Transport and erosional processes in the Taurus-Littrow Valley -- Inferences from neutron fluences in surface soils

Abstract

Isotopic abundances of Gd and Sm have been measured in 15 surface soils from the Apollo 17 site in the Taurus-Littrow Valley. Generally the fluences at this site are similar to fluences measured in surface soils from other sites. Fluences calculated from the isotopic shifts directly reflect the origin of the soils. Materials with very low fluences are from the ejecta blankets of fresh deep craters. Soils composed predominantly of comminuted basalt, other than the ejecta from deep craters, have fluences that are distinctly lower than soils which contain significant proportions of highland materials. Differences in the chemical composition of the irradiated material can account for differences in measured fluence in basaltic soils from the valley floor and highland soils from the South Massif. To permit a direct comparison of the irradiation history of materials with different chemical compositions, we have calculated "normalized" fluences, i.e., the neutron fluence which the sample would have if it had been irradiated in a standard chemical composition. The similarity of the resulting normalized fluences indicates similar regolith residence times for these two types of materials. Higher than average normalized fluences in soils containing a constituent from the North Massif may reflect erosion and/or burial rates on the slopes of the massif that are less than the average burial rate of all the soils that have been sampled. A correlation between fluence and distance from the bottom of the North Massif suggests the erosional processes deposit highland soils from near the surface far into the valley, while materials of deeper origin are deposited close to the bottom of the massif.

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