Effects of Powder Coatings on the Reflectance Spectra and Photometric Properties of Igneous Rocks

1. Introduction

The surfaces of many solar system airless bodies are covered by a layer of fine regolith formed by long-term space weathering processes, and traditional planetary remote sensing studies primarily focused on the research of optical properties of regolith layers. With the recent advances in space explorations, more disk-resolved spectral imagery data of the Moon, Mars, and asteroids have been returned and there is an increasing need to understand the reflectance properties of rocks and complex mixtures of rocks, fragments and powders (Pieters & Noble, 2016). The optical properties of rocks covered by fine dust layers can be very different from that of the layers and the underlying material (Kiddell et al. 2018; Johnson & Grundy, 2001; Johnson et al., 2002,). Therefore, it is important to understand the spectral, photometric and polarimetric properties of igneous rocks covered by powders with various thicknesses.

2. Samples and methods

We measured the reflectance of four igneous rocks (Trachyte, Peridotite, Diabase, Picrite Porphyrite, ranging from high to low reflectance) as bare slabs and slabs covered with powder layers of varying thicknesses (0.1 mm, 0.2 mm, 1 mm, and 2 mm) and particle sizes (0–45 µm, 90–125 µm).

We used two spectro-goniometric systems and one spectrometer to obtain the bidirectional reflectance spectra. CUG spectro-goniometric system: The emergence zenith angle varied from 0° to 70°, and the relative azimuth angle from 0° to 360° (Jiang et al., 2022). This configuration yields 121 viewing geometries for an incidence angle of 0° and 165 geometries for 55°. The spectral range covered is 0.35-2.1 µm. SHADOWS: We selected 15 specific wavelengths between 0.4 and 4.2 µm and collected 71 viewing geometries (Potin et al., 2018). SHINE spectrometer: This instrument measures spectra across a wavelength range of 0.35-4.2 µm with an incident angle of 30° and an emergency angle of 0° (Brissaud et al., 2004).

3. Result

Fig. 1 and Fig. 2 show the upper hemisphere reflectance and spectra of four rock slabs and the slab covered with varying powder layer thicknesses. The results demonstrate that rock slabs exhibit low reflectance and strong forward scattering, and even very thin dust layers (0.1mm) can significantly alter the overall reflectance and mask the strong forward scattering of rock slabs. Reflectance increases with powder thickness until reaching a "saturation thickness", after which further coating has little effect on reflectance spectra. Larger particles (90-125 µm) reach “saturation” faster than smaller ones (0-45 µm). Lower reflectance rocks and spectral regions are more susceptible to surface scattering effects than higher-reflectance rocks.

Fig. 3 displays the dependent of the phase angle on the spectral slope (0.6-1.8 µm) of the four rocks. It shows that rock slabs exhibit strong specular reflection and little phase reddening up in small phase angle, while powder-coated surfaces show a gradual and linear increase in spectral slope with phase angle. We analyzed the 2.7 µm hydration feature for Peridotite and the results show 2.7 µm hydration band decreases in strength with increasing phase angle and is weaker under strong specular reflection angle.

Using the Hapke two-layer model and Planet-GLLiM tool (Hapke, 2012; Douté et al., 2023), we retrieved the phase function parameters and optical constants of Peridotite, and reconstructed the spectra and bidirectional reflectance for bare and powder-coated slabs. Fig. 4 illustrates the dependence of reflectance on thickness, showing that reflectance increases exponentially with increasing optical thickness. We calculate the ratios of surface scattering to total scattering and retrieved the relationship of phase function parameters, indicate stronger surface scattering and forward scattering in bare rocks compared to powder-coated samples.

4. Conclusion:

We have measured the reflectance of four igneous rocks—Trachyte, Peridotite, Diabase, and Picrite Porphyrite—as bare slabs and as slabs coated with powder layers of four varying thicknesses and two particle sizes. We found that even very thin powder layers can significantly alter the reflectance and photometric behavior of rock surfaces. Reflectance increases with powder thickness until reaching a "saturation thickness", with larger particles reaching this threshold more quickly. And powder coatings reduce strong forward scattering. Bare rock slabs exhibit little phase reddening up in small phase angle, while powder-coated surfaces show a gradual and linear increase in spectral slope with phase angle. Modeling results reveal that reflectance grows exponentially with optical thickness, and that coated powder layers reduce surface scattering and weaken forward scattering behavior.

Fig. 1. Reflectance comparison of four rocks covered with different thicknesses of powder and bare rock slab. Purple: bare rock slab; Red: 0.1 mm powder layer coated; Orange: 0.2 mm; Blue: 1 mm; Dark blue: 2 mm.

Fig. 2. Reflectance spectra of the 4 igneous rocks and their powders in 2 size fractions. i = 0°, e = 30°.

Fig. 3. Phase angle dependent of the spectra l slope in the 0.6 to1.8 µm region of particle coated rock.

Fig. 4. The dependence of reflectance on optical thickness for Peridotite. (a) and (b) correspond to 0-45 µm size distribution, and (c) and (d) to 90-125 µm size distribution.

 

 

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