Abstract
In 2010, fine regolith particles on asteroid Itokawa were recovered by the Hayabusa mission. The three-dimensional microstructure of 48 Itokawa particles smaller than 120 µm was examined in previous studies. The shape distribution of Itokawa particles is distributed around the mean values of the axial ratio 2:√2:1, which is similar to laboratory impact fragments larger than several mm created in catastrophic disruptions. Thus, the Itokawa particles are considered to be impact fragments on the asteroid's surface. However, there have never been any laboratory impact experiments investigating the shapes of fine fragments smaller than 120 µm, and little is known about the relation between the shapes of fine fragments and the petrographic textures within those fragments. In this study, in order to investigate the relation between the petrographic textures and the shapes of fine fragments by impacts, the shapes of 2163 fine fragments smaller than 120 µm are examined by synchrotron radiation-based microtomography at SPring-8. Most samples are fine fragments from basalt targets, obtained in previous laboratory impact experiments by Michikami et al. (2016). Moreover, two impacts into L5 chondrite targets were carried out and the shapes of their fine fragments are examined for comparison. The results show that the shape distributions of fine fragments in basalt targets are similar regardless of impact energy per target mass (in contract to the shape distribution of relatively large fragments, which are affected by impact energy), and are similar to those in L5 chondrite targets and Itokawa regolith particles. The physical process producing these fine fragments would be due to multiple rarefaction waves in the target. Besides, the petrographic textures do not significantly affect the shapes of fine fragments in our experiments. On the other hand, according to Molaro et al. (2015), the shapes of the fragments produced by thermal fatigue by the day-night temperature cycles on the asteroid surface are influenced by the petrographic textures. Therefore, we conclude that the Itokawa particles are not the products of thermal fatigue but impact fragments on the asteroid surface.
Highlights
Since the pioneering work of Fujiwara et al (1978), the shapes of fragments in laboratory impact experiments have often been characterized by axes a, b and c, these being the maximum dimensions of the fragment in three mutually orthogonal planes (a ≥ b ≥ c)
Degree of fragmentation and fragment shapes In the adopted experimental data of the fragments (Michikami et al, 2016), the degrees of fragmentation of the target are observed in a wide range from cratering to catastrophic disruption
In line with Michikami et al (2016), we grouped the degrees of fragmentation of the target into four types in the order of decreasing Q: Type I, cratering with no catastrophic disruption [Q < 250 J/kg]; Type II, a transition type where parts of the side surfaces are chipped off [250 ≤ Q < 1050 J/kg]; Type III, the so-called core type, signified by whole surfaces of the target being spalled off with only the core in the central part of the target remaining [1050 ≤ Q < 8000 J/kg]; and
Summary
Since the pioneering work of Fujiwara et al (1978), the shapes of fragments in laboratory impact experiments have often been characterized by axes a, b and c, these being the maximum dimensions of the fragment in three mutually orthogonal planes (a ≥ b ≥ c). The axial ratios of fragments are distributed around mean values of the axial ratios b/a. ∼po0r.t7ioann2d:√c/2a:1∼(0F.5u,jiwi.ea.racoertreasl.p, o1n9d7i8n;gCtaopaac:cbi:ocniinetthale., simple pro1984, 1986; Durda et al, 2015; Michikami et al, 2016). On the other hand, the shapes of fragments are flatter, i.e. the mean axial ratios of b/a and c/a are ∼0.7 and ∼0.2, respectively (Michikami et al, 2016). The shapes of fragments in impact cratering are different from those in catastrophic disruption. T. Michikami et al / Icarus 302 (2018) 109–125
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