5:15 PM - 7:15 PM
[PPS07-P10] 3D visualization and observation of chondrules by destructive tomography
Keywords:chondrule, 3D visualization, tomography, observation, planet formation
Introduction: It is essential to understand how dust particles accumulate into planetesimal scale objects to understand the initial conditions for planet formation [e.g., 1]. Chondrule is a spherical-shaped inclusion observed in chondrites, with a diameter of about 1 mm. In addition to typical shapes, chondrules with irregular shapes are often found [e.g., 2]. The deformation of chondrules represents the heating and impact processes that the chondrule underwent in either the protoplanetary disk or the parent body [3,4]. Therefore, the origin of the diverse shapes provides clues to reveal physical conditions in dust accumulation. However, it is impossible to accurately measure the particles' three-dimensional (3D) shape from cross-sectional images. Measuring the 3D shape using tomographic methods, which reconstruct the cross-sectional image in three dimensions, allows us to discuss the causes of the particle shapes of chondrules. In addition, the color of the particle surface and interior can be obtained to distinguish and recognize secondary processes that have occurred after the formation of the particle shape.
Method: First, a sample of the Allende meteorite (CV3 chondrite) was prepared by mounting a piece (1880 mm3) of the meteorite in resin. The specimens were polished every 5 µm, and each polished surface was photographed with a high-performance digital camera (owned by Hokkaido University). A total of 2343 images (24-bit RGB TIFF format) were created. In this study, we focused on two chondrules with a circular shape and two chondrules with an irregular shape in the cross-sectional images. The cross-sectional images of each of the four particles were stacked and reconstructed on the software (Amira) so that the surface and interior of each particle's full color 3D model could be observed in a 360° direction.
Results: The 3D visualization showed that two of the four particles of interest were spherical chondrules, and two were non-spherical chondrules. We observed the spherical chondrule with a circular depression with a depth of about 0.1 mm in one place on the surface. In the cross-sectional images, the chondrule with a dimple showed completely melted tissue, while the compound chondrule showed mottled tissue. On the other hand, non-spherical chondrules with a flat surface in one direction and square chondrules were observed. On the cross-sectional images, the flat-faced chondrule showed a completely melted structure, whereas the angular chondrule showed a mottled structure.
Discussion: Generally, chondrules are formed from the dust heated and melted in the protoplanetary disk, followed by rapid cooling [e.g., 5]. Gravity and centrifugal forces acting during the melting process basically result in the formation of spherical chondrules. In this study, circular depression was observed on the surface of the spherical chondrule. The depression observed on the spherical chondrules may reflect secondary events after chondrule formation, such as collisions of dust particles [6]. This means that the secondary deformation of this spherical chondrule occurred in the protoplanetary disk. On the other hand, the multiple chondrules observed in this study are considered to have formed when two chondrules collided before reaching the same temperature [6].
Non-spherical chondrules are thought to have experienced larger collisional and impact processes in the protoplanetary disk and/or parent body than spherical chondrules. Two theories have been discussed for the formation of flat-shaped chondrules [7,8]. One is the deformation process on the disk, which is known to be caused by the centrifugal force of the high-speed rotation of the droplet before chondrule formation and the dynamic pressure of the nebular gas [7]. The structure (completely melted) of flat-faced chondrules suggests that they may have been exposed to the disk environment as a droplet during the chondrule formation. Also, the flat structure may be formed via the impact process within the parent body [8]. At present, the two theories cannot be completely distinguished, but future analysis of the shape and dimples of 3D models of chondrules should help to distinguish between the two theories quantitatively. The formation of angular chondrules is considered secondary destruction by dust particle collisions [2], but this has not been fully discussed. In the future, the constraints on the formation process will be assessed by observing the fracture surfaces of angular chondrules.
[1] Okuzumi et al., 2012 [2] Kitamura, 1989 [3] Nakamura et al., 2012 [4] Forman et al., 2023
[5] Wakita, 2022 [6] Tsuchiyama et al., 2000 [7] Uesugi et al., 2006 [8] Lindgren et al., 2015
Method: First, a sample of the Allende meteorite (CV3 chondrite) was prepared by mounting a piece (1880 mm3) of the meteorite in resin. The specimens were polished every 5 µm, and each polished surface was photographed with a high-performance digital camera (owned by Hokkaido University). A total of 2343 images (24-bit RGB TIFF format) were created. In this study, we focused on two chondrules with a circular shape and two chondrules with an irregular shape in the cross-sectional images. The cross-sectional images of each of the four particles were stacked and reconstructed on the software (Amira) so that the surface and interior of each particle's full color 3D model could be observed in a 360° direction.
Results: The 3D visualization showed that two of the four particles of interest were spherical chondrules, and two were non-spherical chondrules. We observed the spherical chondrule with a circular depression with a depth of about 0.1 mm in one place on the surface. In the cross-sectional images, the chondrule with a dimple showed completely melted tissue, while the compound chondrule showed mottled tissue. On the other hand, non-spherical chondrules with a flat surface in one direction and square chondrules were observed. On the cross-sectional images, the flat-faced chondrule showed a completely melted structure, whereas the angular chondrule showed a mottled structure.
Discussion: Generally, chondrules are formed from the dust heated and melted in the protoplanetary disk, followed by rapid cooling [e.g., 5]. Gravity and centrifugal forces acting during the melting process basically result in the formation of spherical chondrules. In this study, circular depression was observed on the surface of the spherical chondrule. The depression observed on the spherical chondrules may reflect secondary events after chondrule formation, such as collisions of dust particles [6]. This means that the secondary deformation of this spherical chondrule occurred in the protoplanetary disk. On the other hand, the multiple chondrules observed in this study are considered to have formed when two chondrules collided before reaching the same temperature [6].
Non-spherical chondrules are thought to have experienced larger collisional and impact processes in the protoplanetary disk and/or parent body than spherical chondrules. Two theories have been discussed for the formation of flat-shaped chondrules [7,8]. One is the deformation process on the disk, which is known to be caused by the centrifugal force of the high-speed rotation of the droplet before chondrule formation and the dynamic pressure of the nebular gas [7]. The structure (completely melted) of flat-faced chondrules suggests that they may have been exposed to the disk environment as a droplet during the chondrule formation. Also, the flat structure may be formed via the impact process within the parent body [8]. At present, the two theories cannot be completely distinguished, but future analysis of the shape and dimples of 3D models of chondrules should help to distinguish between the two theories quantitatively. The formation of angular chondrules is considered secondary destruction by dust particle collisions [2], but this has not been fully discussed. In the future, the constraints on the formation process will be assessed by observing the fracture surfaces of angular chondrules.
[1] Okuzumi et al., 2012 [2] Kitamura, 1989 [3] Nakamura et al., 2012 [4] Forman et al., 2023
[5] Wakita, 2022 [6] Tsuchiyama et al., 2000 [7] Uesugi et al., 2006 [8] Lindgren et al., 2015