Do twin spectral peaks of olivine particles in the thermal infrared diagnose their sizes and porosities?

Abstract

A well-established constraint on the size of non-porous olivine grains or the porosity of aggregates consisting of small olivine grains from prominent narrow peaks in thermal infrared spectra characteristic of crystalline silicates is reexamined. To thoroughly investigate thermal infrared peaks, we make theoretical argument for the absorption and scattering of light by non-porous, non-spherical olivine particles, which is followed by numerical verification. Our study provides perfectly rational explanations of the physics behind the small-particle effect of emission peaks in the framework of classical electrodynamics and convincing evidence of small-particle's emission peaks in the literature. While resonant absorption excited by surface roughness on the order of submicrometer scales can be identified even for non-porous olivine particles with a radius of $10\mu \mathrm{m}$, it makes only a negligible contribution to thermal infrared spectra of the particles. In contrast, the porosity of non-spherical particles has a significant impact on the strength and wavelength of the peaks, while the resonant absorption excited by an ensemble of small grains takes place at a wavelength different than one expects for surface roughness. We finally reaffirm that twin peaks of olivine in thermal infrared spectra of dust particles in astronomical environments are the intrinsic diagnostic characters of submicrometer-sized small grains and their aggregate particles in fluffy and porous configurations.

16 pages, 8 figures, to appear in Icarus

References in corpus (8)

• The discrete dipole approximation for periodic targets I. theory and tests
• Two different evolutionary types of comets proved by polarimetric and infrared properties of their dust
• The tensile strength of dust aggregates consisting of small elastic grains: Constraints on the size of condensates in protoplanetary disks
• New constraints on the dust surrounding HR 4796A
• Low-phase spectral reflectance and equivalent geometric albedo of meteorites powders
• The Morphological, Elastic, and Electric Properties of Dust Aggregates in Comets: A Close Look at COSIMA/Rosetta's Data on Dust in Comet 67P/Churyumov-Gerasimenko
• Comet 9P/Tempel 1: Interpretation with the Deep Impact Results
• Mid-infrared observations of the nucleus of comet P/2016 BA14 (PANSTARRS)

Cited by in corpus (1)

• Probing the surface environment of large T-type asteroids