New Measurements of the Particle Size Distribution of Apollo

Introduction: We have initiated a major new program to determine the grain size distribution of nearly all lunar soils collected in the Apollo program. Following the return of Apollo soil and core samples, a number of investigators including our own group performed grain size distribution studies and published the results [1-11]. Nearly all of these studies were done by sieving the samples, usually with a working fluid such as Freon™ or water. We have measured the particle size distribution of lunar soil 10084,2005 in water, using a Microtrac™ laser diffraction instrument. Details of our own sieving technique and protocol (also used in [11]). are given in [4]. While sieving usually produces accurate and reproducible results, it has disadvantages. It is very labor intensive and requires hours to days to perform properly. Even using automated sieve shaking devices, four or five days may be needed to sieve each sample, although multiple sieve stacks increase productivity. Second, sieving is subject to loss of grains through handling and weighing operations, and these losses are concentrated in the finest grain sizes. Loss from handling becomes a more acute problem when smaller amounts of material are used. While we were able to quantitatively sieve into 6 or 8 size fractions using starting soil masses as low as 50mg, attrition and handling problems limit the practicality of sieving smaller amounts. Third, sieving below 10 or 20µm is not practical because of the problems of grain loss, and smaller grains sticking to coarser grains. Sieving is completely impractical below about 5-10µm. Consequently, sieving gives no information on the size distribution below ~10 µm, which includes the important submicrometer and nanoparticle size ranges. Finally, sieving creates a limited number of size bins and may therefore miss fine structure of the distribution which would be revealed by other methods that produce many smaller size bins. Because of the known complexity of lunar surface soil-forming processes, particles in the finer size range may be created and modified by processes totally different from the processes that create coarser particles, and such differences may leave a record in the grain size distribution. The only data in this finer size range is from Coulter counter or optical/electron microscope techniques [4, 12]. Knowledge of the grain size distribution of material finer than about 10µm is necessary to help evaluate possible exposure health hazards to humans at a lunar location. Nanometer size particles may cause …