10:00 〜 10:15
[PPS02-04] Experimental Study of the Behavior of Granular Materials on Small Bodies under Vibrations using X-ray Computed Tomography
キーワード：小天体、レゴリス、振動、X線CT
Small bodies are believed to hold important information about the formation and early evolution of the Solar System [e.g., 1, 2]. Most of the explored small bodies have been found to be covered by unconsolidated gravel deposits called regolith [e.g., 3]. The regolith appears to be a homogeneous chunk of gravel, but careful investigations indicate that it varies from body to body and/or region to region. We interpret this to be important because such variations may reflect mass transport processes due to disturbances that occurred after the formation of the bodies. For example, the presence of fine ponds and the statistical deficiency in small craters on Eros are interpreted as the result of regolith migration [4]. Itokawa has two distinct areas, one filled with large boulders and the other with fine-grained regolith, indicating the segregation of fines due to the gravel migration [5]. Compared to Itokawa, Ryugu and Bennu shows less apparent regional differences, but many traces of mass movements exist, suggesting that regolith migration may have contributed significantly to the formation of the seemingly homogeneous surfaces [1, 6].
The nature of mass transport on asteroids, however, still remains ambiguous especially in the vertical direction, although some perturbations including gardening effects may exist [e.g., 7]. This is due to the fact that observations of asteroids are basically limited to their surfaces. In this study, therefore, we experimentally investigate the nature of mass movement on an asteroid, specifically when it is perturbed by impacts such as micrometeorites, and how the movement processes of the regolith affect the surface.
While experimental studies of the motion of granular material are well established in the field of granular physics [8, 9, 10], direct applications of their results to the behavior of regolith of small bodies are challenging due mainly to the following issues; First, most previous studies have used spherical particles of a uniform size, whereas regolith is generally composed of complex-shaped particles of different sizes. Second, the wall effect should be removed; the behavior of powdery materials is often controlled by frictional differences between the particles and the surrounding walls. Finally, the behavior of particles inside the container cannot be directly observed from the outside of the container. Thus, we prepared a rock set using freshly crushed rock particles to simulate appropriate regolith size and shape distributions. The rock set was placed into a container, where vertical vibrations were applied only at the central region of the container to suppress the wall effect. X-ray CT was then used to directly observe the particles inside the container.
The behavior of rock particles was observed from the images showing the surface of the granular sample and the X-ray CT images revealing the interior. As a result, we successfully observed the behavior of granular materials under vibration, indicating the occurrence of the granular convection. The behavior of each particle was traced from point cloud data obtained by mapping the coarse-grained particles in X-ray CT images. The position of the center of gravity and the orientation of the longest axis of each particle were used to compare the rocks in our experiment and those existing on the surface of asteroid Ryugu, suggesting their similarity in behavior under vibrations. The findings of this work have significant contributions to understanding the behavior of unconsolidated granular materials in the microgravity environment of small bodies.
References
[1] Sugita et al., Science 364, eaaw0422 (2019). [2] Lauretta et al., Nature 212, 925-984 (2019). [3] Murdoch et al., Asteroids IV, 767-794(2014). [4] Dombard et al., Icarus 210(2), 713-721 (2010). [5] Miyamoto et al., Science 316, 1011-1014 (2007). [6] Jawin et al., JGR-Planets 125(9), e2020JE006475(2020). [7] Willman et al., Icarus 208(2), 758-772 (2010). [8] Knight et al., Physical Review Letters 70(24), 3728 (1997). [9] Breu et al., Physical Review Letters 90(1), 014302 (2003). [10] Maurel et al., Monthly Notices of the Royal Astronomical Society, stw2641 (2016).
The nature of mass transport on asteroids, however, still remains ambiguous especially in the vertical direction, although some perturbations including gardening effects may exist [e.g., 7]. This is due to the fact that observations of asteroids are basically limited to their surfaces. In this study, therefore, we experimentally investigate the nature of mass movement on an asteroid, specifically when it is perturbed by impacts such as micrometeorites, and how the movement processes of the regolith affect the surface.
While experimental studies of the motion of granular material are well established in the field of granular physics [8, 9, 10], direct applications of their results to the behavior of regolith of small bodies are challenging due mainly to the following issues; First, most previous studies have used spherical particles of a uniform size, whereas regolith is generally composed of complex-shaped particles of different sizes. Second, the wall effect should be removed; the behavior of powdery materials is often controlled by frictional differences between the particles and the surrounding walls. Finally, the behavior of particles inside the container cannot be directly observed from the outside of the container. Thus, we prepared a rock set using freshly crushed rock particles to simulate appropriate regolith size and shape distributions. The rock set was placed into a container, where vertical vibrations were applied only at the central region of the container to suppress the wall effect. X-ray CT was then used to directly observe the particles inside the container.
The behavior of rock particles was observed from the images showing the surface of the granular sample and the X-ray CT images revealing the interior. As a result, we successfully observed the behavior of granular materials under vibration, indicating the occurrence of the granular convection. The behavior of each particle was traced from point cloud data obtained by mapping the coarse-grained particles in X-ray CT images. The position of the center of gravity and the orientation of the longest axis of each particle were used to compare the rocks in our experiment and those existing on the surface of asteroid Ryugu, suggesting their similarity in behavior under vibrations. The findings of this work have significant contributions to understanding the behavior of unconsolidated granular materials in the microgravity environment of small bodies.
References
[1] Sugita et al., Science 364, eaaw0422 (2019). [2] Lauretta et al., Nature 212, 925-984 (2019). [3] Murdoch et al., Asteroids IV, 767-794(2014). [4] Dombard et al., Icarus 210(2), 713-721 (2010). [5] Miyamoto et al., Science 316, 1011-1014 (2007). [6] Jawin et al., JGR-Planets 125(9), e2020JE006475(2020). [7] Willman et al., Icarus 208(2), 758-772 (2010). [8] Knight et al., Physical Review Letters 70(24), 3728 (1997). [9] Breu et al., Physical Review Letters 90(1), 014302 (2003). [10] Maurel et al., Monthly Notices of the Royal Astronomical Society, stw2641 (2016).