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

1Hiroshi Kimura,2Johannes Markkanen,3Ludmilla Kolokolova,4Martin Hilchenbach,1Koji Wada,1Yasumasa Kanada,1Takafumi Matsuia
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114964]
1Planetary Exploration Research Center (PERC), Chiba Institute of Technology, Tsudanuma 2-17-1, Narashino, Chiba 275-0016, Japan
2Institut für Geophysik und Extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
3 Planetary Data System Group, Department of Astronomy, Rm. 2337, Computer and Space Science Bldg., University of Maryland, College Park, MD, 20742, USA
4 Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077, Göttingen, Germany
Copyright Elsevier

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 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.

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