Visible and near-infrared reflectance spectra of igneous rocks and their powders

Most solid planetary bodies in the solar system are covered by a layer of fine particles and the topic of light scattering by small particles has been thoroughly studied in the past decades. In contrast, light reflection from intact rocks has received much less attention, though the spectral features of fresh rocks are more diagnostic than that of highly space-weathered regolith grains. As high spatial-resolution spectral images obtained by modern space-borne and in-situ sensors have become available, it is important to understand the spectral feature links between rocks and powders made by crushing the rocks. In this work, we selected 13 terrestrial igneous rocks with a 1 mu m absorption feature and measured the visible and near-infrared reflectance spectra of their slabs and powders in three size fractions, 0-45 mu m, 90-125 mu m, and 450-900 mu m. We have found that the spectral char-acteristics of these samples can be divided into two groups. For slabs with reflectance lower than 0.1 at 0.5 mu m, they have less pronounced 1 mu m absorption feature. For slabs with reflectance higher than 0.1, they have pro-nounced 1 mu m feature, consistent with that of their powdered counterparts. By using the equivalent-slab and the Hapke model, we obtained the optical constants and single scattering albedo values of the samples. The dependence of single scattering albedo on effective absorption thickness indicates that the differences between the spectral characteristics of rock slabs and powdered samples are likely controlled by the degree of weak surface scattering contributions. We reconstructed the spectrum of a powdered lunar meteorite which best matches the Chang'E-4 rock and found that the reconstructed rock spectra are very close to the rock spectrum observed in-situ by Chang'E-4.

Automated summary

Light reflection from intact rocks has been less studied compared to light scattering by small particles. With the availability of high-resolution spectral images obtained by modern space-borne and in-situ sensors, it is important to understand the spectral feature links between rocks and crushed powders made from them.