Destruction of rocks by low velocity impact and its implications for accretion and fragmentation processes of planetesimals

Abstract

Recent experimental results on impact destruction of rocks (basalt and tuff) are summarized. The mode of destruction and some empirical relations between the mass of the largest fragment Ml, the released kinetic energy divided by target mass Ei, and the work done during impact W are presented. The mass Ml can be related to both Ei and W:log (Ml/Mt) = a ‐ β log Ei and log (Ml/Mt) = b ‐ γ log W. An effect due to the difference in impact velocity on the Ml versus Ei relation can be observed, but there seems to be no effect due to impact velocity on the Ml versus W relation. Thus it may be suggested that W is a useful means of describing phenomena associated with the impact destruction of rocks. Target shape also affects the Ml versusEi relation but no large differences have been observed between various rocks. The size distribution of fragments can be adequately expressed in terms of an inverse power law relation, in which slope of the size distribution α (i.e., exponent) increases with increasing Ei:α = 0.90 + 0.36 log Ei for basalt and α = 1.94 + 0.24 log Ei for tuff. Based on the empirical α ‐ Ei relations, a fragmentation model of the parent bodies of Eos and Koronis families is presented. It is suggested that the parent bodies of Eos and Koronis families had been differentiated prior to fragmentation and were completely fragmented at Ei = ∼2 × 104 joule/kg and ∼2 × 103 joule/kg.