Meteorite Ablation Evaluated from Data on the Distribution of Cosmogenic Neon Isotopes

The type of the functional dependence of the ratio of the production rates of the cosmogenic isotopes ²²Ne/²¹Ne_c on their location depth d (cm) in ordinary chondrites with a radius R 60 cm was determined on the basis of experimental data on the elemental production rates of cosmogenic Ne isotopes in chondrites (Leya et al., 2000a). The dependence found is of the type ²²Ne/²¹Ne_c = Aexp(–Bd) + C, where the parameters A, B, and C are determined from the relationships: B = 0.560exp(–0.0105R) – 0.187, C = 0.170exp(–0.092R) + 1.083, and 0.170exp(–0.092R) + 1.144. These relationships were used to calculate the average weighted values of the ²²Ne/²¹Ne_c ratio for the volume of the fallen meteorite depending on its given preatmospheric radius. The data obtained served as a basis for plotting a nomogram that makes it possible to estimate the mass lost during passage through the Earth's atmosphere (ablation quantity) from the mass of the fallen meteorite and the average value of the ²²Ne/²¹Ne_c ratio measured in it. The average (median) value of ablation found for 262 chondrites was 91.5^+2.1_–2.6%. In addition to the earlier-established (Alexeev, 2001a; Alexeev, 2001b) peculiarities of H5-chondrites that distinguish them from H-chondrites of other petrologic types, H5-chondrites appeared to exhibit a higher degree of ablation. The observed effect and other distinctive features of H5-chondrites may be due to the specific evolution of the parent body of H-chondrites in the process of its disintegration, reaccumulation, and subsequent reworking of the surface layers.