To obtain a quantitative measure of the effects of lunar environmental conditions on the properties of soil, it is necessary to consider the state of cleanliness on the surfaces of the individual particles rather than the vacuum level per se of the surrounding environment. Experiments were performed to investigate the nature of the adsorbate in soil and then determine the relationship between the cleanliness of the particle surfaces and soil strength. The relative cleanliness was observed to have a very pronounced influence on strength because of an increase in interparticle forces after the desorption of gas. These forces were predominantly van der Waals forces, and bridging and at high values of relative cleanliness chemical bonding would probably be significant.
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