This full-scale model with fully operational lander cameras and surface-sampler subsystem was installed adjacent to a large sand box representing the area in reach of the surface sampler. The Science Test Lander was used during the mission to develop and verify surface-sampler commands and to perform some experiments for the Physical Properties Investigation. CircularS-band radio antenna of lander is 0.76 m wide. Locations of spacecraft parts, cameras, and surface-sampler subsystem are shown in figures 1 and 3. PHYSICAL PROPERTIES OF THE SURFACE MATERIALS AT CONTENTS i\bstract ---------------------------Introduction--------------------------Viking missions ----------------------Supplemental sections-------------------i\cknowledgments---------------------Mission description and conventions -------------Viking lander -----------------------Spacecraft parts ---------------------Surface-sampler subsystem ----------------Mission operations --------------------Other Viking experiments and investigations -------Coordinate systems and measurements ----------Time --------·_ ------------------Sample-field definition ------------------The landing sites ----·-------------------Orbiter views -----------------------Lander panoramas --------------------Sample fields -----------------------Landing ---'-------------------------I>escent -----------------------~ --Touchdown ------------------------Lander 1 ----------------------Lander 2 ----------------------I>iscussion ---------------------- exhaust erosion ------------------35 Surface-sampler data ---------------------44 i\ctivities -------------------------46 Sample trenches ---------------------48 I>rift material --------------------48 Blocky material -------------------52 Crusty to cloddy material --------------56 I>iscussion ----------------------58 Surface-bearing tests -------------------63 I>rift material --------------------63 Blocky material -------------------66 Crusty to cloddy material --------------68 · Backhoe touchdowns -------------------70 Surface impacts ----------------------77 Slope stability-------------------------81 Trench walls -----------------------81 XRFS 1 trench -------------------82 Physical Properties 1 trench ------------. 82 I>eep Hole 1 ---------------------84 Inferred cohesions------------------84 Natural slopes-----------------------84 Conical piles and tailings -----------------88 Summary -------------------------88 The quest for rocks ----------------------88 Sample acquisitions --------------------90 Purges and rock piles -------------------91 Comminution -----------------------92 Rock pushing -----------------------99 Summary -------------------------101 Temperatures -------------------------101 Surface-sampler diurnal temperatures-----------102 Lander 1 ----------------------102 Lander 2 ----------------------103 Interim period ----------------------104I>iscussion----------------------. :. . . --104 Page Visual observations of changes ----------------105 Conical piles------------------------106 Materials in the footpads ----------------...

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 n:icky 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 ofVL-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 cohesion less frictional soils (c/J "' 36°) or weakly cohesive frictionless soils (C < 2 kN/m 2 ). The soil of Mars has both cohesion and friction.

This paper presents the results of a theoretical and experimental investigation of fluid-particle motion in a partially filled cylindrical tank. The body of fluid is assumed to have a steady-state motion consisting of first nonsymmetric sloshing mode only (lowest J1 mode), which gives rise to a free-surface wave that rotates around the tank at the sloshing natural frequency. It was found that theory predicts a net transport motion of the fluid particles in the direction of the free-surface waves when nonlinear terms are retained in differential equations that describe the fluid-particle displacements. Experimental measurements of the particle motion are compared with theoretical predictions. Fluid angular momentum was computed using the theoretical fluid motion and compared with the angular momentum that the fluid would possess if the fluid moved as a rigid body at the same rate as the free-surface waves. It was found that an upper bound to the ratio of the transport angular momentum to the rigid-body angular momentum was equal to 0.8 (η/a)2, where η is the peak wave height of the free surface waves and a is the tank radius.

Analyses of sample trenches excavated on Mars, using a theory for plowing by narrow blades, provide estimates of the angles of internal friction and the cohesions of the Martian surface materials.Angles of internal friction appear to be the same as those of many terrestrial soils because they are generally between 27° and 39°. Drift material, at the Lander 1 site, has a low angle of internal friction (near 18°). All the materials excavated have low cohesions, generally between 0.2 and 10 kPa. The occurrence of cross bedding, layers of crusts, and blocky slabs shows that these materials are heterogeneous and that they contain planes of weakness. The results reported here have significant implications for future landed missions, Martian eolian processes, and interpretation of infrared temperatures.

After the Viking primary mission the surface samplers and cameras continued to operate during the extended mission until early May 1978. Major extended mission accomplishments include (1) excavation of deep trenches, (2) acquisition of more samples (chiefly for the X ray fluorescence experiment), (3) construction of conical piles of materials in the sample fields, (4) backhoe touchdown experiments, (5) acquisition of contiguous pictures of the surface beneath #2 terminal descent engines using mirrors, (6) pushing and pulling rocks, and (7) other experiments for the Physical Properties Investigation. The landing sites have continued to be monitored with the cameras during the lander continuation automatic mission, and the Lander I site will be monitored for a long period of time during the Viking survey mission (perhaps to December 1990 and beyond). Activities of and experiments performed by the surface samplers have disturbed the equilibrium of the surface so that wind and other processes have produced changes. Both pictures and surface sampler data acquired chiefly during the extended mission indicate that the surface materials in the sample fields of the Viking landers may be grouped into four categories (in order of increasing strength): (1) drift material, (2) crusty to cloddy material, (3) blocky material, and (4) rocks. The response of the surface materials to engine exhaust erosion combined with data from other experiments, rock populations at the sites, and theory indicates that the surface should be relatively stable and resistant to wind erosion. Although relatively stable, the erosion of the surface may occur when wind speeds are sufficiently high and when local conditions permit erosion. During the interval of time from the arrival of the Viking landers on Mars in July (VL 1) and September (VL 2) of 1976 the surfaces in the vicinity of the spacecraft have been altered by the spacecraft so that equilibrium with the natural environment no longer exists. The mere presence of the landers themselves alters the equilibrium configuration. Alterations of the surface were initiated during the landings [Moore et al., 1977; Shorthill et al., 1976a, b, c]. These alterations include the production of small circular depressions, resembling craters, beneath the terminal descent engines by entrainment of debris in the exhaust gases. The entrained debris was deposited in rims around the craters, in the footpads, and at various distances from the spacecraft. Further alterations were induced by the surface sampler as it acquired samples and performed a variety of experiments. Subsequently, unstable situations induced by the surface samplers have manifested themselves in the form of slumping or mass wasting of the walls of trenches excavated by the surface samplers. Other responses induced by natural processes can be expected from other situations. The surface sampler subsystem has been described by Crouch [1977], and surface sampler activities during the primary mission have been summarized previously [Moore et al., 197...

The location of the Viking 1 lander is most ideal for the study of soil properties because it has one footpad in soft material and one on hard material. As each soil sample was acquired, information on soil properties was obtained. Although analysis is still under way, early results on bulk density, particle size, angle of internal friction, cohesion, adhesion, and penetration resistance of the soil of Mars are presented.

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,

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,