Dust Analogues Research Articles

High-Temperature Optical Constants of Dust Analogues for the Solar Nebula

The dust in protoplanetary disks is influenced by a lot of different processes. Besides others, heating processes are the most important ones: they change not only the physical and chemical properties of dust particles, but also their emission spectra. In order to compare observed infrared spectra of young stellar systems with laboratory data of hot (up to 700{\deg}C) circumstellar dust analogues, we investigate materials, which are important constituents of dust in protoplanetary disks. We calculated the optical constants by means of a simple Lorentzian oscillator fit and apply them to simulations of small-particle emission spectra in order to compare our results with real astronomical spectra of AGB-stars and protoplanetary disks.

Variation of the FIR/submm optical properties of silicate dust analogues at low temperature: implications for the observations of interstellar cold dust emission

We present here the latest results from experimental studies aiming at measuring the Mass Absorption Coefficient (MAC) of interstellar silicate dust analogues at low temperatures in the far infrared and submillimetre domains which are necessary for the analysis of astronomical data. These results show that above 100 μ m, the MAC of amorphous silicate nano-grains has a complex dependence with the grain temperature and with the wavelength. This has to be considered when interpreting astronomical observations to avoid errors on the derived parameters such as the dust mass and its physical and chemical properties.

Characterisation of martian soil simulants for the ExoMars rover testbed

The European Space Agency (ESA) ExoMars mission involves landing a rover on the surface of Mars on an exobiology mission to extend the search for life. The locomotion capabilities of the ExoMars rover will enable it to use its scientific instruments in a wide variety of locations. Before it is sent to Mars, this locomotion system must be tested and its performance limitations understood. To test the locomotion performance of the ExoMars rover, three martian regolith simulants were selected: a fine dust analogue, a fine Aeolian sand analogue, and a coarse sand analogue. To predict the performance of the ExoMars rover locomotion system in these three regolith simulants, it is necessary to measure some fundamental macroscopic properties of the materials: cohesion, friction angle, and various bearing capacity constants. This paper presents the tests conducted to determine these properties. During these tests, emphasis was placed on preparing the regolith simulants at different levels of density in order to evaluate its impact on the value of the parameters in particular. It was shown that compaction can influence the Bekker coefficients of pressure-sinkage. The shear properties are consistent with the critical state model at normal stresses similar to those of the ExoMars rover in all but one of the simulants, which showed behaviour more consistent with transitional soil behaviour. It is necessary to give due consideration to these variations to ensure a robust test regime is developed when testing the tractive ability of the ExoMars mobility system.

Measuring the optical properties of astrophysical dust analogues: instrumentation and methods

Dust is found throughout the universe and plays an important role for a wide range of astrophysical phenomena. In recent years, new IR facilities have provided powerful new data for understanding these phenomena. However, interpretation of these data is often complicated by a lack of complementary information about the optical properties of astronomically relevant materials. The Optical Properties of Astronomical Silicates with Infrared Techniques (OPASI-T) program at NASA's Goddard Space Flight Center is designed to provide new high-quality laboratory data from which we can derive the optical properties of astrophysical dust analogues. This program makes use of multiple instruments, including new equipment designed and built specifically for this purpose. The suite of instruments allows us to derive optical properties over a wide wavelength range, from the near-IR through the millimeter, also providing the capability for exploring how these properties depend upon the temperature of the sample. In this paper, we discuss the overall structure of the research program, describe the new instruments that have been developed to meet the science goals, and demonstrate the efficacy of these tools.

Recent Results of Solid-State Spectroscopy

AbstractSolid state spectroscopy continues to be an important source of information on the mineralogical composition and physical properties of dust grains both in space and on planetary surfaces. With only a few exceptions, artificially produced or natural terrestrial analog materials, rather than ‘real’ cosmic dust grains, are the subject of solid state astrophysics. The Jena laboratory has provided a large number of data sets characterizing the UV, optical and infrared properties of such cosmic dust analogs. The present paper highlights recent developments and results achieved in this context, focussing on ‘non-standard conditions’ such as very low temperatures, very high temperatures and very long wavelengths.

Longitudinal variability of the biogeochemical role of Mediterranean aerosols in the Mediterranean Sea

Abstract. The Mediterranean Sea is a semi-enclosed basin characterized by a strong thermal stratification during summer during which the atmosphere is the main source of new nutrients to the nutrient-depleted surface layer. From aerosol sampling and microcosm experiments performed during the TransMed BOUM cruise (June–July 2008) we showed that: (i) the Mediterranean atmosphere composition (Al, Fe, P) was homogeneous over ~28° of longitude and was a mixture with a constant proportion of anthropogenic contribution and a variable but modest contribution of crustal aerosols. This quite stable composition over a one month period and a long transect (~2500 km) allowed to define the Mediterranean atmospheric "background" that characterizes the summer season in the absence of major Saharan event and forest fires, (ii) primary production significantly increased at all tested stations after aerosols addition collected on-board and after Saharan dust analog addition, indicating that both additions relieved on-going (co)-limitations. Although both additions significantly increased the N2 fixation rates at the western station, diazotrophic activity remained very low (~0.2 nmol N L−1 d−1), (iii) due to the presence of anthropogenic particles, the probable higher solubility of nutrients associated with mixed aerosols (crustal + anthropogenic contribution), conferred a higher fertilizing potential to on-board collected aerosol as compared to Saharan dust analog. Finally, those experiments showed that atmospheric inputs from a mixed atmospheric event ("summer rain" type) or from a high-intensity Saharan event would induce comparable response by the biota in the stratified Mediterranean SML, during summer.

A sharp change in the mineralogy of annealed protoplanetary dust at the glass transition temperature

The crystallinity of silicate dust detected in protoplanetary disks contrasts with the dominantly amorphous nature of dust in the interstellar medium. The amorphous-to-crystal transition is therefore a valuable probe to constrain physical properties of disks such as temperature gradients or the extent of radial mixing. However, it requires a comprehensive knowledge of the behaviour of amorphous Mg silicates during thermal processing. In this respect, amorphous analogues of enstatite composition (MgSiO3) were thermally annealed around the glass transition temperature (Tg ∼ 1040 K). We show that enstatite is not produced below Tg. Instead, the annealing leads to a mineralogical assemblage dominated by forsterite (Mg2SiO4). A sharp change is observed at Tg, and pyroxene becomes the dominant mineral species. The annealing conditions marginally change the mineralogical assemblages produced above Tg. A notable exception is the recovery of protoenstatite (the high-temperature polymorph), which appears to be easily quenched in dust analogues. Detection of this phase would then provide an excellent probe for fast thermal events in disks. Our data suggest that below ∼1000 K, the mineralogy of silicate dust of solar composition should naturally be dominated by olivine while above this temperature pyroxenes should dominate. They also show that silica polymorphs recently detected in cold regions of disks are the natural by-products of the formation of forsterite out of enstatite precursor. The low-temperature crystallization pathway reported, combined with the evaporation/condensation process, captures the essential features of the zoned mineralogy of protoplanetary disks.

Soil simulant sourcing for the ExoMars rover testbed

ExoMars is the European Space Agency (ESA) mission to Mars planned for launch in 2018, focusing on exobiology with the primary objective of searching for any traces of extant or extinct carbon-based micro-organisms. The on-surface mission is performed by a near-autonomous mobile robotic vehicle (also referred to as the rover) with a mission design life of 180 sols (Patel et al., 2010). In order to obtain useful data on the tractive performance of the ExoMars rover before flight, it is necessary to perform mobility tests on representative soil simulant materials producing a Martian terrain analogue under terrestrial laboratory conditions. Three individual types of regolith shown to be found extensively on the Martian surface were identified for replication using commercially available terrestrial materials, sourced from UK sites in order to ensure easy supply and reduce lead times for delivery. These materials (also referred to as the Engineering Soil (ES-x) simulants) are: a fine dust analogue (ES-1); a fine aeolian sand analogue (ES-2); and a coarse sand analogue (ES-3). Following a detailed analysis, three fine sand regolith types were identified from commercially available products. Each material was used in its off-the-shelf state, except for ES-2, where further processing methods were used to reduce the particle size range. These materials were tested to determine their physical characteristics, including the particle size distribution, particle density, particle shape (including angularity/sphericity) and moisture content. The results are analysed to allow comparative analysis with existing soil simulants and the published results regarding in situ analysis of Martian soil on previous NASA (National Aeronautics and Space Administration) missions. The findings have shown that in some cases material properties vary significantly from the specifications provided by material suppliers. This has confirmed the need for laboratory testing to determine the actual parameters to prove that standard geotechnical processes are indeed suitable. The outcomes have allowed the confirmation of each simulant material as suitable for replicating their respective regolith types.

A photo-chemical method for the production of olivine nanoparticles as cosmic dust analogues

This paper describes a new experimental method to synthesise metal silicate particles in the laboratory with compositions and structures which reflect those likely to form in planetary atmospheres and in relatively cool regions of oxygen-rich stellar outflows. Fe–Mg-silicate nanoparticles of olivine composition were produced by the photo-oxidation of a mixture of Fe(CO)5, Mg(OC2H5)2 and Si(OC2H5)4 vapours in the presence of O3 at room temperature and atmospheric pressure. Transmission electron microscope X-ray and electron energy loss analysis of the particles from a number of experiments run with different precursor vapour mixture ratios show that Mg2xFe2−2xSiO4 particles can be produced ranging from x=0 to 1, where x is linearly proportional to the ratio of Mg(OC2H5)2/(Fe(CO)5+Mg(OC2H5)2). Electronic structure calculations with hybrid density functional/Hartree–Fock theory are then used to explore the pathways involved in producing olivine particles from the FeO3, MgO3 and SiO2 produced from the photolysis of the organometallic precursors in O3. These calculations indicate that highly exothermic reactions lead to the formation of Mg2SiO4, MgFeSiO4 and Fe2SiO4 molecules, which then polymerize. An alternative pathway, also strongly favoured thermodynamically, is the polymerization of MgSiO3 and FeSiO3 to form pyroxenes, which then undergo structural rearrangement to olivine and silica phases. The implications for metal silicate formation in planetary atmosphere and stellar outflows are then discussed.

Observations of Interstellar Carbon Compounds

Infrared absorption and emission features observed spectroscopically in our Galaxy allow to probe the composition of solid dust grains, their evolution and thus follow the cycling of matter in the Galaxy. Many observables do reveal the presence of large amounts of carbonaceous particles in space, other than the PAH-like emission lines. The carbonaceous materials observed include amorphous carbons, diamondoids showing in emission for a few specific sources, and the recently detected fullerenes. An important hydrogenated amorphous carbon component (HAC or a-C:H), traced by the 2940 cm-1 structured absorption feature is observed against Galactic background sources. Since the discovery of this feature in the early eighties (Allen ), the observation of a-C:H has been extended to the mid-infrared by space observatories, giving insight into additional associated features. They are also observed in external galaxies, showing the ubiquitous nature of these components. We will focus on astronomical observations of organic matter other than PAHs, amorphous carbons and associated laboratory dust analogues relevant to astrophysical applications.

Formation and Evolution of Circumstellar and Interstellar PAHs: A Laboratory Study

Studies of dust analogs formed from hydrocarbon ( C H 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 ) and PAH precursors have been performed using a new facility that we have developed to simulate interstellar and circumstellar processes. The species formed in a plasma are detected, characterized and monitored in situ with high-sensitivity techniques, which provide both spectroscopic and ion mass information. From these measurements we derive information on the nature, the size and the structure of dust particles, as well as a better understanding of the growth and destruction processes of extraterrestrial dust.

Laboratory Astrophysics of Dust

Infrared spectroscopy is the best astronomical tool for studying the composition of cosmic dust. Thanks to the Herschel satellite, dust properties from the FIR to mm wavelength range will be sampled in different astrophysical environments. In the laboratory, the study of the temperature and structural dependence of FIR absorption of cosmic dust analogs including agglomeration is essential to interpret observational spectra. For crystalline materials, FIR single phonon bands are temperature dependent due to the anharmonicity of the vibrational potentials. This strong temperature dependence of the FIR bands’ positions can be used as a thermometer of the dust temperature. In amorphous material, the FIR absorption is dominated by disorder-induced single phonon processes and in the submillimeter and millimeter range by highly temperature-dependent low energy processes, e.g. tunneling transitions in two-level systems. The effect of these processes on the FIR absorptivity in amorphous silicates will be demonstrated.

INTERSTELLAR ANALOGS FROM DEFECTIVE CARBON NANOSTRUCTURES ACCOUNT FOR INTERSTELLAR EXTINCTION

Because interstellar dust is closely related to the evolution of matter in the galactic environment and many other astrophysical phenomena, the laboratory synthesis of interstellar dust analogs has received significant attention over the past decade. To simulate the ultraviolet (UV) interstellar extinction feature at 217.5 nm originating from carbonaceous interstellar dust, many reports focused on the UV absorption properties of laboratory-synthesized interstellar dust analogs. However, no general relation has been established between UV interstellar extinction and artificial interstellar dust analogs. Here, we show that defective carbon nanostructures prepared by high-energy collisions exhibit a UV absorption feature at 220 nm which we suggest accounts for the UV interstellar extinction at 217.5 nm. The morphology of some carbon nanostructures is similar to that of nanocarbons discovered in the Allende meteorite. The similarity between the absorption feature of the defective carbon nanostructures and UV interstellar extinction indicates a strong correlation between the defective carbon nanostructures and interstellar dust.

Large clean mesocosms and simulated dust deposition: a new methodology to investigate responses of marine oligotrophic ecosystems to atmospheric inputs

Abstract. Intense Saharan dust deposition occurs over large oligotrophic areas in the Mediterranean Sea and in the Tropical Atlantic, and its impact on the biogeochemical functioning of such oligotrophic ecosystems needs to be understood. However, due to the logistical difficulties of investigating in situ natural dust events, and due to the inherent limitations of microcosm laboratory experiments, new experimental approaches need to be developed. In this paper, we present a new experimental setup based on large, clean mesocoms deployed in the frame of the DUNE (a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem) project. We demonstrate that these tools are highly relevant and provide a powerful new strategy to in situ studies of the response of an oligotrophic ecosystem to chemical forcing by atmospheric deposition of African dust. First, we describe how to cope with the large amount of dust aerosol needed to conduct the seeding experiments by producing an analogue from soil collected in a source area and by performing subsequent appropriate physico-chemical treatments in the laboratory, including an eventual processing by simulated cloud water. The comparison of the physico-chemical characteristics of produced dust analogues with the literature confirms that our experimental simulations are representative of dust, aging during atmospheric transport, and subsequent deposition to the Mediterranean. Second, we demonstrate the feasibility in coastal areas to installing, in situ, a series of large (6 × 52 m3) mesocosms without perturbing the local ecosystem. The setup, containing no metallic parts and with the least possible induced perturbation during the sampling sequence, provides an approach for working with the required conditions for biogeochemical studies in oligotrophic environments, where nutrient and micronutrients are at nano- or subnano-molar levels. Two, distinct "seeding experiments" were conducted by deploying three mesocosms serving as controls (CONTROLS-Meso = no addition) and three mesocosms seeded with the same amount of Saharan dust (DUST-Meso = 10 g m−2 of sprayed dust). A large panel of biogeochemical parameters was measured at 0.1 m, at 5 m and 10 m in all of the mesocosms and at a selected site outside the mesocosms before seeding and at regular intervals afterward. Statistical analyses of the results show that data from three mesocosms that received the same treatment are highly reproducible (variability < 30%) and that there is no significant difference between data obtained from CONTROLS-Meso and data obtained outside the mesocosms. This paper demonstrates that the methodology developed in the DUNE project is suitable to quantifying and parameterizing the impact of atmospheric chemical forcing in a low-nutrient, low-chlorophyll (LNLC) ecosystem. Such large mesocosms can be considered as 1-D ecosystems so that the parameterization obtained from these experiments can be integrated into ecosystem models.

Photoluminescence of hydrogenated amorphous carbons

Context. Hydrogenated amorphous carbons (a-C:H or HAC) have proved to be excellent analogs of interstellar dust observed in galaxies diffuse interstellar medium (DISM) through infrared vibrational absorption bands (3.4 μ m, 6.8 μ m, and 7.2 μ m bands). They exhibit photoluminescence (PL) after excitation by UV-visible photons, and are possible carriers for the extended red emission (ERE), a broad red emission band observed in various interstellar environments.Aims. As many candidate materials/molecules can photoluminesce in the visible, along with the carrier abundance, the PL efficiency represents one of the strongest constraints set by such ERE observations. We wish to precisely characterize the PL behavior of a-C:H as a family of materials.Methods. The a-C:H samples are produced in the form of films deposited on substrates by plasma-enhanced chemical vapor deposition. The produced films were analyzed in transmission by UV-visible and IR spectroscopy, and the wavelength dependent PL spectra were recorded. The intrinsic absolute quantum yield η was then rigorously calculated taking self-absorption of the PL by the film and interfaces effects into account.Results. A wide range of different laboratory synthesized a-C:H were analyzed. Their PL properties are dependent on the optical gap : when decreases from 4.3 eV to 2.8 eV, the a-C:H vary from highly () yellow photoluminescent soft materials to hard materials that emit a wider PL band in the red spectral range, with a lower efficiency (η ~ 0.01–0.1%). For any given a-C:H, the PL characteristics (central wavelength, band width and efficiency) are found to be essentially constant over the explored excitation range ( 250 nm). We compared the characteristics of the produced interstellar dust analog to the constraints imposed by the ERE observations.Conclusions. As for ERE observations, PL efficiencies and band widths of a-C:H are both correlated to the PL central wavelengths. The excitation responsible for the a-C:H emission is efficient over a wide spectral range that matches the ERE excitation. The present a-C:H encounter difficulties for the diffuse ISM ERE observations (%) in simultaneously satisfying the high quantum yield criteria and PL spectral characteristics. We still need to investigate the role of a small number of residual oxygen atoms in the laboratory-produced a-C:H network in quenching the PL yield, as well as to consider the interstellar temperature effect for our analogs.

Optical properties of cosmic dust analogs: a review

Nanometer- and micrometer-sized solid particles play an important role in the evolutionary cycle of stars and interstellar matter. The optical properties of cosmic grains determine the interaction of the radiation field with the solids, thereby regulating the temperature structure and spectral appearance of dusty regions. Radiation pressure on dust grains and their collisions with the gas atoms and molecules can drive powerful winds. The analysis of observed spectral features, especially in the infrared wavelength range, provides important information on grain size, composition and structure as well as temperature and spatial distribution of the material. The relevant optical data for interstellar, circumstellar, and protoplanetary grains can be obtained by measurements on cosmic dust analogs in the laboratory or can be calculated from grain models based on optical constants. Both approaches have made progress in the last years, triggered by the need to interpret increasingly detailed high-quality astronomical observations. The statistical theoretical approach, spectroscopic experiments at variable temperature and absorption spectroscopy of aerosol particulates play an important role for the successful application of the data in dust astrophysics.

Interpretation of Wild 2 dust fine structure: Comparison of Stardust aluminum foil craters to the three‐dimensional shape of experimental impacts by artificial aggregate particles and meteorite powders

Abstract— New experimental results show that Stardust crater morphology is consistent with interpretation of many larger Wild 2 dust grains being aggregates, albeit most of low porosity and therefore relatively high density. The majority of large Stardust grains (i.e. those carrying most of the cometary dust mass) probably had density of 2.4 g cm−3 (similar to soda‐lime glass used in earlier calibration experiments) or greater, and porosity of 25% or less, akin to consolidated carbonaceous chondrite meteorites, and much lower than the 80% suggested for fractal dust aggregates. Although better size calibration is required for interpretation of the very smallest impacting grains, we suggest that aggregates could have dense components dominated by μm‐scale and smaller sub‐grains. If porosity of the Wild 2 nucleus is high, with similar bulk density to other comets, much of the pore space may be at a scale of tens of micrometers, between coarser, denser grains.Successful demonstration of aggregate projectile impacts in the laboratory now opens the possibility of experiments to further constrain the conditions for creation of bulbous (Type C) tracks in aerogel, which we have observed in recent shots. We are also using mixed mineral aggregates to document differential survival of pristine composition and crystalline structure in diverse finegrained components of aggregate cometary dust analogues, impacted onto both foil and aerogel under Stardust encounter conditions.

Characterization of circumstellar carbonaceous dust analogues produced by pyrolysis of acetylene in a porous graphite reactor

Carbon particles synthesized by acetylene pyrolysis in a porous graphite reactor have been investigated. The intimate chemical and physical structures of the particles were probed by proton nuclear magnetic resonance spectroscopy, infrared Fourier transform spectroscopy and X-ray diffraction. The analysis points towards a chemical structure composed of soluble low-mass aromatics surrounding small insoluble larger aromatic islands bridged by aliphatic groups. The diffraction profile indicates that the particles are mostly amorphous with small crystalline domains of ∼6.5 Å composed of a few stacked graphene layers. The properties of these particles are compared with these obtained with other types of production methods such as laser pyrolysis and combustion flames. The results are briefly discussed in the context of the evolution of infrared interstellar emitters. Possible uses of the reactor are proposed.

Laboratory studies of cosmic carbon dust analogs

Laboratory measurements aimed at studying the properties of carbonaceous materials and their evolution under simulated space conditions support the idea that interstellar carbon dust evolves through exposure to UV and cosmic rays, gas and heat. After summarizing the principal aspects of the laboratory approach to the problem of cosmic carbon dust, we discuss recent laboratory results showing how carbon grains influence the properties and evolution of ices mantles and the formation of molecular hydrogen.

The H 2O and CO 2 adsorption properties of phyllosilicate-poor palagonitic dust and smectites under martian environmental conditions

The recent detection of up to ∼10 wt% water-equivalent H heterogeneously distributed in the upper meter of the equatorial regions of the martian surface and the presence of the 3-μm hydrations feature across the entire planet raises the question whether martian surficial dust can account for this water-equivalent H. We have investigated the H 2O and CO 2 adsorption properties of palagonitic dust (<5 μm size fraction of phyllosilicate-poor palagonitic tephra HWMK919) as a martian dust analog and two smectites under simulated martian equatorial surface conditions. Our results show that the palagonitic dust, which contains hydrated and hydroxylated volcanic glass of basaltic composition, accommodates significantly more H 2O under comparable humidity and temperature conditions than do the smectites nontronite and montmorillonite.