J. Mayo Greenberg

Approaching the Interstellar Grain Organic Refractory Component

Infrared spectra have been obtained for laboratory residues of photoprocessed low-temperature ices which have been exposed to long-term solar ultraviolet radiation on the EURECA satellite. This is an analog to the ultraviolet processing of interstellar dust mantles in diffuse clouds after leaving molecular clouds. The 3.4 μm absorption features of these organic materials match those of the diffuse cloud interstellar dust better than any other previously suggested analog to the interstellar organics.

New Extragalactic Perspectives in the New South Africa

What is the nature and composition of the dust grains responsible for the visual extinction in our Galaxy and in other galaxies beyond? What are the ranges in temperature of dust grains? Can these be less than 2.7K? Can the distribution of cold grains be studied optically at unprecedented arcsecond resolution? How does the presence of dust affect the morphology of a galaxy? Is this new dust-penetrated view bringing us to the verge of a breakthrough in understanding the connection between galaxy morphology and the underlying physics of galaxies? How large are the amounts of cold molecular hydrogen gas and cold dust in galactic disks? These are some of the key issues addressed in this book, which takes the postgraduate reader and professional researcher to the cutting edge of this rapidly developing field. Unique features of the book include fourteen in-depth invited review papers and twenty-six pages of discussion transcribed from a television tape. The contributions reflect the entire proceedings of an intensive one week International Conference on cold dust and galaxy morphology held in Johannesburg, South Africa, during January 1996.

Cosmic dust and our origins

Abstract The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized.

Interstellar dust, chirality, comets and the origins of life: life from dead stars?

The physical, chemical and astrophysical processes by which chiral prebiotic molecules can be produced in interstellar dust and later delivered “safely” to the earth are considered. A laboratory analog experiment on the irradiation by circularly polarized UV light of mirror image molecules at the low temperatures of interstellar dust demonstrates that a substantial degree of chirality can be produced by irradiation of the dust by circularly polarized light from pulsars whose mean brightness and distribution in the Milky Way provide the energetic photons. The chirality is then preserved by cold aggregation of the dust into low density fragile nuclei. The thermal evolution of comets following them from birth through billions of years in the Oort cloud and back to the inner solar system results in preservation of dust organics in largely pristine form — even including effects of radiogenic heating. Physical justification for the cushioned transfer of fragments of the fluffy comets impacting the earths atmosphere provides a conceptual basis for depositing significant concentrations of interstellar prebiotic molecules. Chiral amplification in water on the earth is presumed to be enhanced by this local concentration. If chiral molecules are discovered in comet nucleus material which will some day be returned to the laboratory, we may have in our hands the same building blocks from which we evolved.

From Interstellar Dust to Comets: Infrared Emission from Comet Hale-Bopp (C/1995 O1)

The 3-20 μm IR thermal emission spectrum of comet Hale-Bopp (C/1995 O1) at a heliocentric distance of rh=1.15 AU has been calculated using fluffy aggregates of silicate core-organic refractory mantle interstellar dust as the comet dust model. The principal dust properties, similar to those obtained for other comets, provide good fits to the observation. Our results are significantly different from the general opinion that Hale-Bopp is rich in submicron grains. The mean grain masses derived here are 2-3 orders of magnitude higher than those estimated from the empirical superheat method or from the IR emission modeling, both of which use compact silicate/carbon particles. It has been shown that the IR emission alone cannot give a reliable dust production rate. We have also modeled the crystalline silicate emission spectrum.

Thermal history of comets during residence in the Oort Cloud: Effect of radiogenic heating in combination with the very low thermal conductivity of amorphous ice

The thermal history of cometary nuclei during residence in the Oort cloud is studied with the use of the very low thermal conductivity of amorphous ice recently obtained by Kouchi et al. [1992a]. The heat sources included are (1) radioactive nuclides 40K, 232Th, 235U, and 238U with their chondritic abundances, and (2) latent heat released in transition from amorphous ice to crystalline ice. We model the cometary nucleus as a porous aggregate of grains with each individual grain being composed of a refractory core and an icy mantle. It is assumed that the ice is initially amorphous. The bulk thermal conductivity of a cometary nucleus is assumed to be expressed by the product of the thermal conductivity of individual grains and a reduction factor resulting from the porous structure of the nucleus. Numerical results of the thermal history are presented for various conditions including one case which includes heating by 26Al decay. It is shown that the thermal histories are clearly classified into two distinct types depending mainly on the nucleus thermal conductivity κ. (1) Comets with small κ experience a runaway increase in the internal temperature to higher than 120 K during residence in the Oort cloud, in which case most of the ice in the nucleus crystallizes. (2) Comets with a sufficiently large κ on the other hand, do not exhibit a runaway heating and the temperature is limited to < 100 K so that the initial amorphous ice is almost completely preserved. A criterion of nuclear ice crystallization is presented in an analytic expression derived from the analysis of the physical processes of the crystallization. A brief discussion is given on the implications of the results for the sources of volatile molecules observed in the coma.

Ultraviolet Photoprocessing of Interstellar Dust Mantles as a Source of Polycyclic Aromatic Hydrocarbons and Other Conjugated Molecules

By co-depositing a gas mixture of simple carbon- and nitrogen-containing molecules with water on a 10 K surface and exposing it to ultraviolet radiation, we were able to form a residue. This residue was then placed aboard the EURECA satellite behind a magnesium fluoride window and exposed to solar radiation for 4 months before it was returned and analyzed. The resulting residue is believed to simulate the photoprocessing of organic dust mantles in the interstellar medium. Mass spectrometry indicated that the photoprocessing created a rich mixture of polycyclic aromatic hydrocarbons (PAHs) and other conjugated organic molecules, which may explain how PAHs are replenished in space.

Morphological Structure and Chemical Composition of Cometary Nuclei and Dust

The chemical composition of comet nuclei derived from current data on interstellar dust ingredients and comet dust and coma molecules are shown to be substantially consistent with each other in both refractory and volatile components. When limited by relative cosmic abundances the water in comet nuclei is constrained to be close to 30% by mass and the refractory to volatile ratio is close to 1:1. The morphological structure of comet nuclei, as deduced from comet dust infrared continuum and spectral emission properties, is described by a fluffy (porous) aggregate of tenth micron silicate core-organic refractory mantle particle on which outer mantles of predominantly H2O ices contain embedded carbonaceous and polycyclic aromatic hydrocarbon (PAH) type particles of size in the of 1 — 10 nm range.

Formation and evolution of solids in space

Observational Constraints on Interstellar Dust Composition T.P. Snow. The Nature and Evolution of Interstellar Organics J.E. Chiar, Y.J. Pendleton. Tracking the Organic Refractory Component of Interstellar Dust J.M. Greenberg. MID Infrared Spectropolarimetry and the Composition of Cosmic Dust C.M. Wright, et al. Method of Regularization and Models of Interstellar Dust V.G. Zubko. Models and Observations of Gas-Grain Interactions in Star-Forming Regions E.F. van Dishoeck. The Organic Chemistry of TMC-1 Revisited S.B. Charnley. Molecular Gas Phase Counterparts to Solid State Grain Mantle Features: Implications for Gas/Grain Chemistry E. Dartois, et al. Diffuse Interstellar Bands -- The 75th Anniversary J. Krelowski. Observational Constraints on the Carriers of the Diffuse Interstellar Bands (DIBs) J. Cami, et al. Laboratory Simulation of Processes in Interstellar Ices W.A. Schutte. Cosmic Dust and Laboratory Simulation: Wishes, Results and Open Problems L. Colangeli, et al. Stardust Mineralogy: The Laboratory Approach J. Dorschner. Infrared Spectroscopy of Cosmic Dust Analogues at Low Temperatures H. Mutschke, Th. Henning. Matrix Isolation of Amorphous Carbon Grains in Boron-Oxide Glass: Far UV Spectroscopy M.S. Robinson, et al. Matrix-Isolated Nano-Sized Soot Grains and Their Relation to Solid Carbon in Space M. Schnaiter, et al. Temperature Dependence of the FIR Absorption Coefficient for Carbon and Silicate Grains J.R. Brucato, et al. Morphology, Composition and MIR-FIR Spectroscopy of SIlicates of Astrophysical Interest G. Ferrini, et al. IR Reflectance Spectroscopy of Martian Analogues F. Esposito, et al. Properties of Dust Producing Stars H.J. Habing. Dust Formation in Supernovae P.-O. Lagage. The Destruction of Interstellar Dust A.G.G.M. Tielens. Dust Coagulation and the Structure of Dust Aggregates in Space C. Dominik. Solids and Volatiles in Comets: From Cometary Nuclei to Cometary Atmospheres J. Crovisier. In Situ Measurements of Evolved Solids in Space with Emphasis on Cometary Particles J. Kissel. Interstellar Material in Meteorites J.F. Kerridge. Interstellar Dust -- Evidence from Interplanetary Dust Particles J.P. Bradley. The Dust around Vega-Like Stars: A Bridge from YSOs to Comets H.M. Butner, et al. Accreting Gas and Dust in Pre-Main Sequence Disk Systems J.L. Sitko, et al. Observations of Dust and Molecules in the Disks and Envelopes of Young Stellar Objects M.R. Hogerheijde, et al. Release Mechanisms of Volatile Molecules in Amorphous H2O Ice S. Sirono, T. Yamamoto. Scattering by Complex Systems I: Methods B.A.S. Gustafson. Scattering by Complex Systems II: Results from Microwave Measurements B.A.S. Gustafson, et al. Dynamical Structure of the Zodiacal Cloud S.F. Dermott, et al.

Tracking the organic refractory component from interstellar dust to comets.

The abundance and composition of complex organic (carbonaceous) material in the interstellar dust is followed as the dust evolves in its cyclic evolution between diffuse and dense clouds. Interstellar extinction, laboratory and space analog experiments, dust infrared absorption spectra, the cosmic abundance of the condensible atoms, and space and ground-based observations of comet dust are used to impose constraints on the organic dust component as mantles on silicate cores.