich organic chemistry observed in interstellar clouds, because it leads to the production of C+. In this process, the He+ formed by the cosmic-ray ionization of He generates a C+ in a process 103 times faster than the direct cosmic-ray ionization of CO. This result is a direct consequence of the lack of reactivity of He+ with H2. C+ is an important product because it can insert itself into other carbon-containing molecules to increase carbon chain length. For example, the typical synthesis for building larger organic molecules involves the reaction of hydrocarbon radicals such that: followed by the addition of two hydrogens, leading to Cn+1H3+. The neutral species is finally generated by dissociative electron recombination or by proton transfer to a suitable base. Such carbon insertion reactions most likely lead to the wide variety of carbon chains found in interstellar gas. Ion-molecule radiative association becomes increasingly efficient with increasing molecular size. This type of process may also lead to the larger organic species. For example, methanol is thought to be created via the radiative association process followed by dissociative electron recombination: Other ion-molecule reactions can create even larger compounds. Methyl formate is synthesized from The neutral species is then produced again by proton transfer to a suitable base or by dissociative recombination with an electron, creating HCOOCH3 and H. The main point is that, in principle, gas-phase ion-molecule reactions can create organic molecules in interstellar gas. Extensive chemical modeling of ion-molecule chemistry has been carried out and has been relatively successful in reproducing the abundances observed in interstellar space. It is not known, however, what degree of molecular complexity can be achieved through such reactions. This uncertainty remains one of the open questions for astrochemistry. Although the bulk of the reactions in the ISM are initiated by cosmic rays or ultraviolet light, some reactions are believed to be initiated by neutral free radicals. For example, the amount of cyanoacetylene (HC3N) present is modeled more accurately by the addition of a cyano radical (CN.) to acetylene than by ion-molecule reactions: where the symbol . indicates a free radical.
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Exploring Organic Environments in the Solar System It is worth noting that ion-molecule reactions are believed to be a route by which significant deuterium, and some 13C, enhancement occurs in organic molecules in the ISM. This fractionation effect arises from small differences in the molecular binding energies.3 For example, the ratio of deuterated isotopomers to their normal counterparts may be enhanced by up to four orders of magnitude compared to elemental deuterium/hydrogen abundances in the ISM.4 Surface Reactions in the Interstellar Medium Reactions of compounds on dust grains are another source of organic compounds in the ISM. The dust grains are produced in the circumstellar shells of red giant stars that condense from the hot material (mainly silicates) emitted from the stellar surface. Carbon stars eject carbon and partially hydrogenated carbon into the ISM by this route. Stellar ejecta from supernovas are also believed to be a source of grains. A mantle of ice consisting of water, CO, CO2, and organics condenses on these grains at the low temperatures (~10 K) present in the dense clouds (Table 2.2). Ultraviolet light in the ISM initiates reactions that lead to the formation of more complex structures from the simpler compounds in the mantle on the grains. One source of the ultraviolet is the radiation emitted by molecular hydrogen, following collisional excitation by electrons produced by cosmic-ray ionization.5 The 100- to 200-nm-wavelength light has an average flux of 103 photons cm−2s−1 that is about 10−5 that of the ultraviolet flux in the diffuse ISM. Stars are a second source of ultraviolet light that is impinging on the dust in the outer regions of the dark ISM. A third source is the ultraviolet emissions from young stellar objects (newly formed stars) in the dark ISM that irradiate the dust in their vicinity. The radiation not only initiates chemical reactions but also causes the evaporation of the icy mantles from the dust grains. The ultraviolet processing proceeds by dissociating the molecules in the mantle into free radicals. These reaction intermediates are stable at 10 K, but if the grain is warmed by absorption of additional ultraviolet photons, the radicals move around in the mantle and react with the other molecules present. If the reaction is exothermic, the TABLE 2.2 An Inventory of Interstellar Ices Based on Infrared Spectroscopy Young Stellar Objects Species Dark Cloud Low Mass High Mass H2O 100 100 100 NH3 ≤10 ≤8 2-15 CH4 — <2 2 CO 25 0-60 0-25 CO2 21 20-30 10-35 CH3OH <3 ≤5 3-30 H2CO — <2 2-6: HCOOH — <1 2-6: XCN <1 0-2 0-6 OCS <0.2 <0.5 0.2 NOTE: Abundances are expressed as percentages of the H2O abundance for three categories of sight-line. A range of values generally indicates real spatial variation; where followed by a colon, it may merely reflect observational uncertainty. Values for XCN, an unidentified molecule containing C≡N bonds, are based on an assumed band strength (D.C.B. Whittet, P.A. Gerakines, J.H. Hough, and S.S. Shenoy, “Interstellar Extinction and Polarization in the Taurus Dark Clouds: The Optical Properties of Dust Near the Diffuse/Dense Cloud Interface,” Astrophysical Journal 547(1): 872-884, 2001). A dash indicates that no data are currently available. SOURCE: After D.C.B. Whittet, Dust in the Galactic Environment, 2nd Edition, Series in Astronomy and Astrophysics, Institute of Physics Publishing, London, U.K., 1992.
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Exploring Organic Environments in the Solar System energy released may evaporate the grain mantle, thus ejecting the compounds present into space. Mantles may also be heated and evaporated by collisions with other grains and the shock waves from supernovas. The presence of silicates in the grains together with water, CO, and CO2 in their mantles, with smaller amounts of methanol, formaldehyde, formic acid, and methane, has been detected by infrared spectral studies.6,7 The C-H stretching frequency of organics is visible in absorption bands at 3.4 µm characteristic of CH3 and CH2 groups. The similarities between the aliphatic C-H stretch region as seen in spectra of dust clouds in our own galaxy and as seen in the spectra of more distant galaxies suggest that the organic component of the dust in the ISM is widespread and may be an important universal residue of abiotic carbon. The formation of molecular hydrogen from hydrogen atoms cannot occur by binary gas-phase reactions. However, hydrogen atoms have high surface mobility on the grain mantle and can readily combine on a surface to create H2. The energy released in the process of H2 formation is sufficient to desorb the molecule from the grain surface, even at 10 K. The reduction of CO to formaldehyde and methanol is unlikely to occur by the action of cosmic rays because of the high activation energy required for the reactions.8 In addition, some researchers have suggested that the amount of methanol in the grain mantles (5 to 10 percent of the water present) is much greater than that in the gas phase, suggesting that the mantle methanol was formed in the solid phase,9,10 a claim that is in conflict with the previously proposed synthesis of methanol in the gas phase by radiative association (see above). It is likely that methanol is formed by both processes. It is possible that the reduction of CO in the mantle by hydrogen atoms is the source of the formaldehyde and methanol. The reaction with hydrogen atoms has no activation energy because hydrogen atoms can tunnel through the activation barriers on the mantle surface. Extensive laboratory studies have shown that the ultraviolet irradiation of simulated grain mantles results in the generation of more complex organics.11-14 Unfortunately, the laboratory studies are by necessity carried out with a high ultraviolet flux and thus are not representative of interstellar conditions. Since the precise composition of the grain mantles is not known, it is not possible to accurately extrapolate from the laboratory simulations to the amounts of these compounds in the dark ISM. For example, the higher yields of amino acids formed in the experiments of Munoz Caro et al.15 probably reflect the use of a 10-fold lower ratio of water to the other reactants than was used in the comparable study by Bernstein et al.16 It is difficult to compare the extent of formation of organics in the ISM by comparison of the products formed by cosmic rays and ultraviolet. The bulk of the compounds listed in Table 2.1 were formed in gas-phase reactions driven by cosmic rays. The presence of these compounds in the ISM was determined by high-resolution radio astronomy, a technique that is very sensitive and also makes it possible to determine the structures of the compounds. Infrared spectroscopy is much less sensitive than radio astronomy and is a technique that provides information about the functional groups in the organics and not an exact structure when a mixture of compounds is present. The different characteristics of the two spectral measurements make it difficult to compare the amounts and the diversity of compounds formed. A large number of modeling programs exist to predict gas-phase reactions and molecular abundances. The extension of the general chemical modeling programs to surface chemistry faces a number of problems and uncertainties not encountered with gas-phase binary reactions. The variables in the kinetic models are generally the densities of the reactants in the gas phase. The incorporation of surface reactions with known gas-phase reactions into a master reaction scheme presents some significant difficulties. There is a large asymmetry in the fundamental understanding of binary (gas-phase) encounters and processes on surfaces. Several problems require experimental and theoretical resolution before a quantitative model, such as discussed for the gas-phase chemistry ion-molecule chemistry, will be obtained for surface reactions. The problems include the following: The size and surface area of the grains as well as the chemical composition are not well characterized. It is difficult to model a surface of unknown composition. The exact mechanisms for reactions on surfaces are not well characterized. Also, it is difficult to find desorption processes for reaction products that are effective at low temperatures (10 K). The usual gas-phase reaction rate theory is not applicable to gas–surface reactions. Probability theory must be applied, and hence there are no exact solutions.
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Exploring Organic Environments in the Solar System Broad Interstellar Features and the Organic Inventory Broad emission features have been routinely observed in molecular clouds at infrared wavelengths, using spectroscopic techniques. The origin of these features, known as unidentified infrared bands, are most likely emissions from polycyclic aromatic hydrocarbons (PAHs). They occur at wavelengths that are suggestive of both the aliphatic and aromatic C-H stretching frequencies, as well as C-H deformation modes. These data indicate that organic material is present in interstellar gas that consists of large unsaturated hydrocarbons. Indeed, as already mentioned in the previous section, the spectral similarities between the aliphatic C-H stretch feature seen in interstellar dust in our galaxy and corresponding features seen in the spectra of more distant galaxies suggest that the organic component of the dust may represent an important universal residue of abiotic carbon.17 So-called diffuse interstellar bands—i.e., spectral features arising due to the absorption of visible light—are also observed. Hundreds of these bands have been observed, but none have been assigned to specific compounds. Currently PAHs appear to be the most likely structures absorbing the visible light, but this assignment remains to be verified. Carbon stars are the sources proposed for the presence of PAHs in the ISM. The emission spectra of interstellar dust clouds indicate that PAHs are widespread but contribute only 5 to 10 percent of the total carbon. They have been found in interstellar dust grains, in unequilibrated chondrites, and in the martian meteorite ALH84001. Proposed Research on Organics in the Interstellar Medium Ion-molecule reactions are the fastest gas-phase processes known. Their properties make them prime candidates for producing interstellar molecules. Consequently, understanding these reactions is essential for evaluating the chemistry of the ISM, especially considering that this is a low-temperature environment. Reaction rates are not known for many ion-molecule, radiative association, and even certain neutral-neutral reactions that involve rather abundant interstellar carbon-bearing species. Nor are many of the branching ratios known for the products of dissociative electron recombination reactions, the main mechanism by which neutral organic species are produced. How material formed initially in dense, interstellar molecular clouds and evolved through star formation and subsequent nebular condensation is at present highly speculative. Reaction rates should be measured experimentally in the laboratory, especially at low temperatures. Also, theoretical calculations of reaction rates and reaction potential surfaces would be helpful for those processes that are too difficult to be determined by experiment, or for comparison with the experimental results. These data will enable models of interstellar chemistry to be more accurate in the calculation of abundances and in the prediction of possible new organic species. Such data will also help elucidate the major reaction pathways for the production of carbon-bearing molecules. In the laboratory, high-resolution infrared spectral measurements, including pure vibrational and rovibrational studies, of possible organic molecules will suggest other possible interstellar organics. Investigations are needed of carbon-bearing radicals and ions that might function as reaction intermediates in interstellar processes. The data obtained will enable astronomers to study additional carbon-bearing compounds in the ISM and therefore complete the inventory of organic material outside the solar system. The laboratory investigations thus should be followed up with the appropriate astronomical studies, using available telescope facilities, both ground-based and future air- and space-borne platforms such as the Stratospheric Observatory for Infrared Astronomy (SOFIA) and Herschel, as well as the Spitzer Space Telescope. Laboratory work includes both gas-phase and solid-state experiments. Astronomers additionally need to establish more complete databases for organic compounds in interstellar objects. Currently, molecular abundances are known for only a small subset of sources, and often only one such object. Hence, it is currently impossible to evaluate the diversity of organics in the ISM. Systematic observations of the key organic compounds in a statistical sample of molecular sources will be helpful in this regard. PROTOPLANETARY DISKS The early evolution of a young stellar object proceeds with rapid and dramatic changes.18 Stars begin their lives in molecular clouds. As the cloud starts to fragment and collapse, a dense opaque protostellar core forms,
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Exploring Organic Environments in the Solar System typically a few thousand to 10,000 astronomical units (AU) across, and this core falls inward, supplying material (dust, gas, ices—including a rich array of organic molecules) to a central star. Because it is difficult to remove angular momentum from the gas during infall, the material accretes onto a rotationally supported disk surrounding the protostar. Dozens of these protoplanetary disks have been observed. This main accretion phase is often simultaneously accompanied by prominent outflows of material (jets). When the star has accreted approximately 90 percent of its final mass, it will become a pre-main sequence star, just below the mass/temperature limit for hydrogen fusion. The disks can be up to a few hundred AU across with low densities (106 particles cm−3) in the outer region and with densities increasing to 109 particles cm−3 near 100 AU. Temperatures remain low (~10 K) at these distances but increase close to the central protostar. The evolution of the core and formation of the disk around the star are only broadly understood and not yet well constrained by observations. The evolution of high-mass stellar systems and low-mass systems proceeds somewhat differently. Less is known about high-mass star formation because most of the formation phase occurs while the star is embedded in an optically thick cloud of material and is therefore unobservable. One epoch in the formation of high-mass stars that has been observed is the so-called hot-core phase. In this transition phase before a hot massive young stellar object ionizes its surroundings, the object just begins to heat the surrounding neutral gases and can vaporize grain mantles. A hot-core phase can also occur during the formation of low-mass stars like the Sun, if these stars are formed in the proximity of a massive star. The radiation from the massive star will vaporize the icy grain mantles of the small protostar and generate a hot core. The volatiles released from the hot core of a small or massive star will be subjected to the ultraviolet radiation that drives the formation of more complex organics from the volatiles released from the icy mantles. The study of the chemical processes taking place in protoplanetary disks is limited by the angular resolution of submillimeter instrumentation, although the construction of larger submillimeter telescopes and more sensitive arrays, such as Atacama Large Millimeter Array (ALMA) in Chile, will help tremendously at these wavelengths. In addition, the Spitzer Space Telescope, SOFIA, and the James Webb Space Telescope will help in the mid and far-infrared. Numerical simulations and chemical models are able, in combination with observations, to help examine the chemistry in the disks, although there may be differences in the chemical processes for high- and low-mass objects. The chemistry and chemical processes in young stellar objects may be considered in several different regimes as shown in Table 2.3. TABLE 2.3 The Chemistry and Chemical Processes in Young Stellar Objects Components of the Protoplanetary Disk Molecules Detected in Millimeter/Submillimeter Wavelength Region Molecules in the Infrared Wavelength Region Principal Chemical Processes Believed to Occur Formation Stage of the Protoplanetary Disk Dense cloud Molecular ions, carbon chains HC3N, CH3OH, SO, SO2 Simple ices H2O, CO2, CO, CH3OH, HCOOH, H2CO Low-temperature chemistry Ion-molecule reactions Cloud fragmentation collapse to protostar Cold envelope around protostar Simple species H2CO Ices H2O, CO2, CH3OH Low-temperature chemistry Grain surface reactions Enters main accretion phase (class 0) Inner warm envelope High-excitation temperatures High gas/solid ratios C2H2 Sublimation, gas-phase reactions Protostar and disk accretion (class I) Outflows (T-Tauri) Ions/radicals CN, CCH, CO+ Atomic and ionic lines CO Shocks, sputtering, photodissociation, ionization Protostar accretion, bipolar outflows, envelope dissipation (class II) SOURCE: Data from E.F. Van Dishoeck and F.F.S. van der Tak, “Chemistry in Envelopes Around Massive Young Stars,” pp. 97-112 in Astrochemistry: From Molecular Clouds to Planetary Systems, IAU Symposium 197 (Y.C. Minh and E.F. van Dishoeck, eds.), Astronomical Society of the Pacific, San Francisco, Calif., 2000.
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Exploring Organic Environments in the Solar System Low-Temperature Chemical Processes in Protoplanetary Disks In the cold regions of the precursor molecular cloud, the chemistry is dominated by ion-molecule reactions, resulting in small radicals and unsaturated molecules. As the disk warms up, molecules are released from the grains via sublimation, and this initiates gas-phase reactions, which can produce molecules such as H2CO, C2H2, CH3OH, and others.19 These processes can lead to deuterium fractionation in the disk, so that the high deuterium/hydrogen ratios seen in comets may not necessarily imply preservation of interstellar material. Numerical simulations and chemical models will benefit greatly from high spectral and spatial observations made from new interferometric millimeter arrays (e.g., ALMA). This new observing tool will enable an understanding of potential chemical gradients in the disks in planet-forming zones. It should be noted that, once beyond the cold-core stage, the chemistry of the protostellar disk cannot be understood outside the context of a specific dynamical disk model. High-Energy Processes in Protoplanetary Disks Dynamic magnetic fields in young stellar objects can lead to violent reconnection phenomena (analogous to solar flares) that accelerate particles to high energies (MeV to GeV; i.e., similar to cosmic-ray energies), which can heat gases to x-ray temperatures. This process has been observationally detected from satellites with x-ray detectors and from nonthermal radio continuum radiation. In addition to heating, the x-rays cause ionization and excitation of molecules. The secondary electrons from the ionization can produce molecular ions such as H3+ and HeH+. Millimeter emissions from CO, HCN, CN, and HCO+ have been seen around young stellar objects and attributed to x-ray-induced chemistry. The x-ray ionization dominates the ionization caused by external cosmic rays out to about 100 to 1000 AU. High-spatial-resolution spectra can distinguish the mechanism inducing the chemistry because the cosmic-ray and x-ray ionizations operate on different spatial scales in the disks. In addition, x-ray irradiation of disk dust grains, which may contain a variety of carbonaceous compounds (such as PAHs, aliphatic hydrocarbons, and so on) can result in dehydrogenation and breaking of aromatic rings. External cosmic-ray irradiation does not play much of a role in the dense inner disk, but beyond 10 AU, where the density becomes low enough that the rays are not completely attenuated, cosmic-ray irradiation produces H3+ and He+ ions that can convert CO and H2 to CO2, CH4, NH3, and HCN. Shock Waves in Protoplanetary Disks Shock waves occur in protoplanetary disks where the outflow from the protostar collides with the surrounding cloud material, and there are also accretion shocks from the material infalling onto the disk. The accretion shocks occur where the material rains down on the disk at speeds greater than the local speed of sound. The shock waves compress and heat the gas and can therefore affect the chemistry in the disk. In high-speed shocks (producing abrupt discontinuities in the conditions in the gases), temperatures can reach 104 to 105 K, and molecules will dissociate. These can reform in the warm wake of the shock. Ices can recondense as an amorphous solid on cold grains, and this process will result in enhanced volatile trapping in the ices. Thus a mixture of unaltered and modified grains in the disk can result. In lower-speed shocks, temperatures are not as high (~103 K), and endothermic reactions can produce new species not usually seen in the ambient medium. In addition, in the low-velocity shocks, ices can be removed from grain mantles as grains are driven through the medium via sputtering. Theoretical modeling of protoplanetary disks is in its infancy, and much more research is required in order to predict the chemical reactions of the interstellar molecules when subjected to the energy sources associated with the process of star and planet formation. Laboratory research is also needed on ion-molecule chemistry, photochemistry, and reactions in shock-heated gases that model the changing conditions in solar system formation. These interdisciplinary studies may best be carried out in collaborative efforts involving planetary scientists, astronomers, and chemists.
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Exploring Organic Environments in the Solar System INTERPLANETARY AND INTERSTELLAR DUST Interplanetary dust particles (IDPs), which are formed by impacts between asteroids and by the sublimation of cometary materials, provide an opportunity to study extraterrestrial organic chemistry. Multiple flights to Earth’s stratosphere (at altitudes of ~20 km via the use of U2 aircraft) have yielded a relatively large collection of IDPs ranging in size from ~5 to 50 µm. Given the small size of these particles, efficient radiative heat transfer successfully offsets the frictional heating developed during atmospheric entry, thus minimizing thermal alteration of both inorganic and organic phases and preserving what may be the most pristine extraterrestrial organic matter accessible for study on Earth. The integrated flux has been estimated at 4 × 1010 gC/yr. IDPs are classified as anhydrous or hydrous. Anhydrous IDPs contain silicate glass and minerals such as pyroxene and olivine. A cometary origin is likely21 and would have protected these particles from hydrothermal processing on parent bodies. Consequently, they may be the most primitive material available for study on Earth. Hydrous IDPs are dominated by clays and probably derive from asteroids.22 Both types of IDPs contain carbon at abundances ranging up to 90 percent by weight.23,24 Analysis of this carbon—even determining whether it is an oxide, organic matter, or graphite—is difficult because of the small particle size. This problem has been partly circumvented by use of Fourier transform infrared (FTIR) microspectrophotometry, analytical transmission electron microscopy (TEM; with electron energy loss spectroscopy to derive chemical information), and scanning transition x-ray microscopy (STXM), the last utilizing synchrotron-based soft x-ray sources in order to examine the carbon-1s absorption edge. These techniques have shown that the organic matter in both anhydrous and hydrous IDPs is similar in that both types include aromatic and aliphatic carbon skeletons as well as ketones and carboxylic acids.25,26 In comparison with extraterrestrial organic matter in carbonaceous chondrites, the organic matter in IDPs is, in general, less aromatic and more oxidized (predominantly as carboxyl groups). Studies employing an ion probe27 as well as analytical TEM28 reveal that the abundance of nitrogen in IDP organic matter is considerably greater than that observed in organic residues from carbonaceous chondrites. One of the more intriguing aspects of organic matter in IDPs, revealed by the recent development of powerful microscopic analyses,29-32 is the level of microscopic heterogeneity, in terms of both organic structure and isotopic abundances (e.g., H/D and 14N/15N). Significant variation in aromatic, aliphatic, and carboxyl concentrations has been revealed using analytical TEM and STXM33,34 (Note that micro-FTIR lacks the spatial resolution to reveal such spatial heterogeneity in functional group distribution.) Enormous hydrogen and nitrogen stable isotopic anomalies (2H and 15N) have also been observed. Hydrogen isotopic anomalies have been correlated with organic-rich domains.35 Work by Keller et al.36 concludes that aliphatic carbon is the dominant carrier of deuterium. Moreover, high-deuterium anomalies have also been correlated with organic-poor regions of IDPs, i.e., hydrated silicates.37 Nitrogen isotopic anomalies correlate spatially with the organic phases,38 and recent analytical TEM reveals that this nitrogen is likely an amine, possibly a substituent on aromatic moieties. Some anhydrous IDPs have revealed localized deuterium anomalies (measured relative to hydrogen and normalized to the deuterium:hydrogen ratio in Earth’s oceans) as high as 11,000‰ (1‰ is 1 part in 1,000),39 and, in extreme cases, entire IDPs record bulk deuterium anomalies as high as ~25,000‰. Such high deuterium contents are probably derived from molecular-cloud material. The record of solar system evolution encoded by organic matter in IDPs may therefore exceed that recorded by the organic constituents of carbonaceous chondrites. Most of the advances in IDP research have occurred in the past several years. In the near future, the application of new technologies—e.g., the nanoscale secondary ion mass spectrometer (NanoSIMS)—and advances in synchrotron-based instrumentation are likely to yield further, highly significant results. Interstellar (as opposed to interplanetary) dust grains are less well characterized. While the latter are believed to have formed in the solar system, the former formed via condensation in circumstellar regions around evolved stars, including red giants, carbon stars, asymptotic giant branch stars, novas, and supernovas. Information on the nature of interstellar grains is available from two sources, astronomical observations and laboratory studies of meteorites. The astronomical evidence is derived primarily from observations of the infrared emissions from the dust itself or studies of the dust’s ability to scatter, polarize, or redden the light of background stars (the so-called interstellar extinction). Laboratory studies of meteorites have revealed nanoparticles of, for example, diamond
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Exploring Organic Environments in the Solar System (300 parts per million), silicon carbide (~5 parts per million), graphite (~1 part per million), corumdum (30 parts per billion), and silicon nitride (2 parts per billion), whose peculiar isotopic compositions suggest that they formed in interstellar space and were later incorporated into the solar system during its formation.20 Although meteoritic grains provide a window on processes occurring in the stellar environments in which they were created and the subsequent chemical evolution of the galaxy, they cannot be considered representative of all interstellar grains because of unknown selection effects. Several characteristic features of the interstellar extinction curve—i.e., the plot of interstellar reddening as a function of wavelength—provide clues to the identity of interstellar grains. These features are as follows: The general shape of the extinction curve at far-ultraviolet wavelengths requires grains smaller than 0.01 µm in size. A very prominent broad hump in the extinction curve at ~0.22 µm is usually attributed to some form of graphitic carbon. This explanation is not, however, universally accepted. The general shape of the extinction curve at visible wavelengths requires grains larger than 0.1 µm in size. A strong absorption in the extinction curve at ~9.7 µm and another feature at 18 µm can be best explained by silicates. Most of the silicates, perhaps as much as 95 percent, are in an amorphous as opposed to a crystalline form. While the composition of the silicate remains uncertain, olivine (MgFeSiO4) has been suggested as a likely candidate. Whatever the exact nature of the silicates, they almost certainly represent a considerable fraction of the mass of the interstellar dust. In addition to the graphitic carbon and silicates, other astronomical evidence has led researchers to suggest the presence of a variety of other grain candidates, including diamonds, ultraviolet-processed hydrogenated amorphous carbon, onion-like hyper-fullerenes, glassy carbon, aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, carbonates, and silicon carbide. In summary, no one particular set of chemical or physical characteristics fits all of the available evidence. Research Opportunities An outstanding problem in the study of IDPs involves characterization of the origins of micron-scale chemical and isotopic heterogeneity and relating this information to models of nebular formation and evolution. Does the heterogeneity, for example, reflect sources spanning a large range of heliocentric distances? Additionally, it is crucial to establish whether the organic matter of IDPs is similar to that in comets or more similar to pristine organic matter in carbonaceous chondrites. Thus, both cometary sample-return missions and extension of meteorite studies to a broader suite of carbonaceous chondrites should parallel the acceleration of the analyses of archived IDPs. The Stardust mission successfully collected particles from Comet Wild 2 and returned them to Earth in January 2006. The chemical composition of these particles will provide insight into the importance of comets as a reservoir of organic compounds. For more information, see the section “Summary of Past, Present, and Planned Missions: Implications for Carbon Studies” in Chapter 4. The present program for the collection of interplanetary dust is piggybacked on flights that have other scientific objectives. Consequently, the aircraft used for dust collection are seldom flown at times when Earth is passing through high-dust regions of space. If the flights are scheduled for times when Earth passes through the trails of a comet (Table 2.4), it will be possible to collect larger amounts of dust from a specific source. For example, the Leonid meteor shower in November of each year represents dusty debris from the short period comet Tempel-Tuttle. Dust collected during this meteor shower is likely to provide samples of material from this comet. The meteoritic material deposited in the atmosphere by a particular meteor shower is microscopic in size and slowly settles in the atmosphere over an extended period of time. The rate at which particles from a particular shower reach the altitude at which they will be collected by aircraft can be estimated using the Stokes-Cunningham law.40 The settling times vary with the diameter of the particles and, thus, the average silicate sphere with a
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Exploring Organic Environments in the Solar System TABLE 2.4 Principal Meteor Showers and Their Associated Comets Name of Shower Active Period Location of Radiant Source Quadrantids January 1-5 Bootes 96P/Machholtz and 1491 I? April Lyrids April 16-25 Hercules C/Thatcher (1861 G1) Eta Aquarids April 19-May 28 Aquarius 1P/Halley Arietidsa May 29-June 19 Aries 96P/Machholtz and 1491 I? Zeta Perseidsa June 1-17 Taurus 2P/Enckeb Beta Tauridsa June 7-July 7 Taurus — Alpha Capricornids July 3-August 19 Capricorn — S Delta Aquarids July 15-August 28 Aquarius 96P/Machholtz and 1491 I? Perseids July 17-August 25 Perseus/Cassiopeia 109P/Swift-Tuttle Kappa Cygnids August 3-31 Draco — S. Taurids September 15-November 25 Taurus 2P/Enckeb N. Taurids September 15-November 25 Taurus — Orionids October 2-November 7 Orion 1P/Halley Draconids/Giacobinids October 6-10 Draco 21P/Giacobini-Zinner Leonids November 14-21 Leo 55P/Tempel-Tuttle Geminids December 7-17 Gemini (3200) Phaethonc Ursids December 17-267 Ursa Minor 8P/Tuttle aDaytime showers. bAlso associated with various asteroids. cPossible extinct comet or asteroid. SOURCE: Data from R.P. Binzel, M.S. Hanner, and D.I. Steel, “Solar System Small Bodies,” pp. 315-337 in Allen’s Astrophysical Quantities, 4th edition (A.N. Cox, ed.), Springer-Verlag, New York, 2000. diameter of 10.4 µm has a settling time of about 12 days. It will also be helpful to compare the analytical data from dust particles attributed previously to a particular comet to confirm the connection between the dust particle and the comet. Enhancement of the stratospheric-collection program would yield a range of materials great enough to allow meaningful comparisons with putative IDPs collected from seafloor sediments and Antarctic ice sheets. Effects of alteration in those environments may be recognized and quantified, thus providing a greatly expanded time scale and more broadly representative database. Recommendation: A program specifically designed to collect dust in the stratosphere during meteor showers should be implemented. NOTES 1. For a comprehensive review see, for example, B.T. Draine, “Interstellar Dust Grains,” Annual Reviews of Astronomy and Astrophysics 41: 241-289, 2003. 2. For a general review of interstellar chemistry see, for example, E. Herbst, “The Chemistry of the Interstellar Medium,” Annual Reviews of Physical Chemistry 46: 27-53, 1995. 3. T.L. Wilson and R.T. Rood, “Abundances in the Interstellar Medium,” Annual Review of Astronomy and Astrophysics 32: 191-226, 1994. 4. E. Herbst, “Isotopic Fractionation by Ion-Molecule Reactions,” Space Science Reviews 106(1-4): 293-304, 2003. 5. S.S. Prasad and S.P. Tarafdar, “UV Radiation Field Inside Dense Clouds—Its Possible Existence and Chemical Implications,” Astrophysical Journal 267: 603, 1983. 6. W.A. Schutte, A.C.A. Boogert, A.G.G.M. Tielens, D.C.B. Whittet, P.A. Gerakines, J.E. Chiar, P. Ehrenfreund, J.M. Greenberg, E.F. van Dishoeck, and Th. de Graauw, “Weak Ice Absorption Features at 7.24 and 7.41 Microns in the Spectrum of the Obscured Young Stellar Object W 33A,” Astronomy and Astrophysics 343: 966, 1999.
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Exploring Organic Environments in the Solar System 7. E.L. Gibb, D.C.B. Whittet, A.C.A. Boogert, and A.G.G.M. Tielens, “Interstellar Ice: The Infrared Space Observatory Legacy,” Astrophysical Journal Supplement Series 151: 35G, 2004. 8. T.J. Millar, E. Herbst, and S.B. Charnley, “The Formation of Oxygen-Containing Organic Molecules in the Orion Compact Ridge,” Astrophysical Journal 369: 147, 1991. 9. A.G.G.M. Tielens and S.B. Charnley, “Circumstellar and Interstellar Synthesis of Organic Molecules,” Origins of Life and Evolution of the Biosphere 27: 23-51, 1997. 10. R.J.A. Grim, F. Baas, T.R. Geballle, J.M. Greenberg, and W. Schutte, “Detection of Solid Methanol Toward W33A,” Astronomy and Astrophysics 243: 473, 1997. 11. V.K. Agarwal, W. Schutte, J.M. Greenberg, J.P. Ferris, R. Briggs, S. Conner, C.P.E.M. Van De Bult, and F. Bass, “Photochemical Reactions in Interstellar Grains, Photolysis of CO, NH3 and H2O,” Origins of Life 16: 21-40, 1985. 12. R. Briggs, G. Ertem, J.P. Ferris, J.M. Greenberg, P.J. McCain, C.X. Mendoza-Gomez, and W. Schutte, “Comet Halley as an Aggregate of Interstellar Dust and Further Evidence for the Photochemical Formation of Organics in the Interstellar Medium,” Origins of Life and Evolution of the Biosphere 22: 287-307, 1992. 13. M.P. Bernstein, J.P. Dworkin, S.A. Sandford, G.W. Cooper, and L.J. Allamandola, “Racemic Amino Acids from the Ultraviolet Photolysis of Interstellar Ice Analogues,” Nature 416: 401-403, 2002. 14. G.M. Munoz Caro, U.J. Meierhenrich, W.A Schutte, B. Barbier, A. Arcones Segovia, H. Rosenbauer, W.H.P. Thiemann, A. Brack, and J.M. Greenberg, “Amino Acids from Ultraviolet Irradiation of Interstellar Ice Analogues,” Nature 416: 403-406, 2002. 15. G.M. Munoz Caro, U.J. Meierhenrich, W.A Schutte, B. Barbier, A. Arcones Segovia, H. Rosenbauer, W.H.P. Thiemann, A. Brack, and J.M. Greenberg, “Amino Acids from Ultraviolet Irradiation of Interstellar Ice Analogues,” Nature 416: 403-406, 2002. 16. M.P. Bernstein, J.P. Dworkin, S.A. Sandford, G.W. Cooper, and L.J. Allamandola, “Racemic Amino Acids from the Ultraviolet Photolysis of Interstellar Ice Analogues,” Nature 416: 401-403, 2002. 17. For a review, see Y.J. Pendleton and L.J. Allamandola, “The Organic Refractory Material in the Diffuse Interstellar Medium: Mid-infrared Spectroscopic Constraints,” Astrophysical Journal Supplement Series 138: 75-98, 2002. 18. V. Mannings, A.P. Boss, and S.S. Russell, eds., Protostars and Planets IV, University of Arizona Press, Tucson, Ariz., 2002. 19. Y. Aikawa and E. Herbst, “Chemical Models of Circumstellar Disks,” pp. 425-434 in Astrochemistry: From Molecular Clouds to Planetary Systems (Y.C. Minh and E.F. van Dishoeck, eds.), IAU Symposium 197, Astronomical Society of the Pacific, San Francisco, Calif., 2000. 20. For a recent review, see, for example, P.M. Hoppe and E. Zinner, “Presolar Dust Grains from Meteorites and Their Stellar Sources,” Journal of Geophysical Research 105: 10371-10386, 2000. 21. D.E. Brownlee, “Cosmic Dust: Collection and Research,” Annual Review of Earth and Planetary Science 13: 147-173, 1985. 22. L.P. Keller, K.L. Thomas, and D.S. McKay, “An Interplanetary Dust Particle with Links to CI Chondrites,” Geochimica et Cosmochimica Acta 56: 1409-1412, 1992. 23. K.L. Thomas, L.P. Keller, G.E. Blanford, and D.S. McKay, “Quantitative Analyses of Carbon in Anhydrous and Hydrated Interplanetary Dust Particles,” pp. 165-172 in Analysis of Interplanetary Dust (M.E. Zolensky, T.L. Wilson, F.J.M. Rietmeijer, and G.J. Flynn, eds.), AIP Conference Proceedings 310, American Institute of Physics, Woodbury, N.Y., 1994. 24. G.J. Flynn, L.P. Keller, C. Jacobsen, S. Wirick, and M.A. Miller, “Organic Carbon in Interplanetary Dust Particles,” pp. 191-194 in A New Era in Bioastronomy, ASP Conference Series, Vol. 213, Astronomical Society of the Pacific Press, San Francisco, Calif., 2000. 25. L.P. Keller, S. Messenger, G.J. Flynn, S. Clemett, S. Wirick, and C. Jacobsen, “The Nature of Molecular Cloud Material in Interplanetary Dust,” Geochimica et Cosmochimica Acta 68(11): 2577-2589, 2004. 26. G.J. Flynn, L.P. Keller, M. Feser, S. Wirick, and C. Jacobsen, “The Origin of Organic Matter in the Solar System: Evidence from the Interplanetary Dust Particles,” Geochimica et Cosmochimica Acta 67(24): 4791-4806, 2003. 27. J. Aleon, F. Robert, M. Chaussidon, and B. Marty, “Nitrogen Isotopic Composition of Macromolecular Organic Matter in Interplanetary Dust Particles,” Geochimica et Cosmochimica Acta 67: 3773-3783, 2003. 28. L.P. Keller, S. Messenger, G.J. Flynn, S. Clemett, S. Wirick, and C. Jacobsen, “The Nature of Molecular Cloud Material in Interplanetary Dust,” Geochimica et Cosmochimica Acta 68(11): 2577-2589, 2004. 29. L.P. Keller, S. Messenger, G.J. Flynn, S. Clemett, S. Wirick, and C. Jacobsen, “The Nature of Molecular Cloud Material in Interplanetary Dust,” Geochimica et Cosmochimica Acta 68(11): 2577-2589, 2004. 30. J. Aleon, C. Engrand, F. Robert, and M. Chaussidon, “Clues to the Origin of Interplanetary Dust Particles from the Isotopic Study of Their Hydrogen Bearing Phases,” Geochimica et Cosmochimica Acta 65: 4399-4412, 2001. 31. J. Aleon, F. Robert, M. Chaussidon, and B. Marty, “Nitrogen Isotopic Composition of Macromolecular Organic Matter in Interplanetary Dust Particles,” Geochimica et Cosmochimica Acta 67: 3773-3783, 2003. 32. G.J. Flynn, L.P. Keller, M. Feser, S. Wirick, and C. Jacobsen, “The Origin of Organic Matter in the Solar System: Evidence from the Interplanetary Dust Particles,” p. 275 in Bioastronomy 2002: Life Among the Stars, IAU Symposium 213 (R. Norris and F. Stootman, eds.), Astronomical Society of the Pacific, San Francisco, Calif., 2003. 33. L.P. Keller, S. Messenger, G.J. Flynn, S. Clemett, S. Wirick, and C. Jacobsen, “The Nature of Molecular Cloud Material in Interplanetary Dust,” Geochimica et Cosmochimica Acta 68(11): 2577-2589, 2004. 34. G.J. Flynn, L.P. Keller, M. Feser, S. Wirick, and C. Jacobsen, “The Origin of Organic Matter in the Solar System: Evidence from the Interplanetary Dust Particles,” Geochimica et Cosmochimica Acta 67(24): 4791-4806, 2003. 35. J. Aleon, C. Engrand, F. Robert, and M. Chaussidon, “Clues to the Origin of Interplanetary Dust Particles from the Isotopic Study of their Hydrogen Bearing Phases,” Geochimica et Cosmochimica Acta 65: 4399-4412, 2001.
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Exploring Organic Environments in the Solar System 36. L.P. Keller, S. Messenger, G.J. Flynn, S. Clemett, S. Wirick, and C. Jacobsen, “The Nature of Molecular Cloud Material in Interplanetary Dust,” Geochimica et Cosmochimica Acta 68(11): 2577-2589, 2004. 37. L. Nittler, Carnegie Institution of Washington, personal communication, 2003. 38. J. Aleon, F. Robert, M. Chaussidon, and B. Marty, “Nitrogen Isotopic Composition of Macromolecular Organic Matter in Interplanetary Dust Particles,” Geochimica et Cosmochimica Acta 67: 3773-3783, 2003. 39. See, for example, S. Messenger, “Identification of Molecular-cloud Material in Interplanetary Dust Particles,” Nature 404: 968-971, 2000. 40. See, for example, F.J.M. Rietmeijer and P. Jenniskens, “Recognizing Leonid Meteoroids Among the Collected Stratospheric Dust,” Earth, Moon, and Planets 82-83: 505-524, 1998.
Representative terms from entire chapter: