Limits for the Accretion Time of the Earth from Cosmogenic 21Ne Produced in Planetesimals
Physics and Astronomy
Arguments are presented which show that no more than 23% of all neon-21 in the atmosphere consists of spallation Ne-21 formed by the bombardment of the earth's accreting objects by galactic cosmic rays. The amount of spallation Ne-21 depends strongly on the interpretation of the primitive, 'unirradiated' neon in the atmosphere. On the assumptions that (1) the planetesimals were irradiated by galactic cosmic rays of present-day intensity, (2) their mean lifetimes against accretion were 0.1 billion years, and (3) spallation Ne-21 was quantitatively retained by the accreting earth, and on the basis of Anders' (1965) reconstructed size distribution No. 2 of asteroids, the number of spallation Ne-21 atoms yielded amounts to 8.7% of the total number of Ne-21 atoms now on earth.
Proceedings of the Seventh Lunar Science Conference
D. Heymann, M. Dziczkaniec and R. Palma, 1976. Limits for the Accretion Time of the Earth from Cosmogenic 21Ne Produced in Planetesimals. Proceedings of the Seventh Lunar Science Conference 3, 3411-3419.