PHENOMENA ASSOCIATED WITH LOW-VELOCITY (10 Q 300 m/sec) IMPACT AGAINST ROCK. Takafumi Matsui*, Toshihiko Waza*, Kouki Kani** and Shoji Suzuki***, * Geophysical Institute, Faculty of Science, Univ. of Tokyo, ** Faculty of Education, Okayama Univ., *** Faculty of Engineering, Kyoto Univ..

It is believed that planets were formed by accretion of planetesimals. Matsui and Mizutani (1977) pointed out that the collisional behaviour of rocks composing planetesimals have play- ed an important role in the early stage of planetary formation. Collisional evolution-of mass-distribution spectrum of planetes- imals was shown to be influenced significantly by the relations characterizing the collisions-1 phenomena (Matsui, 1978, 1979). However, impact phenomena associated with collision have not been fully understood so far. It has been shown from the recent the- oretical studies on the random velocity distribution of planetes- imals that the mean impact velocity between planetesimals with the mass of 1018 g was in the range of 10 to 100 m/sec (e.g. Nakagawa, 1978). The collisional phenomena associated with such a low-velocity impact have, specifically, not been studied. Therefore, we performed the low-velocity impact experiment on rocks to reveal a nature of the collision process of planetes- imals. Using the 15 mm powder gun facility of faculty of Engi- neering, Kyoto University, projectiles of the cylindrical mild- steel (15CK, carbon content is 0.15 %) and rocks (tuff and basalt) were fired against four kinds of rocks (tuff, basalt, granite and dunite). Impact velocity ranges from%10 to~300m/sec. Mass ratio between projectile and target is almost 1. Thus, the im- parted energy to unit taget mass, ~i,ranges from 1.106 to %lo8 erg/g.- - The phenomena associated with the impact of the mild-steel against the spherical rock target were classified into five cat- egories. The cateqories vary with increase in impact velocity as follows: (I)destruction of the target with producing shatter cone-like fragments, and (5) complete de- struction of the target. These classifications do not depend on the rock type. In Fig. 1 are shown the above four (2, 3, 4 and 5) categories schematically. The phenomena associated with the impact of rock against spherical rock with similar mechanical -properties - are, however, different from the above catesories. For the rbck-rock collision, relative size between projectile and target played an important role on determining the collisional mode: only the smaller body was destructed completely and the larger body suffered no damage even if the released Ei was enough to destruct both projectile and target simultaneously. This ob- servation has an important implication that runaway growth-of the largest body is expected to occur even when the collision condi- tion prefers the destruction to other collisional modes. The size distribution of the fragments produced by the com- plete destruction was well expressed by an inverse power law rela-

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Matsui, T. et.al. -a tion, n(s) ds a s ds, where n(s) is the incremental number of the fragments with the size, s, and a is constant. a seems to increase with increase in ~i.A relationship between a and Ei was well approximated by a = -5.1 + log Ei. The maximum fragment mass normalized by the original target mass (M1/Mt) was also related with Ei. The re1 tionship deter- mined empirically w s expressed by M1/Mt = k Ei-8 . k and 8 are as follows: 5.1~103 and 0.6620.06 for spherical tuff, 8.5~10~ and 0.8520.2 for spherical granite, 4.5~10~and 0.66+0.14 for spheri a1 basalt, 3.1~10~and 1.5320.58 for cubic basalt, and 8.2~105 and 0.7+0.05 for all spherical targets. Fujiwara et al. have reported tEe empirical formula for the cubic basalt based on the high-velocity (a few km/sec) impact experiment on rocks (Fujiwara et al., 1977). The above result is different from that derived by Fujiwara et al.. The maximum fragment mass is a good index for marking the boundries between categories. It is suggested that destruction of the rocks initiates once Ei ex- ceeds %lo6 erg/g. This is consistent with the result derived by Hartmann (1978) for the basalt ball. The shape distribution of the fragments produced by the shatter cone-like destruction (the category 4) is investigated. The length ratios of the fragments between the shortest and the longest axes, a/c, and between the intermediate and the longest axes, b/a, are distributed in the region greater than 0.2 and 0.3, respectively. Average ratio of a:b:c is about 2:fi:l. These results are consistent with those derived from the high- velocity impact experiment on rocks (Fujiwara et al. , 1978) . At the velocity range of ~~100m/sec or less, impact process is interpreted in quasi-static regime, since the impact velocity is much smaller than the shock wave velocity. The core type ' destruction, which means that the central part is left intact as a core, cannot-be observed in our experiment but instead we ob- served the shatter cone-like destruction. Such a difference in fracture mode suggests that the latter can be interpreted by quasi-static regime. The authors wish to express thanks to Prof. Yamada of fac- ulty og Engineering, Kyoto Univ. for his permission to use the gun facility of his laboratory through the course of this study. REFERENCES: Fujiwara, A. et al. (1977) Icarus 2,277-288. Fujiwara, A. et al. (1978) Nature -272, 602-603. Hartmann, W.K. (1978) Icarus 33, 50-61. Matsui, T. (1978) Proc. Lunar and Planet. Sci. con£. -9th, 1-13. Matsui, T. (1979) Proc. Lunar and Planet. Sci. Conf. loth, 1881-1895. Matsui, T.and Mizutani H. (1977) Nature -270, 5-07. Nakagawa, Y. (1978) Prog. Theor. Phys. 58, 1834-1851. 1. ~.bwmdrlth radbllng 1Issur.s . N. C.l.sIr~phI~

Fig. 1 The four categories observed in the low- velocity impact phe- nomena are shown schematically. Il.R.bound rlih longlludllul splllllng

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