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Dense molecular clouds in the ISM
The dense interstellar clouds are often called molecular clouds as their composition is primarily molecular. The gas in these regions is predominantly H2. Nevertheless observations at infrared, radio, millimetre and submillimetre frequencies show large variety of gas phase organic molecules (Irvine, Goldsmith and Hjalmarson 1987, Ohishi and Kaifu 1998, Winnewisser and Kramer 1999) with densities between 100 cm-3 to 108 cm-3. Here we can include nitriles, aldehydes, alcohols, acids ethers, ketones, amines, amides along with long (n<10) chain hydrocarbon compounds. As dense interstellar clouds structure is quite complex, three different areas with organic molecules can be distinguished: grain surfaces, cold dark clouds and hot molecular cores where the organic compounds are formed and observed. These regions will be separately described in the following. Grain surface. The dust particles occurring in cold molecular clouds are an important “catalyst” which is involved in long reactions of atoms or molecules from the gas (Charnley, Tielens and Rodgers 1997, Tielens and Charnley 1997). There are three major mechanisms for surface reactions: the diffuse (Langmuir-Hinshelwood) or the Eley-Rideal (mechanism proposed in 1938 by D. D. Eley and E. K. Rideal (Eley 1948)) or the hot atom interaction (Fig. 1.7). Nevertheless, astrochemical models often include only the diffusive one as it is the best studied (Kolasinski 2008).
The Solar System – protoplanetary disk
The outer Solar System, for instance on the surface of asteroids belt, in the atmosphere of Saturn’s moon Titan, possibly on the moon Triton and certainly in comets (Cruikshank 1997), seems to be more abundant in carbonaceous matter than previously discussed regions of cold clouds, hot cores and diffuse clouds.
Comets
Great amount of volatile organics is observed in comets (Bockelée-Morvan et al. 2000, Biver et al. 2006) which clearly contain the highest portion of volatiles with respect to other small solar system bodies. It is admitted that the nuclei of comets are composed of ice, rock, and large organic entities (CHON particles – carbon, hydrogen, oxygen, and nitrogen (Mumma, Krasnopolsky and Abbott 1997, Irvine et al. 2000). Their chemical composition has been constrained by Greenberg (Greenberg 1998), who considers that 26% of the mass is combined in silicates, 23% in refractory material, 9% in small carbonaceous molecules, and about 30% in H2O ice (with small contributions of CO, CO2, CH3OH, and other simple molecules). By remote observations of cometary gas and dust by telescopes throughout the electromagnetic spectrum are analysed to establish constraints about the formation and composition of comets. The volatile matter in comets was shown to be dominated by water, followed by CO and CO2, with trace amounts of other chemical species such as CH3OH, CH4, C2H2, HCN, C2H6, CH3OH, C2H6 and many others (Crovisier and Bockelée-Morvan 1999, Mumma, Weissman and Stern 1993, Rodgers and Charnley 1998)). Most of these compounds are parent species evaporating directly from the ices. The origin of comets is still quite far from being fully understood. Three models are considered: the interstellar model, which suggests that interstellar grains agglomerated to form cometary nuclei in the cold outer solar nebula far from the protosun (Wilkening and Matthews 1982); the complete chemical equilibrium model, in which presolar material is altered and chemically equilibrated (Lunine et al. 1991); and an intermediate model in which presolar material has been chemically and physically processed (Chick and Cassen 1997, Fegley 1999). At present it is presumed that comets are a mixture of interstellar and nebular material (Mumma et al. 1993, Irvine 1999) and their composition may vary according to their place of origin. Finally, it has to be noted, that after the Stardust mission many complex organics have been found with heterogeneous distribution in abundance and composition among particles (Matrajt et al. 2008). Here there are organic species like PAHs (with few ring aromatic rings – generally smaller then PAHs detected in diffuse clouds) (Sandford et al. 2006) and a new class of aromatic-poor compounds comparable with these in meteorites and interplanetary dust particles (IDPs).
Carbonaceous chondrites
The most chemically complex and well characterized organic matter has been found in carbonaceous chondrites. These most primitive and least processed meteorites contain organics which hold key information about the chemical composition of the gas that formed our Sun and the planets.
Carbonaceous chondrites comprise up to 4% of carbon, mostly in the form of organic matter, with minor fractions of carbonates, elemental carbon phases (graphite and nanodiamonds) and refractory carbides. The organic matter occurs throughout the matrix along with clay minerals and oxides. It is composed mostly of carbon with hydrogen, oxygen, nitrogen or sulfur. It can be divided into two fractions: the soluble and insoluble components Less abundant, accounting for up to 30% of meteoritic organic matter in CI1 (Orgueil) and CM2 (Murchison) meteorites (Sephton 2002), the soluble matter (SOM) can be isolated from meteorites by extraction in common organic solvents. Thanks to many available analytical techniques, SOM has been extensively analyzed for the last 50 years and a large variety of soluble organic compounds was revealed (Botta and Bada 2002, Sephton and Botta 2005, Hayes 1967, Hayatsu and Anders 1981, Mullie and Reisse 1987, Cronin and Chang 1993) (Fig. 1.10). In Murchison the most abundant soluble compounds are carboxylic acids (Yuen and Kvenvolden 1973, Yuen et al. 1984, Krishnamurthy et al. 1992) amino acids (Kvenvolden et al. 1970) sugar derivatives (Cooper et al. 2001) and nucleobases (Martins et al. 2008). Other oxidized compounds have been detected: alcohols, aldehydes and ketones. Aromatic and aliphatic hydrocarbons also occur in Murchison (Yuen and Kvenvolden 1973)(. These soluble compounds share several molecular properties (Pizzarello 2006): (1) they have a complete structural diversity (for each formula, every isomer is detected); (2) their abundance decreases as the carbon number increases; (3) in general, branched chains are more abundant than straight chains.
Murchison and the statistical model of the molecular structure of the IOM
Much of our understanding of meteoritic organic matter has come from the investigation of the Murchison meteorite (Fig. 1.14). It is the most representative carbonaceous chondrite which fell in Australia in 1969 and contains a variety of organic molecules. The total collected mass exceeds 100 kg. The meteorite belongs to the CM group. Murchison is petrologic type 2, which means that it experienced extensive alteration by water-rich fluids on its parent body (Airieau et al. 2005).
Laboratory synthesis of organic matter
The list of organic molecules in space is extensive and sometimes to recognize new species, long lasting observations are needed. The most basic required information is spectroscopy of materials from UV to millimeter wavelengths. Moreover, the obtained data has to be followed up by sometimes most difficult interpretation of observations of astronomical sources. Nevertheless to make the next step in understanding organics the need for laboratory synthesis is compulsory. To obtain rates for various reactions that are expected to form and destroy organics under space conditions is crucial and, along with observational evidence, it will help establishing the cycle of organic molecules in different environments in the Galaxy.
Several attempts to synthesize analogues of extraterrestrial organic matter have been reported so far. They often aim at reconciling the spectroscopic observations on astrophysical media with those obtained on synthetic organic matter. Various targets are distinguished in the literature such as cometary ices, interstellar organic molecules (on grains or in the gas phase gas), interplanetary dust particles (IDPs), meteorites, or Titan’s aerosols. As a result, different experimental devices have been set up.
Table of contents :
Abstract
Résume
Introduction
1 State of the art
1.1 Meteorites
1.1.1 Meteorites – precious extraterrestial bodies
1.1.2 Meteorite classification
1.1.3 Chondrites as the witnesses of the formation of the Solar system .
1.2 Formation of the solar system
1.3 Organics in space
1.3.1 Interstellar Matter
1.3.2 The Solar System – protoplanetary disk
1.4 Laboratory synthesis of organic matter
1.4.1 Tholins
1.4.2 ISM dust
1.4.3 Meteorites
1.5 Most important issues- what can be done?
2 Materials and methods
2.1 Synthesis of organic matter
2.1.1 Plasma basics
2.1.2 Description of the synthesis device
2.2 Soluble organic matter analysis
2.3 Insoluble organic matter analysis
2.3.1 Elemental analysis
2.3.2 FTIR
2.3.3 Raman Micro-spectroscopy
2.3.4 Solid state 13C NMR
2.3.5 EPR
2.3.6 Py GC/MS
2.3.7 Ruthenium Tetroxide Oxidation (RuO4 oxidation)
2.3.8 HRTEM
2.3.9 NanoSIMS
3 Synthesis of organic matter containing carbon and hydrogen
3.1 Introduction
3.2 Chemical structure of OM synthesized from pentane or octane*
3.2.1 Synthesis
3.2.2 Contamination issue
3.2.3 Characterization of the soluble OM by GC-MS
3.2.4 650°C Pyrolysis-GC-MS analysis of the IOM
3.2.5 358°C Pyrolysis-GC-MS analysis of the IOM
3.2.6 HRTEM observation of the IOM
3.2.7 Raman Micro-spectroscopy analysis of the IOM
3.2.8 NMR analysis of the IOM
3.2.9 FTIR analysis of the IOM
3.2.10 RuO4 oxidation of the IOM
3.2.11 GC-MS analysis of methylated RuO4 oxidation octane product
3.2.12 EPR analysis of the IOM
3.2.13 Conclusion
3.3 Chemical structure of the organic matter synthesized from methane .
3.3.1 Characterization of the soluble OM by GC-MS
3.3.2 Characterization of the IOM by Py GC-MS
3.4 NanoSIMS analysis of the D enrichment in the IOM
3.5 Discussion and comparison with meteorite IOM
4 Synthesis and analysis of organic matter containing nitrogen
4.1 Introduction
4.2 Synthesis in the presence of nitrogen hydrides
4.2.1 Synthesis
4.2.2 Characterization of the soluble OM by GC-MS
4.2.3 Characterization of the IOM
4.3 Synthesis in the presence of N2
4.3.1 Synthesis
4.3.2 Characterization of the SOM by GC-MS
4.3.3 Characterization of the IOM
4.4 Comparison with chondritic IOM
5 Aqueous alteration & incorporation of oxygen
5.1 Introduction
5.2 Simulation experiment for aqueous alteration
5.2.1 Experimental
5.2.2 Released soluble compounds
5.2.3 IOM analysis
5.2.4 Pyrolysis-GC-MS analysis of the IOM
5.3 Incorporation of Oxygen upon IOM formation
5.3.1 Synthesis
5.3.2 Characterization of the soluble OM by GC-MS
5.3.3 Analysis of IOM
5.4 Comparison with chondritic IOM
6 Conclusions and future perspectives
6.1 Conclusions
6.2 Future perspectives
7 Bibliography .
