TWO MODELS FOR ISOTOPIC VARIATIONS IN THE LUNAR REGOLITH
RAY, JAMES R.
Doctor of Philosophy
Two independent models are presented which attempt to account for large, apparently secular, isotopic variations recorded in lunar samples. To explain the observed long-term increase of ('15)N/('14)N, the first model considers the consequences of a "spike" contamination of the sun's outer convective zone with material greatly enhanced in ('14)N. We argue that this situation may have arisen through the following scenario: (i) The sun formed in a star cluster which included a nearby (TURN)3M(,(CIRCLE)) member. (ii) After about 10('8)y, the companion star ejected a planetary nebula enriched in ('14)N, ('22)Ne and ('4)He due to prior nuclear processing. (iii) Upon encoutnter with the solar system, the planetary nebula material was accreted onto the sun's surface and possibly into planetary/meteroitic source matter. The predominant effect was a sizeable decrease of ('15)N/('14)N in the solar convective zone and hence in the solar wind. (iv) Subsequent mixing processes in the convective zone have slowly acted to restore the sun's original surface composition. That variation, in turn, has been recorded by solar wind N trapped in lunar soils and breccias. The second model takes note of observational evidence from the Apollo 16 site which indicates that even though TiO(,2)-rich samples from other sites show a long-term increase of ('3)He/('4)He, TiO(,2)-poor samples do not. Consequently, we regard models which envision He isotopic variations as occurring in the solar wind reservoir to be untenable. Instead, the explanation must entail the differing retentivities of high and low TiO(,2) materials for inert gases. We propose that lunar atmospheric He is ultimately responsible for secular isotopic variations in He and deduce the requirements on past solar wind flow conditions. In order for atmospheric He to have been an important source of trapped lunar gas (by ionization and subsequent acceleration a la Manka and Michel, 1970), the ancient solar wind flux must have been about 100 times greater than now. Assuming the bubble diffusion model of Tamhane and Agrawal (1979) to accurately describe gas loss from lunar soil particles, we show that the different He isotopic records of TiO(,2)-rich and -poor samples can be explained if ancient atmospheric He ions were implanted with larger energies than contemporary solar wind ions. This is equivalent to requiring that the ancient interplanetary magnetic field was larger than today by a factor greater than 4.4. Associated secular increases of ('15)N('/14)N and ('13)C/('12)C are expected but the magnitudes depend critically upon the fractions of these elements which evolve into the lunar atmosphere in the atomic form.(' )