D. Deboffle

Non-racemic Amino Acid Production by Ultraviolet Irradiation of Achiral Interstellar Ice Analogs with Circularly Polarized Light

The delivery of organic matter to the primitive Earth via comets and meteorites has long been hypothesized to be an important source for prebiotic compounds such as amino acids or their chemical precursors that contributed to the development of prebiotic chemistry leading, on Earth, to the emergence of life. Photochemistry of inter/circumstellar ices around protostellar objects is a potential process leading to complex organic species, although difficult to establish from limited infrared observations only. Here we report the first abiotic cosmic ice simulation experiments that produce species with enantiomeric excesses (e.e.s). Circularly polarized ultraviolet light (UV-CPL) from a synchrotron source induces asymmetric photochemistry on initially achiral inter/circumstellar ice analogs. Enantioselective multidimensional gas chromatography measurements show significant e.e.s of up to 1.34% for (13C)-alanine, for which the signs and absolute values are related to the helicity and number of CPL photons per deposited molecule. This result, directly comparable with some L excesses measured in meteorites, supports a scenario in which exogenous delivery of organics displaying a slight L excess, produced in an extraterrestrial environment by an asymmetric astrophysical process, is at the origin of biomolecular asymmetry on Earth. As a consequence, a fraction of the meteoritic organic material consisting of non-racemic compounds may well have been formed outside the solar system. Finally, following this hypothesis, we support the idea that the protosolar nebula has indeed been formed in a region of massive star formation, regions where UV-CPL of the same helicity is actually observed over large spatial areas.

Ultraviolet photoproduction of ISM dust Laboratory characterisation and astrophysical relevance

The production of a hydrogenated amorphous carbon polymer (a-C:H) via the photolysis of a series of organic molecule precursors at low temperature is described. Such amorphous material is synthesised under interstellar conditions (10 K and Lyman-α photons) and represents the best candidate to explain the Diffuse Interstellar Medium absorption observed in our Galaxy and in other galaxies. We perform a series of laboratory analyses (Infrared spectroscopy, µspectroscopy, Raman, Photoluminescence and UV-visible spectroscopy) which allow a full characterisation of such polymers. This allows us to assess the importance of the polymer and possible scenarios for its role in crucial aspects of the lifecycle of dust. Such material has implications for the carbon budget at galactic scales, hydrogen formation, extended red emission, as a PAH precursor, and in explaining the 2175 A extinction bump.

FTIR and Raman analyses of the Tagish Lake meteorite: Relationship with the aliphatic hydrocarbons observed in the Diffuse Interstellar Medium

Using FTIR and Raman microspectroscopies we have analysed 6 fragments of the Tagish Lake meteorite. The data obtained show that all the fragments belong to the carbonate-rich lithology, where an organic material, including a highly disordered macromolecular carbonaceous component is found. The FTIR approach shows that part of the organic material present in Tagish Lake is aliphatic. The Raman approach shows that there is also highly disordered polyaromatic organic material, which is abundant. Furthermore, the comparison of Raman data of Tagish Lake to other carbonaceous chondrites (CI, CM2, CR2) shows that the carbon in Tagish Lake is different, supporting the assertion that this meteorite is a unique and new type of carbonaceous chondrite. The comparison of the aliphatic hydrocarbon FTIR data found in the Tagish Lake meteorite with the aliphatic hydrocarbon IR data of the carbonaceous chondrites Orgueil and Murchison and with the diffuse Interstellar Medium (ISM) shows that they are different, in that the Tagish Lake meteorite has longer aliphatic chains.

The effects of circularly polarized light on amino acid enantiomers produced by the UV irradiation of interstellar ice analogs

Two irradiation experiments on interstellar ice analogs at 80 K under interstellar-like conditions were performed with the LURE SU5 synchrotron beamline to assess, for the first time, the photochemical effect of circularly polarized ultraviolet light (UV CPL) at 167 nm (7.45 eV) with right and left polarizations on such ice mixtures. Methods. This effect was measured by determining the enantiomeric excesses (e.e.s) for two amino acids formed in the solid organic residues produced during the subsequent warm-up of the irradiated samples to room temperature: alanine, the most abundant chiral proteinaceous amino acid produced (both polarizations) and 2,3-diaminopropanoic acid (DAP), a non-proteinaceous amino acid (rightpolarization experiment). These excesses were compared to those measured for the same amino acids produced after unpolarized UV irradiation of the same ice mixtures (expected to be zero), in order to determine the contribution of CPL only. A careful estimate of all the associated uncertainties (statistical and systematic errors) was also developed. Results. It appears that the enantiomeric photochemical effect at this wavelength is weak, since both alanine and DAP e.e.s were found to be small, at most of the order of 1% in absolute values, and tends to be inconclusive since the effects obtained for both amino acids and both polarizations are not those expected. In light of these results, the hypothesis that CPL may be one source responsible for the e.e.s measured for such amino acids in some meteorites and, more generally, that CPL may be directly related to the origin of biomolecular homochirality on Earth is discussed.

Organic matter in Seyfert 2 nuclei: Comparison with our Galactic center lines of sight

We present ESO - Very Large Telescope and ESA - Infrared Space Observatory 3 to 4 µm spectra of Seyfert 2 nuclei as compared to our galactic center lines of sight. The diffuse interstellar medium probed in both environments displays the characteristic 3.4 µm aliphatic CH stretch absorptions of refractory carbonaceous material. The profile of this absorption feature is similar in all sources, indicating the CH2/CH3 ratios of the carbon chains present in the refractory components of the grains are the same in Seyfert 2 inner regions. At longer wavelengths the circumstellar contamination of most of the galactic lines of sight precludes the identification of other absorption bands arising from the groups constitutive of the aliphatics seen at 3.4 µm. The clearer continuum produced by the Seyfert 2 nuclei represents promising lines of sight to constrain the existence or absence of strongly infrared active chemical groups such as the carbonyl one, important to understand the role of oxygen insertion in interstellar grains. The Spitzer Space Telescope spectrometer will soon allow one to investigate the importance of aliphatics on a much larger extragalactic sample.

Revisiting the solid HDO/H2O abundances

We revisit the reported detection and upper limits on HDO in ice mantles present in the molecular cloud environments of the massive young protostars Gl 2136 and W33 A, using independent VLT-ISAAC and UKIRT-CGS4 spectroscopic observations. We also present VLT and UKIRT spectra of RAFGL 7009 near the HDO absorption wavelength and reanalyze the ISO-SWS spectral data for NGC 7538 IRS9, Orion-BN and S140. We demonstrate that the previously reported detections of HDO in W33 A and NGC 7538 IRS9 are incorrect. We present an in-depth analysis that shows that, besides the sensitivity limits, detection of low levels of HDO is difficult in amorphous ice mantles when features from solid methanol, a common grain mantle constituent, are present. We discuss the specific problems arising in the ISO data in this wavelength range for NGC 7538 IRS9. Using VLT-ISAAC observations, we also investigate the HDO/H 2 O ratio toward the intermediate mass stars IRAS 05329-0728 and IRAS 08448-4343. Our derived upper limits for the D/H ratio in water ice range from HDO/H 2 O < 1% to 0.2% in the different sources, and we discuss these limits in comparison with values derived in other environments.

The origin of GEMS in IDPs as deduced from microstructural evolution of amorphous silicates with annealing

Aims. We present laboratory studies of the micro-structural evolution of an amorphous ferro-magnesian silicate, of olivine composition, following thermal annealing under vacuum. Methods. The amorphous silicate was prepared as a thin film on a diamond substrate. Annealing under vacuum was performed at temperatures ranging from 870 to 1020 K. After annealing the thin films were extracted from the substrate and analysed by transmission electron microscopy to infer their microstructural and compositional evolution. Results. Spheroidal metallic nano-particles (2−50 nm) are found within the silicate films, which are still amorphous after annealing at 870 K and partially crystallized into forsterite for annealing up to 1020 K. We interpret this microstructure in terms of a reduction of the initial amorphous silicate FeO component, because of the carbon-rich partial pressure in the furnace due to pumping mechanism. Annealing in a controlled oxygen-rich atmosphere confirms this interpretation. Conclusions. The observed microstructures closely resemble those of the GEMS (Glass with Embedded Metal and Sulphides) found in chondritic IDPs (Interplanetary Dust Particles). Since IDPs contain abundant carbonaceous matter, a solid-state reduction reaction may have occurred during heating in the hot inner regions of the proto-solar disc. Related to this, the presence of forsterite grains grown from the amorphous precursor material clearly demonstrates that condensation from gaseous species is not required to explain the occurrence of forsterite around young protostars and in comets. Forsterite grains in these environments can be formed directly in the solid phase by thermal annealing of amorphous ferro-magnesian silicates precursor under reducing conditions. Finally, locking iron as metallic particles within the silicates explains why astronomical silicates always appear observationally Mg-rich.

Methane clathrate hydrate FTIR spectrum - Implications for its cometary and planetary detection

Context. The physical behaviour of methane clathrate hydrate, a crystallographic ice crystal is of major importance for both the earth and the stability of gases in many astrophysical bodies (planets, comets, etc.). Aims. We provide an infrared spectroscopic identification for astrophysical methane clathrate hydrates and investigate the crystal field experienced by the trapped molecule. Methods. A methane clathrate crystal was produced in a moderate-pressure optical cell. Using FTIR spectroscopy, the v 3 asymmetric CH-stretching mode of the entrapped methane molecule is recorded from 7 K to 80 K, then back to 7 K. Results. It is shown that the trapped methane molecules in the clathrate hydrate is a quasi rotor, displaying gaseous behaviour at low temperatures. A series of ro-vibrational specific lines is observed, shifted in frequency by the water-ice cage interactions with the trapped methane molecules. Because these transitions are unique to methane clathrate hydrate, they represent a crucial identification pattern for astrophysical icy bodies at low temperatures, such as comets and/or interstellar grains.

Methane clathrate hydrate infrared spectrum II. Near-infrared overtones, combination modes and cages assignments

Context. Recently, we recorded the infrared spectrum of the methane clathrate hydrate stretching mode at low temperature, a caged compound of possible interest for solar system studies as well as interstellar ice mantles. Aims. We provide a practical infrared spectroscopic identification for methane clathrate hydrate to examine its astrophysical presence or absence. We investigate the crystal field induced shifts, and assign the different transitions to the different encaged molecules environments in this clathrate hydrate. Methods. A methane clathrate crystal is produced in an infrared transmitting moderate-pressure closed cell. Using Fourier transform infrared (FTIR) spectroscopy, the overtones (3ν4 ,2 ν3) and combination modes (ν2 + 2ν4, ν1 + ν4, ν3 + ν4, ν2 + ν3,ν3 + 2ν4,2ν2 + 2ν4,ν2 + ν3 +ν4) falling in the 6000–3000 cm −1 (∼1.65–3.4 μm) and their temperature behaviour are investigated. In addition, non-astrophysical CH4/CF4 gas mixtures are used to build clathrates with different methane large and small cage occupancies to help in assignments. Results. Combination modes show the two distinct cages and the quasi-free rotor low temperature ro-vibrational structure expected for methane clathrate hydrates. A comparison with the pure phase I is performed. Implications for methane clathrate hydrate detection are clearly identified. Conclusions. Solid methane actual remote observations of solar system objects surfaces do not display the clathrate hydrates’ specific shift and occupancy signatures. Observationnally, a search for their infrared spectroscopic specific signatures should be performed, focusing on thermodynamically favourable objects like trans- neptunian objects (TNOs) or recently exposed (e.g. fresh impact) planet (or their satellites) surfaces. On the modeling side, efforts must be undertaken to progressively implement clathrate formation kinetics.

Low temperature CH 4 and CO 2 clathrate hydrate near to mid-IR spectra

The physical behaviour of methane and carbon dioxide clathrate hydrates, specific crystallographic ice crystals are of major importance for the earth and may control the stability of gases in many astrophysical bodies such as the planets, comets and possibly interstellar grains. Such models claim they provide an alternative trapping mechanism modifying the absolute and relative composition of icy bodies and can be at the source of late time injection of gaseous species in planetary atmospheres. However, there is a clear need to detect them directly. We provide in this study the laboratory recorded signatures of clathrate hydrates in the near to midinfrared for astrophysical remote detection. These laboratory experiments will in a near future allow to follow the kinetic formation by diffusion in dedicated experiments, another important step to implement, to understand and model their possible presence in space.