Impact Process of Boulders on the Surface of Asteroid 25143 Itokawa— Fragments from Collisional Disruption

Earth Planets Space, 60, 7–12, 2008 Impact process of boulders on the surface of asteroid 25143 Itokawa— fragments from collisional disruption A. M. Nakamura1, T. Michikami2, N. Hirata3, A. Fujiwara4, R. Nakamura5, M. Ishiguro6, H. Miyamoto7,8, H. Demura3, K. Hiraoka1, T. Honda1, C. Honda4, J. Saito9, T. Hashimoto4, and T. Kubota4 1Graduate School of Science, Kobe University, Kobe, 657-8501, Japan 2Fukushima National College of Technology, Iwaki, Fukushima 970-8034, Japan 3School of Computer Science and Engineering, University of Aizu, Aizuwakamatsu, Fukushima 965-8580, Japan 4Institute of Space and Astronautical Sciences (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Kanagawa 229-8510, Japan 5National Institute of Advanced Industrial Science and Technology, Tsukuba 306-8568, Japan 6Astronomy Department, Seoul National University, Seoul 151-747, Korea 7The University Museum, University of Tokyo, Tokyo 113-0033, Japan 8Planetary Science Institute, Tucson, AZ 85719, USA 9School of Engineering, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan (Received November 3, 2006; Revised March 29, 2007; Accepted April 11, 2007; Online published February 12, 2008) The subkilometer-size asteroid 25143 Itokawa is considered to have a gravitationally bounded rubble-pile structure. Boulders appearing in high-resolution images retrieved by the Hayabusa mission revealed the genuine outcome of the collisional event involving the asteroid’s parent body. Here we report that the boulders’ shapes and structures are strikingly similar to laboratory rock impact fragments despite differences of orders of magnitude in scale and complexities of the physical processes. These similarities suggest the universal character of the process throughout the range of these scales, and the brittle and structurally continuous nature regarding the parent body of the boulders. The similarity was likely preserved because of relatively lesser comminuting processes acting on individual boulders; the close assemblages of similar appearing boulders (a boulder family) represent the impact destruction of boulders on the surface. Key words: Asteroid, boulder, impact, fragmentation. 1. Introduction teorite’s fall have suggested that meter-class, stony, near- The collisional growth and disruption of solid bodies take Earth asteroids (NEAs) have tensile strengths more than an place over many magnitudes of scale, ranging from dust order of magnitude lower than those measured for ordinary particles to planets. Small bodies in the solar system such chondrites (Brown et al., 2004). as asteroids experience mutually direct and destructive col- Laboratory studies on the impact process of asteroids lisions (Chapman and Davis, 1975). Asteroids are consid- have been undertaken for the past decades using light-gas ered to have internal structures resulting from collisions that gun and other facilities (Fujiwara et al., 1989; Holsapple range from monolithic, fractured and shattered due to mod- et al., 2002). Although these experimental studies have erate impacts to gravitationally reagglomerated rubble piles provided quantitative data and insights into many aspects following major disruption. The outcome of a collisional of centimeter-scale impact processes, difficulty in applying disruption depends largely on the mechanical property of these findings to asteroids does exist due to the very limited the parent body (Holsapple et al., 2002; Richardson et al., scale of the experiments. The laboratory experiments have 2002; Michel et al., 2003), that is, the transmission effi- therefore been extended to the larger scales by numerical ciency of the impact energy throughout the body (porosity) studies and scaling methods. and the bonding strength between the constituent compo- Asteroid 25143 Itokawa, about 500 meters long and nents of the body. However, only limited and indirect infor- 3.5 × 1010 kg in mass, is considered to have a rubble-pile mation about the mechanical property of asteroids has been structure consisting of fragments from an earlier collisional available. Meteorites are the compositionally correspond- disruption of a preexisting parent asteroid (Fujiwara et al., ing materials of asteroids, but our collection has probably 2006). The size of the largest boulder (Yoshinodai) is one- suffered greatly from a selection effect against weaker ma- tenth of Itokawa itself and the number of boulders increases terials due to dynamical pressure during atmospheric entry with decreasing size (Saito et al., 2006). Boulders larger and transit. For example, analyses of the trajectory of a me- than several meters in maximum width number in the hun- dreds. They cannot have been fully supplied by the craters Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sci- on the current surface, but must have originated on the par- ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society ent body or from the Itokawa-forming collisional event. Im- of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci- ences; TERRAPUB. ages of the boulders on Itokawa’s surface with pixel resolu- 7 8 A. M. NAKAMURA et al.: IMPACT PROCESS OF BOULDERS ON ASTEROID ITOKAWA SURFACE tion of ∼6mmto∼70 cm taken by the Asteroid Multi-band Imaging CAmera (AMICA) provide us with the actual outcome of the collisional disruption event of the parent body of a subkilometer-size asteroid. In Section 2, we report the morphological characteris- tics of the boulders on Itokawa and provide comparisons between the boulders and the laboratory impact fragments. In Section 3, we discuss the evidences for possible impact processes of boulders on the surface. Section 4 presents our conclusion. In this paper, we refer to apparently root- less rocks and features with distinctive positive relief larger than a few meters as “boulders” in accordance with a previ- ous definition (Saito et al., 2006). 2. Boulder Shape and Structure Boulders do not cover the surface of Itokawa uniformly. Fig. 1. View of a boulder-rich region on Itokawa (image no. The boulder-dominated region is called “rough terrain”, ST 2481211873). At the upper left, the boulders are in a pile. Scale whereas the remainder is “smooth terrain” (Saito et al., bar indicates 30 m. 2006). Figure 1 shows the boulder-rich region (the rough terrain) on Itokawa’s surface in contrast to the smooth terrain. While both angular and degraded boulders were ob- the parent body of the Yoshinodai boulder. served on asteroid 433 Eros (Thomas et al., 2001), the The most prominent features on the boulders are cracks boulders on Itokawa exhibit a wide spectrum of angularity and fractures (Figs. 2(b), (f)–(h)). A possible origin of the and irregularity (Figs. 2(a)–(i)). Thin, flat-looking boulders cracks and fractures is any process after the boulder was were also seen, although they may be parts of bigger blocks formed, for example, impact fracturing on Itokawa’s sur- that are mostly hidden beneath the surface. One of these face (possibly the cracks on the boulder in Fig. 2(h), since with a spatula-like shape (Figs. 2(d), (e)) was observed in small pieces near the boulder with a similar appearance multiple images and its thickness-to-width ratio was deter- could have been fragmented from the boulder). However, mined to be ∼0.2. A Similarly wide range of shapes is this is unlikely for most of the cracks of the boulders, since also seen in the fragments from laboratory impact disrup- they are not linked with any crater-like depression expected tions. Figures 2(j)–(l) show fragments of centimeter-size for an impact site. Moreover, cracks are commonly ob- collected after laboratory impacts. Basalt spheres of 6 cm served in laboratory impact fragments (Fig. 2(k)) generated in diameter and only 0.3 kg in mass were impacted by a 7- from originally homogeneous targets with no apparent sur- mm-diameter nylon projectile at ∼3.2 km/s (Nakamura and face cracks before impact disruption. These cracks also re- Fujiwara, 1991; Nakamura, 1993; Nakamura et al., 2007). mained on fragments in numerical simulations of solid body Fragments in Fig. 2(j) are angular and conical, which is disruption (Benz and Asphaug, 1994) and were caused by characteristic of boulders in Fig. 2(b). The square frag- the growth of incipient microscopic flaws of the target body. ments in Fig. 2(k) look similar to the boulders in Fig. 2(a). Figure 3 illustrates the largest cracks reach a length up to The irregularly shaped (wavy-shaped) fragment in Fig. 2(k) about 0.8 times the maximum width of fragments, similar is similar to the one in Fig. 2(i). to the apparent cracks in the boulders on Itokawa. The frag- The similarity in shape suggests a brittle and consolidated ments and boulders having cracks of lengths similar to their nature of the boulders’ parent body, in which cracks could own sizes are in a state corresponding to “fully cracked” at propagate. The thin shape is additional evidence sugges- that size scale (Housen and Holsapple, 1999). It is likely tive of such a nature. Thin spall fragments are ejected at that the cracks on the boulder surfaces are mostly due to the an impact event because of tensile fracture caused by a ten- impact event that created the boulders. sile wave reflected from a compressive wave at the free sur- Although there is a difference of many orders of magni- face of the target. In other words, spallation occurs at the tude in the scale and complexity of the physical processes, free surface when the compressive wave generated by an these first-order similarities in shapes and structures of the impact travels to the surface without being severely attenu- boulders and the laboratory fragments bridge laboratory ated by pores and internal discontinuities in the target ma- disruption of solid bodies (governed by the growth and co- terial along the way. The average ratio of the thickness to alescence of microscopic flaws) and the natural collisional maximum length of the spall fragments from cratering ex- disruption process.

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