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At least 451 records · Page 25

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Atom Probe Tomography and Three-Dimensional Atomic Scale Characterisation of Interplanetary Dust Particles: Inorganic and Organic, Hydrous and Anhydrous Assemblages

Interplanetary dust particles (IDPs)preserve primordial fragments from our Solar System and external stellar systems, sampled from a vast range of dust forming bodies that often display smaller degrees of parent body processing than is seen in meteorites. IDPs preserve some of the smallest astromaterials from our Solar System, many of which are believed to be among the most important materials for studying physical and chemical processing, and formation mechanisms occurring within the interstellar medium (ISM), Solar Nebula and external planetary systems[1]. However, their formation processes and provenance are poorly constrained, owing in part to spatial resolution limitations of traditional analytical techniques. Atom probe tomography (APT) uses a pulsed laser to field-evaporate material at the atomic scale from needle-shaped samples, and time-of-flight spectrometry for 3D reconstruction of element and isotope distribution in samples after data acquisition. This technique has the highest spatial resolution available of any technique used within the geoscience field[2]. However, measuring multi-phase specimens and porous media in APT can be challenging, two primary characteristics of IDPs. Here we present a custom method for APT needle design and the first results of a 3Datomic scale study of interplanetary dust particles, salt crystals and extraterrestrial insoluble and soluble organic matter designed to overcome the challenges of studying IDP-like materials.

N D Nevill↗

NASA NTRS1996

The Physics of Molecular Shocks in Star-Forming Regions

Molecular shocks are produced by the impact of the supersonic infall of gas and dust onto protostars and by the interaction of the supersonic outflow from the protostar with the circumstellar material. Infalling gas creates an accretion shock around the circumstellar disk which emits a unique infrared spectrum and which processes the interstellar dust as it enters the disk. The winds and jets from protostars also impact the disk, the infalling material, and the ambient molecular cloud core creating shocks whose spectrum and morphology diagnose the mass loss processes of the protostar and the orientation and structure of the star forming system. We discuss the physics of these shocks, the model spectra derived from theoretical models, and comparisons with observations of H2O masers, H2 emission, as well as other shocks tracers. We show the strong effect of magnetic fields on molecular shock structure, and elucidate the chemical changes induced by the shock heating and compression.

Hollenbach, David↗

DOE OSTI2015

A NEW DETERMINATION OF THE BINDING ENERGY OF ATOMIC OXYGEN ON DUST GRAIN SURFACES: EXPERIMENTAL RESULTS AND SIMULATIONS

The energy to desorb atomic oxygen from an interstellar dust grain surface, E{sub des}, is an important controlling parameter in gas-grain models; its value impacts the temperature range over which oxygen resides on a dust grain. However, no prior measurement has been done of the desorption energy. We report the first direct measurement of E{sub des} for atomic oxygen from dust grain analogs. The values of E{sub des} are 1660 ± 60 and 1850 ± 90 K for porous amorphous water ice and for a bare amorphous silicate film, respectively, or about twice the value previously adopted in simulations of the chemical evolution of a cloud. We use the new values to study oxygen chemistry as a function of depth in a molecular cloud. For n = 10{sup 4} cm{sup −3} and G{sub 0} = 10{sup 2} (G{sub 0} = 1 is the average local interstellar radiation field), the main result of the adoption of the higher oxygen binding energy is that H{sub 2}O can form on grains at lower visual extinction A{sub V}, closer to the cloud surface. A higher binding energy of O results in more formation of OH and H{sub 2}O on grains, which are subsequently desorbed by far-ultraviolet radiation, with consequences for gas-phase chemistry. For higher values of n and G{sub 0}, the higher binding energy can lead to a large increase in the column of H{sub 2}O but a decrease in the column of O{sub 2}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Scattering Properties of Large Irregular Cosmic Dust Particles at Visible Wavelengths

The effect of internal inhomogeneities and surface roughness on the scattering behavior of large cosmic dust particles is studied by comparing model simulations with laboratory measurements. The present work shows the results of an attempt to model a dust sample measured in the laboratory with simulations performed by a ray-optics model code. We consider this dust sample as a good analogue for interplanetary and interstellar dust as it shares its refractive index with known materials in these media. Several sensitivity tests have been performed for both structural cases (internal inclusions and surface roughness). Three different samples have been selected to mimic inclusion/coating inhomogeneities: two measured scattering matrices of hematite and white clay, and a simulated matrix for water ice. These three matrices are selected to cover a wide range of imaginary refractive indices. The selection of these materials also seeks to study astrophysical environments of interest such as Mars, where hematite and clays have been detected, and comets. Based on the results of the sensitivity tests shown in this work, we perform calculations for a size distribution of a silicate-type host particle model with inclusions and surface roughness to reproduce the experimental measurements of a dust sample. The model fits the measurements quite well, proving that surface roughness and internal structure play a role in the scattering pattern of irregular cosmic dust particles.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1988

Comparison of the 3.36 micrometer feature to the ISM

It has been noted that the 3.36 micrometer emission feature is not the same as that of any ISM band at 3.4 micrometer. This is documented herein. There is no convincing analog to the cometary 3.36 micrometer emission feature seen in the Interstellar Matter band. This fact suggests that if the carbonaceous material in comets came from the ISM, it was either further processed in the solar nebula or has a different appearance because of the different excitation environment of the sun and ISM.

Tokunaga, Alan T.↗

NASA NTRS1992

LDEF: 69 Months in Space. First Post-Retrieval Symposium, part 1

This document is a compilation of papers presented at the First Long Duration Exposure Facility (LDEF) Post-Retrieval Symposium. The papers represent the preliminary data analysis of the 57 experiments flown on the LDEF. The experiments include materials, coatings, thermal systems, power and propulsion, science (cosmic ray, interstellar gas, heavy ions, and micrometeoroid), electronics, optics, and life

Space experiment↗

NASA NTRS1992

LDEF: 69 Months in Space. First Post-Retrieval Symposium, part 2

A compilation of papers from the symposium is presented. The preliminary data analysis is presented of the 57 experiments flown on the LDEF. The experiments include materials, coatings, thermal systems, power and propulsion, science (cosmic ray, interstellar gas, heavy ions, and micrometeoroid), electronics, optics, and life science.

Arlene S. Levine↗

NASA NTRS1991

SOFIA (Stratospheric Observatory for Infrared Astronomy) will be a telescope with 2.5 m effective aperture operating at altitudes of 12.5 to 13.7 km in a Boeing 747 aircraft. It will enable astronomers on board to make observations which are impossible from the ground at wavelengths between 0.3 microns and 1.5 mm. Diffraction-limited imaging will be possible at wavelengths beyond about 10 microns. SOFIA's mobility will assure all-sky coverage and access to transient events from anywhere in the world. Some of the science topics to be studied will be star and planet formation, origin and evolution of biogenic materials, comets and planetary atmospheres in the solar system, the interstellar media of other galaxies, and phenomena at our galactic center. The SOFIA concept is based on 21 years of experience with NASA's Kuiper Airborne Observatory (KAO), which has been retired to help fund the project. As with the KAO, state-of-the-art focal plane instruments and a vigorous educational outreach program will be features of this new airborne astronomy facility. A more intensive flight program, approximately 150 flights per year, will accommodate many more guest investigators however. Planned as a collaboration between the United States and Germany, SOFIA is scheduled to begin a 20 year operating lifetime in 2001.

Erickson, E. F.↗