The influence of chondrules on sub-mm fragment shape distributions in Allende impact experiments

The surfaces of sub-kilometer-sized asteroids directly explored by spacecraft, such as Itokawa, Ryugu and Bennu, are covered with blocks and/or regolith particles, whose shapes are considered clues to understanding their formation and evolution on the asteroid's surface. Ryugu particles returned by the Hayabusa2 mission are likely fragments resulting from impacts because their shapes resemble impact fragments from laboratory experiments. However, there is a lack of laboratory impact experiments examining the shapes of fragments in carbonaceous chondrites, thought to originate from carbonaceous asteroids such as Ryugu and Bennu. The measured sizes of Ryugu particles are in the mm and sub-mm range, similar to the sizes of chondrules. Also, carbonaceous chondrites are generally structurally weaker than the basalts and granites often used in previous laboratory impact experiments. Differences in the strength of the chondrules and matrix might affect the overall strength of the meteorite. In this study, as a first step towards a better understanding of impact fragment shapes in carbonaceous chondrites, we conducted impact experiments on the carbonaceous meteorite Allende (CV3). A spherical alumina projectile with 1.0 mm and a glass projectile with 0.80 mm in diameter were fired into 1–2 cm-sized Allende targets at nominal impact velocities of 2.0 and 4.0 km/s, respectively. To investigate the correlation between the chondrules (typically sub-mm in size) and the shapes of fine fragments, we measured the shape distributions of sub-mm impact fragments using X-ray microtomography. We observed several impact fracture surfaces along the chondrule boundaries. In addition, these fragments tended to be rounder than fragments from previous impact experiments. However, because the total number of these fragments is relatively small, the fragments were found to have the same overall shape distribution as previous laboratory impact fragments, Itokawa particles and Ryugu particles. This may imply that impact fragment shapes are independent of the bulk material strength. These findings will be useful for understanding the formation process of regolith layers on asteroid surfaces, Itokawa particles, Ryugu particles, and Bennu particles.