Semiempirical rates of formation of cosmic ray tracks in spherical objects exposed in space: preatmospheric and postatmospheric depth profiles

Abstract

Observations of fossil tracks in meteorites and lunar rocks are used to estimate long-term averaged production rates of tracks due to cosmic ray multicharged nuclei in common silicates in spherical meteorites. Results are presented graphically for track production rate (TPM, tracks/cm² m.y.) versus depth in meteorites with radii of 3-500 cm. Track production profiles are also presented for TPM values expected at the surface of a uniformly ablated meteorite as a function of ablated depth for 'recovered,' i.e., postatmospheric, radii values of 3-500 cm. In the case of meteorites having a 'single' exposure history their cosmic ray exposure age is given by measurements of cosmogenic gases, e.g., ³He and ²²Ne. This procedure allows an estimation of TPM values at the surface of the meteorite from the observed track densities. The TPM curves presented herein then suffice to deduce the ablation during atmospheric transit of a spherical meteorite for cases where the assumption of a uniform burn is a reasonable approximation. Errors in the deduced extent of ablation due to uncertainties in the estimated TPM values and the size of the recovered meteorite are discussed.