Surface materials of the Viking landing sites

Creators: Moore, Henry J.; Hutton, Robert E.; Scott, Ronald F.; Spitzer, Cary R.; Shorthill, Richard W.

Abstract

Martian surface materials viewed by the two Viking landers (VL-1 and VL-2) range from fine-grained nearly cohesionless soils to rocks. Footpad 2 of VL-1, which landed at 2.30 m/s, penetrated 16.5 cm into very fine grained dunelike drift material; footpad 3 rests on a rocky soil which it penetrated ≈3.6 cm. Further penetration by footpad 2 may have been arrested by a hard substrate. Penetration by footpad 3 is less than would be expected for a typical lunar regolith. During landing, retroengine exhausts eroded the surface and propelled grains and rocks which produced craters on impact with the surface. Trenches excavated in drift material by the sampler have steep walls with up to 6 cm of relief. Incipient failure of the walls and failures at the end of the trenches are compatible with a cohesion near 10–10^2 N/m^2. Trenching in rocky soil excavated clods and possibly rocks. In two of five samples, commanded sampler extensions were not achieved, a situation indicating that buried rocks or local areas with large cohesions (≥10 kN/m^2) or both are present. Footpad 2 of VL-2, which landed at a velocity between 1.95 and 2.34 m/s, is partly on a rock, and footpad 3 appears to have struck one; penetration and leg strokes are small. Retroengine exhausts produced more erosion than occurred for VL-1 owing to increased thrust levels just before touchdown. Deformations of the soil by sampler extensions range from doming of the surface without visible fracturing to doming accompanied by fracturing and the production of angular clods. Although rocks larger than 3.0 cm are abundant at VL-1 and VL-2, repeated attempts to collect rocks 0.2–1.2 cm across imbedded in soil indicate that rocks in this size range are scarce. There is no evidence that the surface sampler of VL-2, while it was pushing and nudging rocks ≈25 cm across, spalled, chipped, or fractured the rocks. Preliminary analyses of surface sampler motor currents (≈25 N force resolution) during normal sampling are consistent with cohesionless frictional soils (ϕ ≈ 36°) or weakly cohesive frictionless soils (C < 2 kN/m^2). The soil of Mars has both cohesion and friction.

Additional Information

© 1977 American Geophysical Union. Received March 31, 1977; revised May 23, 1977; accepted May 24, 1977. We acknowledge the continuing aid and support given to the Physical Properties Investigation Team by the Surface Sampler Team, L. V. Clark, D.S. Crouch, L. K. Schwab, K. Z. Bradford, and W. DeShazor. The Imaging Team kindly furnished the images used in this report. We thank R. B. Hargraves, D. W. Collinson, and E. C. Morris, who gave us able assistance throughout the mission. S. Liebes, Jr., provided the mensuration data for the rock pushes and nudges. We also thank I. M. Mack for her assistance as the Physical Properties intern during the month of August and P. Duffy for his assistance for the month of September. For typing the manuscript several times and for attending to many administrative matters we thank L. Crafton. We appreciate the support of R. Goldstein during sol 0 for VL-1 and VL-2 and throughout the mission. We also thank A. Castro and V. Gillespie for their additional support. The help of H. Zimmer and G. Neukum during the acquisition of data is appreciated. The trajectory data were kindly furnished by A. Fontana, F. W. Hopper, J. T. Findley, and J. W. Gerschultz. We acknowledge the help and assistance of S. Dwornik, particularly at the beginning of the project when the Physical Properties Investigation first started. We are especially appreciative of the assistance of P. Cates during the primary mission and particularly during the extended mission. Finally, the Physical Properties Team wishes to express its sincere 'thank you' to the project manager, Jim Martin; the mission director, Tom Young; and the project scientist, G. A. Soffen, for their untiring dedication to the goals of the mission, which maximized the science return. This work was supported by NASA contract NAS1-12705 to the Geospace Sciences Laboratory of the University of Utah Research Institute, NASA order L-9714 to the U.S. Geological Survey, and NASA contract NAS1-10534 to TRW Systems, Inc.

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