Interstellar dust is suggested to be one of the most thoroughly studied of all small-particle systems, whose characteristics have in some ways not been duplicated in our laboratories. The particles are maintained in complete isolation from one another, in ultra-high vacuum and at low temperature, and have been studied spectroscopically from far infra-red to far ultra-violet. Optical properties of the interstellar dust which are surveyed include effects interpreted as due to surface plasmon bands in the ultra-violet, surface phonon bands and other vibrational absorption bands in the infra-red, linear and circular polarization caused by aligned particles, impurity absorption, and defect absorption possibly caused by radiation damage. A discussion of the possible origins for interstellar grains leads to speculation on their relationship to certain meteorites. Absorption and scattering effects from small particles in general are surveyed by way of Mie calculations for a representative insulator, MgO,...

Interstellar grains The interaction of light with a small-particle system

The reststrahlen features in thermal infrared, or vibrational, spectra of Mg-Fe olivines ((Mg,Fe)2SiO4) exhibit trends in position, strength, and number of features that are diagnostic of the relative proportions of the Mg and Fe cations in the minerals. Although band positions move to lower wavenumbers (longer wavelengths) across the forsterite–fayalite compositional binary in a generally linear manner, specific feature shifts in transmittance data are described best by two linear fits with a break in slope near Fo70. The break in slope may be accompanied by an offset as well; both traits are attributed to structural changes in olivine brought about by distortion of the crystal lattice by Fe. Reflectance and emissivity spectra exhibit similar trends in band position with composition, and all three types of data demonstrate that some olivine band strengths change across the Mg–Fe solid solution series and also are diagnostic of composition. Olivines have been identified in a wide array of thermal infrared spectra of planetary materials and have been interpreted as being present on the surfaces of Mercury, the Moon, Mars, and a number of asteroids based on the analysis of thermal infrared spectra. New linear least squares models of the emissivity spectra of olivine-bearing Martian meteorites enable a preliminary estimation of the accuracy with which quantitative estimates of olivine abundance and solid solution composition can be derived from the spectra of mixtures.

Thermal infrared (vibrational) spectroscopy of Mg–Fe olivines: A review and applications to determining the composition of planetary surfaces

A smooth surface layer of highly disordered olivine, (Mg, Fe)2SiO4, has been produced by exposure of polished, natural olivine to a dose of 5×1016 cm−2 of 1.5 MeV neon ions from a Van de Graaff accelerator. The dielectric functions of the disordered silicate in the wavelength range from 8 to 30 μm have been determined from analysis of specular reflectance data, and extinction for Rayleigh particles of such disordered olivine has been calculated. Extinction measurements for amorphous olivine smoke collected on a substrate are also presented. The small particle extinctions of both kinds of structurally disordered olivine are shown to agree well with the main features of the absorption and emission spectra from interstellar grains in the 10 and 20 μm region.

Infrared extinction of heavy ion irradiated and amorphous olivine, with applications to interstellar dust

Infrared transmission spectra from 2.5 to 25 μm of various meteorite classes

Optical constants of chlorite and serpentine between 2.5 and 50 μm

Infrared spectra (7–40 μ) of the carbonaceous chondrites, Cold Bokkeveld, Murray, and Orgueil are compared with phyllosilicate minerals and interstellar grain spectra. Similarities in the position and shape of the silicate feature near 9.8 μ suggest that similar silicate minerals are present in primitive meteorites and the interstellar dust.

A. Zaikowski

Papers

Infrared Spectra of Carbonaceous Chondrites and the Composition of Interstellar Grains