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Scienze Fisiche, Universita di Napoli "Federico II", Mostra d'Oltremare pad. 19, 1-80125 Napoli, ..... temperature is controlled by an electronic device connected to a temperature .... For the FORA sample signatures of a crystalline component ...
Meteoritics & Planetary Science 37, 1623-1635 (2002) Available online at http://www.uark.edu/meteor

Production, processing and characterization techniques for cosmic dust analogues A. ROTUNDII*, J. R. BRUCAT02, L. COLANGELI2, G. FERRINI2,3, V. MENNELLA2, E. PALOMBA2 AND P. PALUMBOI llst, di Mat., Fis. e Appl., Universita "Parthenope", Via A. De Gasperi 5, 1-80133 Napoli, Italy 2INAF, Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 1-80 131 Napoli, Italy 3Dip. Scienze Fisiche, Universita di Napoli "Federico II", Mostra d'Oltremare pad. 19, 1-80125 Napoli, Italy *Correspondence author's e-mail address:[email protected]

(Received 2002 July 6; accepted in revised form 2002 September 12) (Presented at "Laboratory Simulations ofCircumstellar Dust Analogs: Expectationsfor Comet Nucleus Encounters", a special session ofthe 64th annual Meteoritical Society meeting, Vatican City, 2001 September 13)

Abstract-The laboratory analyses of cosmic dust analogues-that in the context ofthis paper include interstellar, circumstellar as well as cometary dust-have a critical role in the study of circumstellar and cometary dust. The morphological, structural and chemical characterization of these analogues are critical for comparisons oftheir infrared and ultraviolet spectra with those obtained by astronomical observations, as well as for modeling purposes. Besides, the results from these laboratory studies are important to the success of space missions to comets when testing and calibrating the payload instruments. The interpretations of returned scientific data would benefit from the comparison with data recorded by the instruments in a laboratory setting for different classes ofpreviously characterized analogues. We produced various types ofcondensed samples: (1) Mg,Fe-silicates, (olivine, pyroxene), (2) carbon-rich dust, and (3) mixed carbon-silicate dust. The samples were prepared using different techniques, viz. (1) laser bombardment of solid targets in an Ar and 02 atmosphere, (2) arc discharge in an Ar and H2 atmosphere, and (3) grinding powders of natural minerals. We simulated various post-condensation processes, such as thermal annealing, ultraviolet irradiation, ion bombardment and exposure to atomic hydrogen. These processes produced compound samples of a wide range of physico-chemical properties. To identify their textures, morphologies, grain compositions and crystallographic properties we used electron microscopy and far-ultraviolet to far-infrared (millimeter range) spectroscopy. INTRODUCTION

Cometary dust includes condensed volatiles consisting of carbon-hydrogen-oxygen-nitrogen (CHON) in mixed silicateCHON particles (Kissel and Krueger, 1987; Jessberger et al., 1988; Fomenkova et al., 1992, 1994). The grains occur in more or less fluffy aggregates with variable proportions of the silicate and organic components, which could be amorphous or crystalline, structurally-mixed, complex dust ranging from pure silicate to pure CHON particles. Comets formed by accreting materials present in a protoplanetary nebula that included silicate dusts, carbonaceous materials and volatile compounds and molecules. These materials are also present in various astronomical environments, such as circumstellar disks and in the interstellar medium. Crystalline or amorphous grains formed depending on the temperature of their environment. Spectral analyses ofcircumstellar dust show that crystalline grains are observed only around evolved stars with high mass-loss rates, whereas low mass-loss rate stars do not

exhibits any crystalline bands (Cami et al., 1997; Sylvester et al., 1999). Annealing of amorphous silicate dust in stellar outflows will result in a series of kinetically controlled reactions. They will eventually lead to the formation of crystalline silicates, providing that the grains remained "hot" for a sufficiently long time (Brucato et al., 1999a; Fabian et al., 2000; Hallenbeck et al., 1998, 2000; Rietmeijer et al., 2002a,b). One would expect the observation ofsome crystalline silicate features in the interstellar medium but no reliable evidence for this has been found yet (Lutz et al., 1996; Demyk et al., 1999). Following the current opinion that silicates coming from the pre-solar cloud and in-falling onto the protosolar nebula were initially amorphous, successive thermal processing at high temperatures would have been necessary to induce their crystallization. Comets were generally considered as objects wherein the pristine amorphous presolar material was preserved. But recently the infrared space observatory (ISO) data showed evidence for the presence of crystalline silicates in at least some Oort cloud comets (Crovisier et al.,

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1997). It suggests that cometary grains experienced thermal processing at some time during their pre-cometary residence. Two possible scenarios could explain the presence ofcrystalline grains in comets, viz. turbulent radial mixing in the solar nebula (Bockelee-Morvan et al., 2002; Nuth et al., 2000) and annealing of dust by nebular shocks (Harker and Desch, 2002). During comet formation, volatiles present in the proto-solar nebula may have been removed and/or reprocessed (Nuth et al., 2000). However, once arrived in the outer region ofthe accretion disk, pre-cometary grains may have coagulated into porous particles that become the sites where fresh volatiles could condense. Grain coagulation built up large cometesimals during the energetic T-Tauri phase of the young Sun. The high flux of photons and particles could have modified the nature of ices and silicate cores forming new molecular species (Strazzulla and Baratta, 1992). Important aspects of cometary dust origin and evolution can be deduced by astronomical observations, by theoretical modeling and by direct investigation of dust with cometary origin. In fact, some fraction of chondritic interplanetary dust particles (IDPs) have a cometary origin and probably contain primordial matter from our planetary system (Rietmeijer, 1998 (and references therein), 2002). Thus, studies ofIDPs that are collected in the stratosphere by means of airborne inertialimpact, flat-plate collectors (Brownlee, 1978) are of critical importance for comet science. The near-Earth orbit environment offers another possibility for the collection of meteoroids of different origins. In the last few years a number ofdust capture experiments were carried out in low-Earth orbit for example onboard the Russian MIR space station (e.g., Westphal et al., 1997) and the long duration exposure facility (Zolensky et al., 1994). The inventory of the extraterrestrial material available presently will be soon enriched by in situ collected dust from small solar system bodies, by sample return missions such as Stardust (Brownlee et al., 2000) and MusesC (Fujiwara et al., 1999). A comprehensive overview of the available analytical techniques, used in experiments on "nanosamples", is given in Zolensky et al. (2000). Laboratory activity devoted to simulate cosmic dust formation and evolution in different space environments represent a concrete tool to improve our knowledge of comets. Presently, a number of laboratories are conducting cosmic silicate and carbon dust analogues condensation experiments (e.g., Nuth et al., 2002; Fabian et al., 2000; Blanco et al., 1996) using similar but also different techniques than those reported in this paper. There are basically two different methods to generate the vapours in these experiments, viz. (1) the instantaneous evaporation of stoichiometric crystalline materials (this work; Fabian et al., 2000) and (2) pre-mixing metal-oxide vapours (Nuth et al., 1999). It seems that the method used to generate the initial vapour might have an impact on the results, which is an interesting topic to be addressed in futureresearch. For example, the smokes prepared by the second method report the condensation of individual, amorphous

nanograins with predictable metastable eutectic compositions (Nuth et al., 2002; Rietmeijer et al., 2002b). Condensed smokes prepared by the first method have generally compositions that closely mimic the metal-oxide ratios of the target materials, such as described in this paper. The production and characterization in the laboratory of cometary dust analogues (CDA) is also crucial to develop space missions directed towards comets. To this aim we produce and process CDA to simulate the building blocks of comet nuclei in different phases of their evolution in a step by step procedure emphasizing the composition and structure of cometary dust including various pure silicates and carbon-based materials. These dust analogues can be amorphous, crystalline and partially crystallized. We then study the compositions and structures of mixed analogues made of these silicates and carbon-rich materials. To account for cometary material crystallization, due to thermal annealing during the perihelion passage, and/or structural changes due to cosmic phenomena like ultraviolet and ion bombardment, we apply the following processing techniques: thermal annealing, ultraviolet irradiation, ion bombardment and exposure to atomic hydrogen. An accurate characterization of each kind of the produced materials is performed using scanning electron microscope (SEM), energy dispersive x-ray (EDX) analysis, spectroscopy ranging from the far ultraviolet into the millimeter range. The main aim is to gain critical knowledgeon increasingly complex, but well-characterized, materials that will be realistic analogous of cometary dust as defined by models developed on the basis of results from (1) previous space missions (e.g., GIOTTO Halley mission; ISO), (2) ground-based observations, and (3) laboratory analyses oflDPs.

EXPERIMENTAL In Table 1 we report the samples produced and studied so far. They are grouped in three main classes of materials (i. e., silicates, carbon-based and mixed carbon-silicates) that each required a specific production technique and post-production processing and analyses. The silicates we used are natural mineral that present low percentages of impurities detected during the chemical analysis that do not interfere with the spectral properties. We labeled the samples with names ofthe corresponding pure mineral. Analogue morphology was characterized using a Stereoscan 360-Cambridge field emission scanning electron microscope (FE-SEM), operating at a maximum accelerating voltage of 25 KeV and with a nominal spatial resolution of 2nm. To analyze the chemical composition ofthe samples we used an EDX detection system attached to the FE-SEM that is capable to detect elements down to Be. For the silicates quantitative EDX data reduction is made following Papike (1987). Since the laser-ablation technique (see "Silicate Dust Analogues: Condensed Samples") might alter the composition during evaporation and condensation, the chemical composition of the condensed

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TABLE 1. List of the samples produced with the relative production and processing technique and a brief description of the resulting sample. Sample

Production

Silicates

Augite Enstatite Fayalite Forsterite Goethite Hematite Ilmenite Jadeite Kaolinite Montmorillonite Opal

Grind, sieve and sediment minerals

Crystalline size selected

Enstatite Fayalite Forsterite

Laser vaporization

Condensate of amorphous grains

Enstatite Forsterite

Laser vaporization

Carbon soot

Laser vaporization in Ar atmosphere

Condensate of amorphous grains

Arc discharge in Ar atmosphere

Condensate of amorphous and crystalline grains

Arc discharge in H2 atmosphere

Condensate of amorphous and crystalline grains

Carbon based

Mixed

Processing

Crystallographic properties

Class

Thermal annealing

Partially crystallized condensed grains

Laser vaporization in Ar atmosphere

Thermal annealing

Amorphous condensates and crystallized grains

Arc discharge in Ar atmosphere

Thermal annealing

Amorphous condensates and crystallized grains

Arc discharge in H2 atmosphere

Thermal annealing Ultraviolet irradiation Ion bombardment Exposure to H 2

Amorphous condensates and crystallized grains

Forsterite plus carbon Fayalite plus carbon

Laser vaporization

Forsterite plus carbon

Laser vaporization

silicates needs to be determined. Chemical analysis is also necessary to characterize the mineral stoichiometry or the nonstoichiometry of amorphous grains produced in the condensation experiments. To study the optical behavior, structure and composition of the samples, infrared spectroscopy has been applied. Transmission spectra in the range 2.5-25 /-lm have been obtained at spectral resolution of2 cm- 1 by means ofan infrared Fourier transform interferometer (Bruker, Mod. IFS 66V) for all produced and processed samples.

Condensate of amorphous grains

Thermal annealing

Amorphous condensates and crystallized grains

Sample Preparation-For FE-SEM characterization of sample morphology and size distributions, the samples are dispersed on, or directly deposited onto, a smooth silicon wafer chip designed for high-resolution SEM analysis. For EDX analyses the samples are placed on a smooth carbon pin-stub or smooth silicon wafer for the silicate and carbon-based samples, respectively. Few hundreds of micrograms of dust analogue sample are embedded in KBr matrix by using the standard pellets technique suitable for infrared spectroscopy, whereas they are dispersed or directly collected on ultraviolet

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grade fused silica substrates for ultraviolet spectral analysis. For thermal processing, the most suitable substrate to use was the smooth silicon wafer chip. Silicate Dust Analogues Natural Single Crystals-Fragments of natural silicate minerals are grounded in agate mills for pre-selected times. The grain size is selected using a sieving apparatus for size ranges of 0-20, 20-50, 50-100, 100-150 and 150-200 ust: For grain sizes