09:15 〜 09:30
[PPS03-07] Specular photometry of opaque minerals in Ryugu returned samples
キーワード：Ryugu, Sample return, Spectroscopy
Remote sensing observations by Hayabusa2 revealed the dark and uniform nature of Ryugu surface materials with a geometric albedo as low as 4.5±0.2% [1], which is common in C-type asteroids [2]. The reflectance measurements of returned samples also corroborated this result [3]. However, the reflectance of Ryugu is significantly lower than typical CI and CM meteorites [1, 3] and the material property responsible for Ryugu’s low reflectance has not been understood well yet. Because the samples returned from Ryugu preserve the physical and chemical properties of Ryugu’s material without receiving contamination and weathering on Earth, investigation on their reflectance at microscopic scales is very useful for understanding what causes the low reflectance. In this study, we mapped the reflectance of Ryugu’s returned samples at the scales of minerals to identify which minerals/phases contribute to the low reflectance.
We used the multi-band stereoscopic imager developed for the JAXA curation facility [4] to map the reflectance at effective resolutions down to ~5 um. This high-resolution measurement allows us to measure the reflectance of some large minerals within sample grains. Tens of apparently bright hexagonal structures, which are likely euhedral minerals, were identified under diffuse epi-illumination (i.e., illumination from various directions). However, we find that these hexagonal structures actually appear dark (reflectance of 1−1.5%) under unidirectional epi-illumination (i.e., illumination with unidirectional light). In contrast, the surrounding matrix had reflectance similar to the particle average under any illumination conditions and followed a typical photometric function for diffuse materials. Such difference in photometric function between hexagonal structures and the surrounding materials suggests that hexagonal structures possess a specular photometric property, which often occur on the surface of opaque minerals. In fact, opaque minerals (e.g., Fe-Ni sulfide and magnetite) have been identified by transmission electron microscope (TEM) observation of Ryugu samples [5]. These observations lead to a hypothesis that most of the bright spots observed on the returned grains [3, 6] may be due to specular reflections on opaque minerals.
To test this hypothesis, we measured change in polarization upon reflection on Ryugu sample surfaces by installing two linear polarization filters: one in front of the incident light and the other in front of the microscope. We found that bright spots with higher apparent brightness exhibit a higher extinction rate when two polarizers are crossed (correlation coefficient 0.64). This result shows that light reflected on bright spots did not change its polarization greatly, indicating that this is dominated by surface reflection. The apparent reflectance of these bright spots can vary by an order of magnitude depending on the viewing geometry. This result suggests that the abundance of such reflective minerals may have a significant effect on the average reflectance of Ryugu.
The above results on the distinctive photometric properties of opaque minerals indicate that we could generate a map of their abundance based on optical measurements, which is non-destructive and much faster than electron microscope measurements. Such map is also useful for identifying intrinsically bright materials on Ryugu [7] owing to its composition. Thus, we conducted additional measurements of sample particles illuminated from different angles and generated maps of “specurality”: defined as the ratio of the highest reflectance to the average over all the measured viewing geometry. In the presentation, we plan to discuss about the result of specularity map analysis.
[1] Sugita et al., 2019, Science. [2] Masiero et al., 2017, Astron. J. [3] Yada et al., 2021, NatAstron. [4] Cho et al., in revison. [5] Noguchi et al., 2022, LPSC. [6] Pilorget et al., 2021, NatAstron. [7] Tatsumi et al., 2021, NatAstron.
We used the multi-band stereoscopic imager developed for the JAXA curation facility [4] to map the reflectance at effective resolutions down to ~5 um. This high-resolution measurement allows us to measure the reflectance of some large minerals within sample grains. Tens of apparently bright hexagonal structures, which are likely euhedral minerals, were identified under diffuse epi-illumination (i.e., illumination from various directions). However, we find that these hexagonal structures actually appear dark (reflectance of 1−1.5%) under unidirectional epi-illumination (i.e., illumination with unidirectional light). In contrast, the surrounding matrix had reflectance similar to the particle average under any illumination conditions and followed a typical photometric function for diffuse materials. Such difference in photometric function between hexagonal structures and the surrounding materials suggests that hexagonal structures possess a specular photometric property, which often occur on the surface of opaque minerals. In fact, opaque minerals (e.g., Fe-Ni sulfide and magnetite) have been identified by transmission electron microscope (TEM) observation of Ryugu samples [5]. These observations lead to a hypothesis that most of the bright spots observed on the returned grains [3, 6] may be due to specular reflections on opaque minerals.
To test this hypothesis, we measured change in polarization upon reflection on Ryugu sample surfaces by installing two linear polarization filters: one in front of the incident light and the other in front of the microscope. We found that bright spots with higher apparent brightness exhibit a higher extinction rate when two polarizers are crossed (correlation coefficient 0.64). This result shows that light reflected on bright spots did not change its polarization greatly, indicating that this is dominated by surface reflection. The apparent reflectance of these bright spots can vary by an order of magnitude depending on the viewing geometry. This result suggests that the abundance of such reflective minerals may have a significant effect on the average reflectance of Ryugu.
The above results on the distinctive photometric properties of opaque minerals indicate that we could generate a map of their abundance based on optical measurements, which is non-destructive and much faster than electron microscope measurements. Such map is also useful for identifying intrinsically bright materials on Ryugu [7] owing to its composition. Thus, we conducted additional measurements of sample particles illuminated from different angles and generated maps of “specurality”: defined as the ratio of the highest reflectance to the average over all the measured viewing geometry. In the presentation, we plan to discuss about the result of specularity map analysis.
[1] Sugita et al., 2019, Science. [2] Masiero et al., 2017, Astron. J. [3] Yada et al., 2021, NatAstron. [4] Cho et al., in revison. [5] Noguchi et al., 2022, LPSC. [6] Pilorget et al., 2021, NatAstron. [7] Tatsumi et al., 2021, NatAstron.