On the Brittle-Ductile Behavior of Iron Meteorites' New Experimental Constraints

High-velocity (-•2 to -•5 km/sec) impacts into octahedrite iron meteorites (Gibeon, El Sampal, and Arispe), steel, and iron targets were made at the NASA-Ames Vertical Gun Facility. The experiments clearly revealed the brittle behavior of meteorites at very low temperatures (<200 ø K) as indicated by spallation of the crater rim and the development of deeply penetrating fractures in the meteorite sample. Although failure also occurred at higher temperatures (290 ø K), it was restricted to near-surface spaIls. High-velocity (-•5 km/sec) impacts resulted in a net mass loss regardless of the brittle-ductile behavior of the target or projectile, whereas low-velocity (-•2 km/sec) impacts resulted in a net mass gain for ductile projectiles into brittle and ductile targets. Consequently, even if iron meteorites (octahedrites) were originally the core of a layered parent body, they might be subsequently destroyed in a brittle manner. Such results are consistent with the hypothesis by Matsui and Mizutani (1977) that the brittle behavior of iron-like planetesimals inthe low-temperature asteroid zone prevents growth into full-size planets.