A new interpretation of the mechanical properties of the Gibeon Meteorite

Abstract

A set of results on the temperature and strain rate dependence of the tensile properties of the Gibeon meteorite reported by Gordon in 1970 is reinterpreted. It is shown that these results indicate that the meteorite material undergoes a ductile-brittle transition in low strain rate tensile tests at about 50°K. It is estimated that under asteroidal impact conditions the transition temperature would be about 200°K. Thus provided that the internal hydrostatic pressure did not depress the transition temperature significantly, the material would fragment in a brittle manner yielding meteoritic fragments free of plastic deformation. An approximate calculation of the effect of pressure on the transition temperature shows that brittle behavior would not occur in asteroids having radii greater than about 140 km. Thus since meteoritic fragments rarely show signs of gross plastic deformation, it is concluded that this is an approximate upper limit to the size of the parent asteroids. Work on the nickel composition profile in octahedrites by J. A. Wood (1964) led to an estimate of the radii of the parent asteroids of about the same magnitude. Our value of 140 km is probably subject to an error of about 50% in either direction. The larger asteriods now present in the solar system are of about this size.