Terrestrial ℗ℓNe and the accretion time of the earth
Palma, Russell L.
Master of Science
The atmospheric composition of neon is considered along the lines of the multicomponent theory for that gas. It is found that an excess amount of ^Ne is indicated if the terrestrial values of the 2^Ne/22Ne and 2^Ne/22Ne ratios are taken to be the result of mixtures of the components Ne-E and Ne-B or Ne-E and Ne$w. This excess ranges from 9-17% of the Ne presently in the earth's atmosphere. Production rates for 21Ne by galactic cosmic ray irradiation are calculated from consideration of meteorite and lunar drill core data. The Ne excess indicated by the selected component mixing lines places constraints on the mass distribution of the earth's protoplanetary bodies and their accretion times. The formulation developed by Safronov and Wetherill for the formation interval of the earth yields a result in concordance with this possible 21^Ne excess. Solar cosmic rays and the influx of meteroids are found to be of negligible importance in the production of Ne, as are all nuclear reactions in the earth after formation, excepting 18O(a ,n)21Ne. This last reaction contributes approximately 2.1% of all Ne over the lifetime of the earth. The possible contribution to terrestrial neon from the passage of the solar system through dense interstellar clouds is discussed, as is the retention of neon during the accretionary process.