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John F. Lindsay (2)
The meteorite flux at the lunar surface provides the primary energy source for the development of the lunar soil. Size, shape and modal analyses of soil returned by Apollo 11, 12, 14 and 15 and Luna 16 indicate that the two most important dynamic processes resulting from meteorite impact are vitrification and comminution of the detrital material. The effects of the two processes are mutually opposed such that, as the glass content of the soil increases over an extended period of time, the statistical parameters of the mature soil tends to stabilize. Comminution probably plays a dominant role early in the development of the soil by reducing the median grain size and producing a logarithmic-normal grain-size distribution. Later the combined effects of vitrification and agglutination of he fines produces a bimodal distribution of glass particles such that the median grain size stabilizes at approximately 3.5 and the standard deviation of size distribution is increased to about 2. Synchronously the graphic skewness of the soil approaches zero and the graphic kurtosis probably decreases.
The shape of the lunar soil particles evolves as the soil matures. In immature soils rock and mineral fragments with simple shapes and high sphericities predominate and the relationship between sphericity and grain size is simple and linear. However, impact vitrification produces two morphologically distinct types of glass particles and superimposes a more complex sinusoidal pattern upon the distribution as the soil matures such that the mean sphericity of the particles is reduced in the 2 to 3, size range and in the 6, and smaller size range.
The evolution of the soil does trot necessarily progress in a regular manner. Both introduction of freshly comminuted bedrock material by small local impact events, as well as local topographic effects, influence the development of the soil and reduce its maturity. Mixed soils produced by local events tend to be coarse grained, positively skewed and very poorly sorted whereas the soils formed on steep slopes have gain size parameters similar to mature soils except that they may be somewhat finer grained. The shape parameters are similarly modified.
The accumulation rate of the soil is not constant nor is deposition a continuous process. The growth of the soil probably proceeds in an exponential manner with a primary accumulation rate of approximately 1.2 10-6 yr-1 which is gradually decelerated at between -3.3 and -1.7 10-16 cm yr-2. This suggests that approximately 90 percent of the energy potentially available for comminuting bedrock is absorbed in the reworking of the soil
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