Laboratory simulation of cometary dust collection and analysis

An experiment for the in situ analysis of cometary dust grains during a rendezvous mission to a comet consists of three elements: (1) Substrate preparation, i.e. cleaning of the substrates in order to reduce the background contamination, (2) dust collection and (3) chemical analysis. All three elements have been simulated in a laboratory experiment. Combined heat treatment (up to 200°C) and surface sputtering of the gold substrates by a glow discharge reduced the surface contamination of Na, Al and K by a factor of 1000 and that of all other contaminants below the detection threshold. During the sputtering of the substrates they acquired a surface roughness of ～5 μm, which improved their collection efficiency for dust particles. Dust particles were shot onto those substrates at speeds up to 200 m s^?1.Chemical analysis using secondary ion mass spectroscopy (SIMS) provided information on elemental abundances, the molecular composition and isotopic ratios of selected elements even at a surface dust coverage of 10^?2–10^?4. Both technical details of the new equipment and results of the first investigation on target surface cleaning and SIMS analysis of dust particles will be reported.     

Correlated microanalysis of cometary organic grains returned by Stardust

Abstract– Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X‐ray absorption near‐edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen‐rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl‐containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule‐like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.     

Composition of cometary dust: The case against silicates

It is argued that the infrared emission including 10 and 18 μ features observed in recent comets is unlikely to be due to silicates. The vaporization temperature of the emitting material ～500 K is consistent with emission by crystalline polyformaldehyde.     

Sources of cometary radicals and their jets: Gases or grains

The recent discovery of CN and C₂ gas jets in comet Halley has led to basic speculation as to their physical source mechanism. A basic quantitative study of the photosputtering of CHON grains and the spatial evolution of trace gas jets is presented here. Two possible single sources, a parent gas and CHON grains, for both the jet and the background gas, are also investigated. It is shown that a parent trace gas jet will remain focused out to distances as large as 10⁵ km from the nucleus and could provide a source for the observed radical jets. Conversely, photosputtering of small CHON grains by solar UV radiation can provide the source not only for cometary CN and C₂ but also possibly for inner coma C atoms and C⁺ ions. However, constraints on the size and/or morphology of the contributing grains themselves are found. Isotropic speed components comparable to the outflow speed are likely to be added to radicals upon production from either the CHON grain or the parent gas source and will yield a radical jet which becomes more diffuse with increasing distance from the nucleus. However, in neither case will the radical jet completely isotropicize; it will be confined generally to a quadrant as projected on the sky plane. Observational tests which can be made once the large set of in situ and remote observations have been analyzed are suggested to distinguish between the two scenarios.     

Ionization of interstellar matter by suprathermal dust grains

Suprathermal dust grains as suggested by Wickramasinghe produce electrons of energies not higher than 20 eV by Coulomb collisions with free electrons in an interstellar medium. These electrons are responsible for the production of singly ionized ions but not effective for that of highly ionized ones. This explains a general feature of the composition of atoms and ions as observed from the Copernicus satellite.     

Production of metagalactic X-rays by relativistic dust grains

The relativistic dust grains which may be responsible for ultra-high energy cosmic rays, as suggested by the present author, interact with the cosmic black-body radiation. This results in the energy loss of the relativistic dust grains, so that their energy spectrum is cut-off at the Lorentz factor as large as 2×10³ (0.1/a), wherea is the grain radius. The black-body radiation is scattered and absorbed by the dust grains. The photons scattered and reemitted contribute to metagalactic X-rays. The X-ray intensity estimated is comparable to the observed one in the soft X-ray region.     

Condensation processes in astrophysical environments: The composition and structure of cometary grains

Abstract— We review the results of our recent experimental studies of astrophysical dust analogs. We discuss the condensation of amorphous silicates from mixed metal vapors, including evidence that such condensates form with metastable eutectic compositions. We consider the spectral evolution of amorphous magnesium silicate condensates as a function of time and temperature. Magnesium silicate smokes anneal readily at temperatures of about 1000–1100 K. In contrast we find that iron silicates require much higher temperatures (?1300 K) to bring about similar changes on the same timescale (days to months). We first apply these results to infrared space observatory observations of crystalline magnesium silicate grains around high‐mass‐outflow asymptotic giant branch stars in order to demonstrate their general utility in a rather simple environment. Finally, we apply these experimental results to infrared observations of comets and protostars in order to derive some interesting conclusions regarding large‐scale nebular dynamics, the natural production of organic molecules in protostellar nebulae, and the use of crystalline magnesium silicates as a relative indicator of a comet's formation age.     

The effect of a sector boundary crossing on the cometary dust tail

Recognizing that grains in the cometary dust tail are electrically charged, we study the effect of an interplanetary sector boundary crossing on their distribution. We specifically consider Halley's comet around the time of encounter by the GIOTTO and VEGA 1 and 2 spacecrafts in March 1986. The smallest dust particles (r _g0.3 m) are strongly effected, and the projection of their distributions in a plane containing the Sun-Comet axis and normal to the orbital plane show a wavy appearance. Also, since reversals in the interplanetary magnetic field occur with a periodicity of 5 to 10 days, the spacecrafts, which follow 3 to 4 days apart are likely to encounter entirely different dust distributions at the lower end of the mass spectrum.     

The expulsion and retention of dust grains by galactic discs

We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 10⁹ yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium.     

PILOT: a balloon-borne experiment to measure the polarized FIR emission of dust grains in the interstellar medium

Future cosmology space missions will concentrate on measuring the polarization of the Cosmic Microwave Background, which potentially carries invaluable information about the earliest phases of the evolution of our universe. Such ambitious projects will ultimately be limited by the sensitivity of the instrument and by the accuracy at which polarized foreground emission from our own Galaxy can be subtracted out. We present the PILOT balloon project, which aims at characterizing one of these foreground sources, the polarized continuum emission by dust in the diffuse interstellar medium. The PILOT experiment also constitutes a test-bed for using multiplexed bolometer arrays for polarization measurements. This paper presents the instrument and its expected performances. Performance measured during ground calibrations of the instrument and in flight will be described in a forthcoming paper.     

Dust in cometary comae: Present understanding of the structure and composition of dust particles

In situ probing of a very few cometary comae has shown that dust particles present a low albedo and a low density, and that they consist of both rocky material and refractory organics. Remote observations of solar light scattered by cometary dust provide information on the properties of dust particles in the coma of a larger set of comets. The observations of the linear polarization in the coma indicate that the dust particles are irregular, with a size greater (on the average) than about 1 μm. Besides, they suggest, through numerical and experimental simulations, that both compact grains and fluffy aggregates (with a power law of the size distribution in the −2.6 to −3 range), and both rather transparent silicates and absorbing organics are present in the coma. Recent analysis of the cometary dust samples collected by the Stardust mission provide a unique ground truth and confirm, for comet 81P/Wild 2, the results from remote sensing observations. Future space missions to comets should, in the next decade, lead to a more precise characterization of the structure and composition of cometary dust particles.     

FT-IR microspectroscopy of extraterrestrial dust grains: Comparison of measurement techniques

Identification of astronomical dust composition rests on comparison of Infrared (IR) spectra with standard laboratory spectra; frequently, however, a single mineralogical composition is assumed for spectral matching. Advances in laboratory instrumentation have enabled very precise IR spectra to be measured on single grains and zones within grains; with a more complete set of spectral data for planetary dust, better compositional matches will be achieved for astronomical dust. We have compared several FT-IR spectroscopy techniques (open path transmission spectroscopy and diffuse reflectance spectroscopy of powders; microspectroscopy of single grains and powders and ATR spectroscopy of thin sections) to determine their utility for the direct measurement of the mid-IR spectra of small amounts of extraterrestrial grains. We have focussed our investigation on the spectra of the olivine series of silicates, (Mg,Fe)₂SiO₄, a species frequently identified as one of the major constituents of interstellar dust. The positions of three characteristic SiO₄ stretching bands at ∼10.4, 11.3 and 12 μm were measured for comparison of the techniques. All methods gave satisfactory results, although care must be taken to guard against artefacts from sample thickness and orientation effects. Single grains hand-picked from meteorites can be analysed, but results are inaccurate if the grain size is too large (>1-10 μm). Spectra for single grains also show variations that arise from sample orientation effects. Once the analytical artefacts are taken into account, we found that measurement of powder with a diamond compression cell is best suited for the analysis of small amounts of materials.     

The size distribution of dust grains in single clouds – II. The analysis of extinction using inhomogeneous grains

The extinction curves of single clouds, seen towards the stars HD 147165, 179406 and 202904, have been modelled using various mixtures containing both the bare and inhomogeneous (composite and/or multilayer) grains. It has been shown that the models composed of the bare graphite and silicate grains together with the multilayer grains containing silicates, organic refractory and water ice, are more useful in explaining extinction under the reduced cosmic abundances. The models based on Mathis' composite grains or on Greenberg & Li's core-mantle grains can also provide quite good fits of the extinction and the measured scattering parameters, but still require an excessive amount of carbon which results in too large a C/O ratio. The inhomogeneous grains essentially contribute to the extinction in practically the whole wavelength range of our extinction curves. As a rule, such grains have quite wide size distributions, centred at around 100 nm, although graphite grains are mainly of small sizes.     

In situ imaging of μm and sub-μm-sized grains in a cometary environment by atomic force microscopy

The micro-imaging dust analysis system (MIDAS) is an essential element among the scientific payload on the international Rosetta mission to comet 46P/Wirtanen. The MIDAS instrument based on an atomic force microscope (AFM) collects small particles drifting outwards from the nucleus surface. AFM is able to image small structures in 3D at nanometer-scale resolution. These images provide morphological and statistical information like grain size distribution on the dust population. In order to support the development of the flight hardware, optimisation of the control functions and consolidation of a proper scheme of data interpretation, laboratory studies with similar instruments were carried out. The obtained data demonstrate the capabilities of this technique. For the first time an instrument is able to observe the smallest (nm-sized) grains which are predicted by models and were to a certain extent deduced from previous measurements on the Giotto and Vega missions to comet 1P/Halley. On larger (μm-sized) particles the complex morphology will be visualised with high precision in 3D, and if present, within these aggregates crystalline materials with defined crystal faces can be identified.     

Solar flare track densities in interplanetary dust particles: The determination of an asteroidal versus cometary source of the zodiacal dust cloud

Dust particles that are larger than 1 μm, when injected into the Solar System from comets and asteroids, will spiral into the Sun due to the Poynting-Robertson effect. During the process of spiraling in, such dust particles accumulate solar flare tracks in their component minerals. The accumulated track density for a given dust grain is a function of the duration of its space exposure and its distance from the Sun. Using a computer model, it was determined that the expected track density distributions from grains produced by comets are very different from those produced by asteroids. Individual asteroids produce populations of particles that arrive at 1 AU with scaled track density distributions containing “spikes,” while comets supply particles with a flatter and wider distribution of track densities. Particles with track densities above 3 × 10⁷ (sϱA/v) tracks/cm² have probably been exposed to solar flare tracks prior to injection into the interplanetary medium and are therefore likely to be asteroidal. Particles with track densities below 0.7 × 10⁷(sϱA/v) tracks/cm² must be derived from comets or Earth-crossing asteroids. Earth-crossing asteroids are not responsible for all the dust collected at 1 AU since they cannot produce the large track densities observed in some of the interplanetary dust particles collected in the stratosphere. The track densities observed in the stratospheric dust fall within the predicted range, but there is at present an insufficient number of carefully determined densities to make strong statements about the sources of the present dust population.     

Dust ring formation due to sublimation of dust grains drifting radially inward by the Poynting-Robertson drag: An analytical model

Dust particles exposed to the stellar radiation and wind drift radially inward by the Poynting-Robertson (P-R) drag and pile up at the zone where they begin to sublime substantially. The reason they pile up or form a ring is that their inward drifts due to the P-R drag are suppressed by stellar radiation pressure when the ratio of radiation pressure to stellar gravity on them increases during their sublimation phases. We present analytic solutions to the orbital and mass evolution of such subliming dust particles, and find their drift velocities at the pileup zone are almost independent of their initial semimajor axes and masses. We derive analytically an enhancement factor of the number density of the particles at the outer edge of the sublimation zone from the solutions. We show that the formula of the enhancement factor reproduces well numerical simulations in the previous studies. The enhancement factor for spherical dust particles of silicate and carbon extends from 3 to more than 20 at stellar luminosities L_?=0.8-500L_⊙, where L_⊙ is solar luminosity. Although the enhancement factor for fluffy dust particles is smaller than that for spherical particles, sublimating particles inevitably form a dust ring as long as their masses decrease faster than their surface areas during sublimation. The formulation is applicable to dust ring formation for arbitrary shape and material of dust in dust-debris disks as well as in the Solar System.     

Origin of cometary extended sources from degradation of refractory organics on grains: polyoxymethylene as formaldehyde parent molecule

The radial distribution of some molecules (CO, H₂CO, HNC, …) observed in the coma of some comets cannot be explained only by a direct sublimation from the nucleus, or by the photolysis of a detected parent compound. Such molecules present a so-called extended source in comae. We show in this paper that extended sources can be explained by refractory organic material slowly releasing gas from grains ejected from the cometary nucleus, due to solar UV photons or heat. The degradation products are produced throughout the coma and therefore are presenting an extended distribution. To model this multiphase chemistry we derive new equations, which are applied to Comet 1P/Halley for the case of the production of formaldehyde from polyoxymethylene (POM), the polymer of formaldehyde (-CH₂-O-)_n. We show that the presence of a few percent of POM on cometary grains (a nominal value of ∼4% in mass of grains is derived from our calculations) is in good agreement with the observed distribution, which so far were not interpreted by the presence of any gaseous parent molecule.     

Light scattering by moving dust grains in the immediate vicinity of young stars

We consider the problem of the distortion of the photospheric spectrum for a young star as its light is scattered in the inner accretion disk in the dust grain evaporation region. In T Tauri stars, this region is at a distance of the order of several stellar radii and is involved in the large-scale motions of matter with velocities of ～100 km s^?1 or higher. The light scattering in such a medium causes the frequency of the scattered radiation to be shifted due to the Doppler effect. We analyze the influence of this effect on the absorption line profiles in the spectra of T Tauri stars using classical results of the theory of radiative transfer. We consider two models of a scattering medium: (i) a homogeneous cylindrical surface and (ii) a cylindrical surface with an azimuth-dependent height (such conditions take place during the accretion of matter onto a star with an oblique magnetic dipole). We show that in the first case, the scattering of the photospheric radiation causes the absorption lines to broaden. If the motion of the circumstellar matter in the dust evaporation region is characterized by two velocity components, then the line profile of the scattered radiation is asymmetric, with the pattern of the asymmetry depending on the direction of the radial velocity. In the second case, the scattered radiation can cause periodic shifts of the absorption line centroid, which can be perceived by an observer as periodic radial-velocity variations in the star. We suggest that precisely this effect is responsible for the low-amplitude radial-velocity variations with periods close to the stellar rotation periods that have recently been found in some of the T Tauri stars.     

A model of cometary gas and dust production and nongravitational forces with application to P/Halley

A program that computes gas and dust production rates and idealized nongravitational force components has been developed and applied to the case of Comet Halley. We use a modified form of our earlier comet model (F.P. Fanale and J.R. Salvali[(1984) Icarus 60, 476–511] to which coma effects and a section on nongravitational forces have been added. The possibility of grain cohesion is also included. These models are used together with observations from 1910 and semiempirically derived data to investigate the effects of obliquity and thermal conductivity of the near thermal conductivity of the nucleus on gas and dust production. The results indicate that the thermal conductivity of the nucleus is of the order of 10⁵ ergs/cm-s-°K, which implies that the ice near the surface is in the crystalline form. A general method is presented for calculating the radii of cometary nuclei using theoretically derived and semiempirically derived nongravitational force components. This method is used to calculate possible radii for Comet Halley that depend on the model variation chosen. The method used and the results presented herein should have greater significance and value when the observational data from Halley's current perihelion passage become available.