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[PPS03-P16] Density Estimation of Ryugu Particles by Stereoscopic Measurement
Keywords:Ryugu, Hayabusa2, Density, Porosity
Background: Comparison between the bulk density of C-type asteroid Ryugu (1.19±0.02 g/cm3) determined from gravity and shape measurement by the Hayabusa2 spacecraft and the lowest known grain density (2.42±0.06 g/cm3) among carbonaceous chondrites led to the estimation that Ryugu’s porosity is >50% [1]. There are two types of pores and porosities in Ryugu: macropores, which exist in between constituent particles (macroporosity), and micropores, which exist within constituent particles (microporosity) [2]. Possible evolution history inferred from Ryugu’s high porosity will differ greatly depending on which type of porosity is dominant. If macroporosity is dominant, high porosity of Ryugu reflects the history of the impact-induced disruption of its parent body and subsequent reaccumulation as a rubble pile. If microporosity is dominant, in contrast, numerous pores were likely to be created in constituent particles on Ryugu’s parent body before the rubble-pile formation. Thus, the nature of such microporosity might be useful for understanding Ryugu’s parent body.
Objectives: By measuring the densities of induvial particles of the Ryugu samples as a part of JAXA’s curational efforts, we aim to determine whether Ryugu’s porosity is mainly macroporosity or microporosity.
Method: In the cleanroom at the curation facility of JAXA/ISAS, images of Ryugu particles illuminated by Light-emitting-diode (LED) lights were taken from many directions using a camera by rotating it around the vertical axis. The camera observed the samples from an angle of 45 degrees with respect to the vertical axis. Since the range of camera rotation angles does not cover 360 deg, sample particles were rotated manually to acquire the images from all azimuthal angles around the particle. The 3-dimensional shape models of the particles were created from the images with the software Metashape (Agisoft LLC). The scale of shape models was calibrated using the images of prescribed grid taken under the same condition as the sample particles, and their volumes were estimated.
Evaluation of improvement of measurement equipment: For the measurement of Ryugu particles at the curation facility, we made improvement, such as increasing the camera angle in existing measurement equipment to improve the accuracy of measurement. To evaluate this improvement, shape measurement and density estimation of a graphite particle as a simulant of Ryugu particles were conducted. Compared with a shape model created from the images of the same graphite particle using our measurement system before this improvement, the accuracy of shape measurement was qualitatively improved since the shape of the underside of the particle became visible, and the noise in the shape model became smaller.
Density estimation of Ryugu particles: Since the Ryugu particles were fragile and measurement time was limited, it was difficult to flip the particles upside down for stereoscopic measurement of all the surfaces of the sample particles. Thus, it was necessary to improve the accuracy of the shape measurement by capturing the near-bottom surfaces only by measuring from oblique upward directions. We could obtain the density (1.06±0.17 g/cm3) of only one sample particle among the five particles measured in November 2021. The estimated density was consistent with the particle density (1.12±0.08 g/cm3) calculated from the measured volume in the previous study [3]. If this particle represents Ryugu, this would suggest that microporosity is dominant in Ryugu’s porosity. The method used in this study to estimate the volume, in principle, can become more accurate than the volume measurement by the ellipsoid approximation used in previous study [3]. We plan to measure the density of more Ryugu particles.
Reference: [1] Watanabe et al., 2019, Science, 364(6437). [2] Grott et al., 2020, J. Geophys. Res.: Planets, 125(12). [3] Yada et al., 2021, Nat. Astron., https://doi.org/10.1038/s41550-021-01550-6
Objectives: By measuring the densities of induvial particles of the Ryugu samples as a part of JAXA’s curational efforts, we aim to determine whether Ryugu’s porosity is mainly macroporosity or microporosity.
Method: In the cleanroom at the curation facility of JAXA/ISAS, images of Ryugu particles illuminated by Light-emitting-diode (LED) lights were taken from many directions using a camera by rotating it around the vertical axis. The camera observed the samples from an angle of 45 degrees with respect to the vertical axis. Since the range of camera rotation angles does not cover 360 deg, sample particles were rotated manually to acquire the images from all azimuthal angles around the particle. The 3-dimensional shape models of the particles were created from the images with the software Metashape (Agisoft LLC). The scale of shape models was calibrated using the images of prescribed grid taken under the same condition as the sample particles, and their volumes were estimated.
Evaluation of improvement of measurement equipment: For the measurement of Ryugu particles at the curation facility, we made improvement, such as increasing the camera angle in existing measurement equipment to improve the accuracy of measurement. To evaluate this improvement, shape measurement and density estimation of a graphite particle as a simulant of Ryugu particles were conducted. Compared with a shape model created from the images of the same graphite particle using our measurement system before this improvement, the accuracy of shape measurement was qualitatively improved since the shape of the underside of the particle became visible, and the noise in the shape model became smaller.
Density estimation of Ryugu particles: Since the Ryugu particles were fragile and measurement time was limited, it was difficult to flip the particles upside down for stereoscopic measurement of all the surfaces of the sample particles. Thus, it was necessary to improve the accuracy of the shape measurement by capturing the near-bottom surfaces only by measuring from oblique upward directions. We could obtain the density (1.06±0.17 g/cm3) of only one sample particle among the five particles measured in November 2021. The estimated density was consistent with the particle density (1.12±0.08 g/cm3) calculated from the measured volume in the previous study [3]. If this particle represents Ryugu, this would suggest that microporosity is dominant in Ryugu’s porosity. The method used in this study to estimate the volume, in principle, can become more accurate than the volume measurement by the ellipsoid approximation used in previous study [3]. We plan to measure the density of more Ryugu particles.
Reference: [1] Watanabe et al., 2019, Science, 364(6437). [2] Grott et al., 2020, J. Geophys. Res.: Planets, 125(12). [3] Yada et al., 2021, Nat. Astron., https://doi.org/10.1038/s41550-021-01550-6