12:00 〜 12:15
[PPS03-11] Possible detection of bright boulder fragments in Ryugu samples based on polarization measurement.
キーワード：Ryugu、Sample return、Polarization-based measurement、Bright boulders
Remote sensing observations by Hayabsua2 revealed that Ryugu has dark and uniform surface material with a geometric albedo as low as 4.5±0.2% [1], which is common in C-type asteroids [2]. The remote sensing observations also revealed that some boulders on the surface of Ryugu have significantly higher reflectance than average [3-5], which are called bright boulders. The reflectance measurements of returned samples had expected to distinguish ‘bright boulder’ like samples from average samples and connect further investigation of ‘bright boulder’ like samples [6, 7]. However, reflectance mapping of returned samples showed that there are many bright spots caused by specular reflection [8]. Bright boulders are not considered due to specular reflectance because the sizes of bright boulders and specular reflectance are different by ~1,000 times. Thus, analyses of ‘bright boulder’ like samples need distinction of specular reflectance. To compensate for such effects, 3D shape measurements were conducted [9], but no effective compensation method has been developed so far. The purpose of this study is to show that the results of the polarization-based analysis, whose purpose is originally to distinguish specular reflection from diffuse reflection, exhibit some irregular polarization possibly connected to intrinsic optical parameters of minerals such as birefringence.
In order to achieve this goal, we added two polarizers to the optical measurement suite we developed for the JAXA curation facility with a multi-band spectroscopic imager at an effective resolution down to ~5 um. One polarizer is placed right in front of the lens barrel of the camera, and the other is placed in the light source to change the polarization of incident light. While the polarizer in front of the camera is fixed, the polarizer in the light source rotates 360°. We measured the change in light intensity from open nicols to crossed-nicols and successfully separated bright spots caused by diffuse reflection from that caused by specular reflection because specular reflection preserves polarization but diffuse reflection (i.e., multiple reflection and body scattering within minerals) changes polarization state (line A, B, and C in the figure) [10].
Additionally, we recently found some bright spots exhibiting polarization apart from the average of the grain. Previously, the reflected light is considered to be brightest when open nicols and darkest when crossed-nicols. Now, it was revealed that most bright spots follow this rule but several bright spots extinct several degrees off from crossed nicols up to near 90°(line D in the figure). Generally, surfaces with specular reflections have different reflectance with p-polarization and s-polarization and rotate the polarization of incident light. This difference is known to be connected to the index of refraction of substrate. Thus, the measurement of this rotation may lead to the index of refraction of the target material. Also, in other research, some polarization-based method is proposed to distinguish the birefringence of the target [11]. Therefore, the results of our polarization measurement may contain more information than the separation of specular and diffuse reflection. Birefringence or index of refraction is connected to shock metamorphism of minerals, so we will conduct further analyses with our polarization-based data and give a presentation about this.
[1] Sugita et al., 2019, Science. [2] Masiero et al., 2017, Astron. J. [3] Tatsumi et al., 2021, Nat. Astron. [4] Sugimoto et al., 2021a, Icarus, 114529. [5] Sugimoto et al., 2021b, Icarus, 114591. [6] Yada et al., 2021, Nat. Astron. [7] Cho et al., 2022, PSS. [8] Yumoto et al., 2022, LPSC. [9] Yabe et al., 2022 LPSC. [10] Yumoto et al., 2022, JpGU. [11] Hee et al., 1992, J. Opt. Soc. Am. B.
In order to achieve this goal, we added two polarizers to the optical measurement suite we developed for the JAXA curation facility with a multi-band spectroscopic imager at an effective resolution down to ~5 um. One polarizer is placed right in front of the lens barrel of the camera, and the other is placed in the light source to change the polarization of incident light. While the polarizer in front of the camera is fixed, the polarizer in the light source rotates 360°. We measured the change in light intensity from open nicols to crossed-nicols and successfully separated bright spots caused by diffuse reflection from that caused by specular reflection because specular reflection preserves polarization but diffuse reflection (i.e., multiple reflection and body scattering within minerals) changes polarization state (line A, B, and C in the figure) [10].
Additionally, we recently found some bright spots exhibiting polarization apart from the average of the grain. Previously, the reflected light is considered to be brightest when open nicols and darkest when crossed-nicols. Now, it was revealed that most bright spots follow this rule but several bright spots extinct several degrees off from crossed nicols up to near 90°(line D in the figure). Generally, surfaces with specular reflections have different reflectance with p-polarization and s-polarization and rotate the polarization of incident light. This difference is known to be connected to the index of refraction of substrate. Thus, the measurement of this rotation may lead to the index of refraction of the target material. Also, in other research, some polarization-based method is proposed to distinguish the birefringence of the target [11]. Therefore, the results of our polarization measurement may contain more information than the separation of specular and diffuse reflection. Birefringence or index of refraction is connected to shock metamorphism of minerals, so we will conduct further analyses with our polarization-based data and give a presentation about this.
[1] Sugita et al., 2019, Science. [2] Masiero et al., 2017, Astron. J. [3] Tatsumi et al., 2021, Nat. Astron. [4] Sugimoto et al., 2021a, Icarus, 114529. [5] Sugimoto et al., 2021b, Icarus, 114591. [6] Yada et al., 2021, Nat. Astron. [7] Cho et al., 2022, PSS. [8] Yumoto et al., 2022, LPSC. [9] Yabe et al., 2022 LPSC. [10] Yumoto et al., 2022, JpGU. [11] Hee et al., 1992, J. Opt. Soc. Am. B.