Apollo 17 neutron stratigraphy — Sedimentation and mixing in the lunar regolith

Abstract

We have measured shifts in the isotopic a bundances of Gd and Sm in soils from the Apollo 17 deep drill stem and calculated the neutron fluence from these measurements. The measurements show two well defined regions of nearly constant fluence: (1) a thick deep section with a very large neutron fluence, and (2) a thinner shallow region with a small fluence. This depth dependence is most plausibly described by a model of rapid accumulation in the last 100–200 m.y., the layered structure reflecting accumulations of isotopically homogeneous source material. This interpretation is compatible with a variety of other characteristics of the soils, including the spallation produced126Xe normalized to target element abundances. An alternative model of deposition, followed by irradiation without mixing, followed by shallow mixing will quantitatively describe the data. The model yields an age of 1.25 AE for the bottom of the drill stem. This model was rejected because of the implausible requirement that the soils from the drill stem be accumulated from a source of unirradiated material. The uniformity of various properties of soils provides criteria for defining major stratigraphic intervals in the drill stem which differ from those identified by the Preliminary Examination Team. Neutron fluences measured on shallow and deep soils from all lunar landing sites have been normalized to irradiation in an arbitrary standard chemical environment. We infer from histograms of the normalized fluences that there is a distinct difference in neutron fluence between shallow and deep samples which implies a general vertical stratification of neutron fluence in the lunar regolith. The regolith can be divided into three vertical regions: (1) a well mixed surface layer, ∼100 g cm^(−2) thick, with an average fluence of 2.3 × 10^(16) n cm^(−2), (2) a poorly mixed zone extending from 100 g cm^(−2) to at least 500 g cm^(−2) with an average fluence of 3.5 × 10^(16) n cm^(−2), and (3) a deep layer of lightly irradiated materials (<10^(16) n cm^(−2)). Analysis of this stratification, using a vertical mixing model, indicates that the probability of mixing to several hundred g cm^(−2) is comparable to the probability of mixing to several kg cm^(−2). This is in contrast to the depth-cratering rate models which have been inferred from crater size frequency distributions using a power law. Alternatively, this discrepancy can be resolved if the true ^(157)Gd capture rate is 1/3 of the value calculated by Lingenfelteret al. (1972). The most plausible interpretation is that vertical mixing models are not an adequate description of relatively rare deep cratering events which result in significant lateral heterogeneity and addition of unirradiated material to the lunar surface.

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