Reflectance spectra of regolith analogs in the mid-infrared: effects of grain size

• Published in: Planetary and space science Vol. 51; no. 4; pp. 281 - 294
• Main Authors: Le Bras, A., Erard, S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.04.2003; Elsevier
• Subjects: Asteroids; Astrophysics; Laboratory experiments; Physics; Planetary surfaces; Remote sensing; Spectroscopy; Thermal-infrared; Spectroscopy; Thermal-infrared; Laboratory experiments; Planetary surfaces; Asteroids; Remote sensing
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/S0032-0633(03)00017-5

This paper focuses on the effects of grain size variations on reflectance spectra in the 2– 40 μm range. Samples of basalt and anorthosite were prepared in several grain size separates ranging from 25 to 800 μm . Varying grain size modifies the spectral contrast of absorption bands in the volume scattering region, in a manner that depends on the strength of the absorption. The spectral contrast of the weakest bands decreases with decreasing grain size, while that the strongest bands usually reaches a maximum for intermediate sizes. However, the 2.7 μm OH band of anorthosite does not behave this way, in contrast to what has been suggested in previous works. In the surface scattering region all absorptions are associated with a Christiansen minimum, a group of reststrahlen peaks, and a transparency peak. This occurs both in the mid-IR range and in the less known 250– 500 cm −1 range, suggesting that lattice mode transitions behave like vibrational transitions. The first Christiansen frequency (∼1250– 1150 cm −1 ) is constant for the anorthosite sample, but shifts to shorter frequencies with decreasing grain size for the basalt sample. This is ascribed to a strong spectral contrast between the mineralogic species present in the basalt sample. The intensity of the reststrahlen peaks decreases with decreasing grain size fractions for grain size smaller than 200 μm , and with increasing grain size for larger grains. Transparency features appear for grain sizes smaller than 50 μm . No correlation between the reststrahlen peaks intensity and the transparency features intensity has been found. Shape and occurrence of these spectral features in the mid-infrared range appear to be strongly grain size dependent.