10:45 〜 11:00
[PPS02-01] X-ray Computed Tomography Visualizes Behaviors of Granular Materials under Vertical Vibrations
キーワード：小惑星、粉粒体の対流、X線CT
Recent robotic small-body explorations have revealed that the observed asteroids are generally covered by loose, unconsolidated, granular materials called regolith [e.g. 1, 2]. These asteroids have rubble-pile structures, where rock fragments are held together by their weak gravities, making the surface states easy to be modified by later processes [e.g. 3]. The resultant surfaces from such surface processes show a specific homogeneity/heterogeneity in compositions [4], mechanical variations [5], and a range of particle sizes from sub-microns to meter-scale, sometimes up to boulder sizes [6]. While asteroid Ryugu has an almost continuous equatorial ridge [7], asteroid Itokawa does not have a large-scale topographic feature [1]. Instead, Itokawa shows two distinctive areas, smooth and rough terrains, resulting from global segregations of fine materials [8]. Such segregations are believed to be caused by impact-induced seismic shakings, which should occur on Ryugu and Bennu. Still, the nature of such granular behavior is not fully understood, even though laboratory experiments and numerical studies have been extensively conducted to observe complex behaviors of granular materials subjected to vertical vibration [e.g., 9, 10]. Part of the reason is the wall effect, where walls of containers or cells determine the behavior of granular materials in a way different from conditions on the surfaces of asteroids. Observing internal structures of granular materials are needed to comprehend this effect but are challenging to observe even if the materials are stored in a transparent box.
Here, we perform vibration experiments of granular materials without any wall effects as confirmed by internal-structure observations by using X-ray computed tomography (CT). We use rock particles with angular shapes, whose size-frequency distributions generally follow those of asteroids obtained studied by previous explorations [6]. The granular materials are stored in a cylindrical acrylic container, and a vibrator is placed at the bottom center of the container to transmit artificial shakings for making particles fluidized. In this manner, we only allow the central part of the container to be vibrated in order to eliminate the wall-induced effect. Internal cross-sectional images are frequently taken by X-ray CT technology. As a result, we can successfully observe the internal motions of every granular particle under granular convection. Granular particles at the sidewalls do not move, indicating the wall effects were sufficiently suppressed. The concentration of fine particles at the central part is confirmed, and this phenomenon can be differently interpreted from previous explanations of segregation in the vertical direction [11]. We conclude that, on rubble-piles asteroids, seismic waves can be generated by meteorite impacts, whose vibrations may cause regolith convection and concentration of fine grains in the subsurface at the center of vibrations.
References
[1] Saito et al., Science 312(5778), 1341-1344 (2006). [2] Lauretta et al., Nature 212, 925-984 (2019). [3] Fujiwara et al., Science 312(5778), 1330-1334 (2006). [4] Kitazato et al., Science 364(6437), 272-275 (2019). [5] Tanbakouei et al., Astronomy & Astrophysics 629, A119 (2019). [6] Michikami et al., Icarus 331, 179-191 (2019). [7] Sugita et al., Science 364, eaaw0422 (2019). [8] Miyamoto et al., Science 316, 1011-1014 (2007). [9] Knight et al., Physical Review Letters 70(24), 3728 (1997). [10] Breu et al., Physical Review Letters 90(1), 014302 (2003). [11] Maurel et al., Monthly Notices of the Royal Astronomical Society, stw2641 (2016).
Here, we perform vibration experiments of granular materials without any wall effects as confirmed by internal-structure observations by using X-ray computed tomography (CT). We use rock particles with angular shapes, whose size-frequency distributions generally follow those of asteroids obtained studied by previous explorations [6]. The granular materials are stored in a cylindrical acrylic container, and a vibrator is placed at the bottom center of the container to transmit artificial shakings for making particles fluidized. In this manner, we only allow the central part of the container to be vibrated in order to eliminate the wall-induced effect. Internal cross-sectional images are frequently taken by X-ray CT technology. As a result, we can successfully observe the internal motions of every granular particle under granular convection. Granular particles at the sidewalls do not move, indicating the wall effects were sufficiently suppressed. The concentration of fine particles at the central part is confirmed, and this phenomenon can be differently interpreted from previous explanations of segregation in the vertical direction [11]. We conclude that, on rubble-piles asteroids, seismic waves can be generated by meteorite impacts, whose vibrations may cause regolith convection and concentration of fine grains in the subsurface at the center of vibrations.
References
[1] Saito et al., Science 312(5778), 1341-1344 (2006). [2] Lauretta et al., Nature 212, 925-984 (2019). [3] Fujiwara et al., Science 312(5778), 1330-1334 (2006). [4] Kitazato et al., Science 364(6437), 272-275 (2019). [5] Tanbakouei et al., Astronomy & Astrophysics 629, A119 (2019). [6] Michikami et al., Icarus 331, 179-191 (2019). [7] Sugita et al., Science 364, eaaw0422 (2019). [8] Miyamoto et al., Science 316, 1011-1014 (2007). [9] Knight et al., Physical Review Letters 70(24), 3728 (1997). [10] Breu et al., Physical Review Letters 90(1), 014302 (2003). [11] Maurel et al., Monthly Notices of the Royal Astronomical Society, stw2641 (2016).