Engineering telemetry data and lunar surface photographs by Surveyor 1 have been evaluated for information on the mechanical properties of the lunar surface material at the Surveyor 1 landing site. Based primarily on photographic evidence, estimates of soil density, cohesion, and other soil characteristics are presented. Also, the mechanisms in which the lunar material is believed to have failed under the footpad impacts are discussed. Because dynamic soil reactions cannot be interpreted directly from the available data, a comparative study using computer-simulated landings was initiated. Preliminary results of this study, which is still in progress, are presented. OBSERVATIONS AND EXPLANATIONSThe interpretation of the lunar surface properties discussed here is based on (1) 801

The surface material at the Surveyor 5 site is granular and slightly cohesive. Spacecraft footpads plowed trenches in this material as the spacecraft slid during landing. For a compressible soil model, a static bearing capacity of 2.7 newtons/cm •' gave best agreement with the observations. Static firing of the vernier engines against the surface moved surface particles; a crater 20 cm in diameter and about 1 cm deep was produced, apparently at engine shutdown. The permeability of the soil to gases, to a depth of about 25 cm, is 1 X 10 -s cm .ø,

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Analyses of engineering and television data returned by Surveyor 3 indicate that the mechanical properties of the lunar surface material inside the crater in which the spacecraft landed are similar to the mechanical properties of the level mare area at the Surveyor 1 landing site. The static bearing strength ranges from 2 to 6 newtons/cm2 for a penetration depth between 2 and 5 cm and a bearing diameter between 20 and 30 cm. The soil cohesion is less than 1 newton/cm2; its rigidity modulus is low. A substantial percentage of the particles of the lunar surface material appear to be smaller than 0.06 mm in diameter, and a significant number are under 0.01 mm. The fine structure of the surface can be deformed substantially by applying a small, distributed, compressive force.

The mechanical properties of the lunar soil at the Surveyor V landing site seem to be generally consistent with values determined for soils at the landing sites of Surveyor I and III. These three maria sites are hundreds of kilometers apart. However, the static bearing capability may be somewhat lower than that at the previous landing sites (2 x 10(5) to 6 x 10(5) dynes per square centimeter or 3 to 8 pounds per square inch). The results of the erosion experiment, the spacecraft landing effects, and other observations indicate that the soil has significant amounts of fine-grained material and a measurable cohesion.