Zahia Djouadi

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Impact Features on Stardust: Implications for Comet 81P/Wild 2 Dust

Particles emanating from comet 81P/Wild 2 collided with the Stardust spacecraft at 6.1 kilometers per second, producing hypervelocity impact features on the collector surfaces that were returned to Earth. The morphologies of these surprisingly diverse features were created by particles varying from dense mineral grains to loosely bound, polymineralic aggregates ranging from tens of nanometers to hundreds of micrometers in size. The cumulative size distribution of Wild 2 dust is shallower than that of comet Halley, yet steeper than that of comet Grigg-Skjellerup.

Elemental compositions of comet 81P/Wild 2 samples collected by Stardust

We measured the elemental compositions of material from 23 particles in aerogel and from residue in seven craters in aluminum foil that was collected during passage of the Stardust spacecraft through the coma of comet 81P/Wild 2. These particles are chemically heterogeneous at the largest size scale analyzed (∼180 ng). The mean elemental composition of this Wild 2 material is consistent with the CI meteorite composition, which is thought to represent the bulk composition of the solar system, for the elements Mg, Si, Mn, Fe, and Ni to 35%, and for Ca and Ti to 60%. The elements Cu, Zn, and Ga appear enriched in this Wild 2 material, which suggests that the CI meteorites may not represent the solar system composition for these moderately volatile minor elements.

Infrared Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

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 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.

Chemical and morphological evolution of a silicate surface under low-energy ion irradiation

Aims. Olivine surfaces have been subjected to low-energy ion irradiation with H + ,H e + and Ar + at energies within the keV range in order to simulate the effects of energetic gas-grain interactions within shocked regions of the interstellar medium. Methods. The induced modifications in the chemical composition and the bonding configuration of the upper and the near surface regions were monitored in situ by X-ray photoelectron spectroscopy (XPS). The associated morphological evolution of the samples was studied by atomic force microscopy (AFM). Results. Results show that the surface chemistry evolves during irradiation with a noticeable Mg enrichment relative to Si. This evolution is interpreted as coming from magnesium atom diffusion driven by the electric field caused by the positive ion implantation. The iron valence state is also strongly affected by irradiation, with reduction occurring at relatively high fluxes. However, at low fluxes the iron is found to oxidise from Fe 2+ to Fe 3+ due to a charge transfer between the incident positive ions and the iron in the sample. The atomic force microscopy results show that the surface roughness tends to increase with irradiation and that this roughness influences the surface chemical reactivity of the grains, as shown by the enhanced formation of carbonate on the surfaces when they are exposed to a CO2 atmosphere. The implications for the evolution of interstellar dust include an enhanced dust catalytic activity. These effects would arise from modification under irradiation of the surface reactivity and an increase in the available grain surface area.

Hydrocarbon Materials of Likely Interstellar Origin from the Paris Meteorite

We have examined some grains from the Paris meteorite through infrared and Raman micro-spectroscopy in order to investigate their carbonaceous and mineralogical components. In the mid- as well as far-infrared regions, the raw and global spectra of Paris resemble those of CM meteorites. However, we have obtained rather peculiar infrared spectra for some aromatic-rich micron-sized fragments of Paris displaying a very good match between its organic signatures both in the 3.4 ?m and 6 ?m regions, and the ones observed from the diffuse interstellar medium infrared sources toward the Galactic center, suggesting that this meteorite may have indeed preserved some organic matter of interstellar origin.

Hydroxyl radical production and storage in analogues of amorphous interstellar silicates: a possible “wet” accretion phase for inner telluric planets

Context. Interstellar silicate grains are thought to be amorphized by interaction with high- and low-energy particle interactions in astrophysical environments. In addition, low energy (a few keV) particles will implant atoms within the grains. Aims. In this paper we experimentally investigate the consequence of the implantation of H + at low irradiation energies into analogues of interstellar silicate grains, and look for the formation of hydroxyl radicals within the silicate matrix. Methods. Thin amorphous silicate films (∼100 nm) were sequentially irradiated with H + ions at low energies (3.5, 2.5 and then 1.5 keV) ensuring an implantation of the ions through the full depth of the films. The fluences used, 3× 10 16 ,1 0 17 and 3× 10 17 H + /cm 2 , are compatible with those expected in shocks in the interstellar medium. We used infrared spectroscopy to monitor and quantify the OH band evolution after irradiation. In order to distinguish the newly formed OH groups from those originating from unavoidable atmospheric contamination, the D/H depth ratios were measured with a NanoSIMS ion microprobe. Results. An increase in the OH band strength in the infrared spectra after irradiation reveals the formation of OH bonds within the irradiated silicate thin films. NanoSIMS measurements of the D/H signature in the region of ion implantation show that the newlyformed OH groups make up about 40% of the observed OH band in the IR, the rest are due to an atmospheric hydroxylation of the sample. Only about 2% of the incident ions lead to OH bond formation and, at most, the irradiated silicates retain about 3% of the incident protons as OH groups within their structure. Conclusions. Our laboratory experimental simulations show a possible production and storage of hydroxyl radicals in amorphous laboratory silicates. In the astrophysical context, such OH radicals, strongly bonded to pre-accretion material, could constitute a non negligible reservoir of -OH, thus water. These experimental results allow us to revisit and reinstate the hypothesis of a possible “wet” accretion of the telluric planets early in the history of the formation of the Solar System.

Ferromagnetic inclusions in silicate thin films: insights into the magnetic properties of cosmic grains

Context. We recently reported the formation of metallic inclusions in an amorphous and/or crystalline silicate matrix by thermal annealing of thin films in reducing atmospheres. Experimentally, the obtained microstructures closely resemble those of the glass with embedded metal and sulphides (GEMS) found in chondritic porous interplanetary dust particles (CP IDPs). We present here the magnetic properties of these synthetic samples. Aims. In this paper we report the detection and measurements of single domain and super paramagnetic ferromagnetic inclusions (SD/SP) in annealed silicate thin films of composition analogous to interstellar silicates and discuss the implications for the alignment of cosmic grains in astrophysical environments, in the presence of weak magnetic fields. Methods. We investigate the magnetic properties of synthesized laboratory silicate samples by measuring their magnetization when subjected to a given magnetic field. The measurements were performed at different temperatures including those compatible with interstellar dust. Results. The high values of remanent magnetization at saturation obtained in this work suggest the ability of our samples to indefinitely maintain a significant magnetization which may contribute to their alignment in weak magnetic fields. Conclusions. From our laboratory experimental simulation we propose that interstellar grains contain iron in form of nm-sized metallic beads. This can explain the non-detection of iron in interstellar grains. These inclusions could play a role in the alignment of grains. We propose a possible scenario for the magnetization of the cosmic grains and give a minimum value for the magnetic susceptibilty for GEMS.