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 0.2-1.2cm 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.
Forty-six days after Viking 1 landed, Viking 2 landed in Utopia Planitia, about 6500 kilometers away from the landing site of Viking 1. Images show that in the immediate vicinity of the Viking 2 landing site the surface is covered with rocks, some of which are partially buried, and fine-grained materials. The surface sampler, the lander cameras, engineering sensors, and some data from the other lander experiments were used to investigate the properties of the surface. Lander 2 has a more homogeneous surface, more coarse-grained material, an extensive crust, small rocks or clods which seem to be difficult to collect, and more extensive erosion by the retro-engine exhaust gases than lander 1. A report on the physical properties of the martian surface based on data obtained through sol 58 on Viking 2 and a brief description of activities on Viking 1 after sol 36 are given.
The amounts of magnetic particles held on the reference test chart and backhoe magnets on lander 2 and lander 1 are comparable, indicating the presence of an estimated 3 to 7 percent by weight of relatively pure, strongly magnetic particles in the soil at the lander 2 sampling site. Preliminary spectrophotometric analysis of the material held on the backhoe magnets on lander 1 indicates that its reflectance characteristics are indistinguishable from material within a sampling trench with which it has been compared. The material on the RTC magnet shows a different spectrum, but it is suspected that the difference is the result of a reflectance contribution from the magnesium metal covering on the magnet. It is argued that the results indicate the presence, now or originally, of magnetite, which may be titaniferous.
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 1 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.
The purpose of the physical properties experiment is to determine the characteristics of the martian "soil" based on the use of the Viking lander imaging system, the surface sampler, and engineering sensors. Viking 1 lander made physical contact with the surface of Mars at 11:53:07.1 hours on 20 July 1976 G.M.T. Twenty-five seconds later a high-resolution image sequence of the area around a footpad was started which contained the first information about surface conditions on Mars. The next image is a survey of the martian landscape in front of the lander, including a view of the top support of two of the landing legs. Each leg has a stroke gauge which extends from the top of the leg support an amount equal to the crushing experienced by the shock absorbers during touchdown. Subsequent images provided views of all three stroke gauges which, together with the knowledge of the impact velocity, allow determination of "soil" properties. In the images there is evidence of surface erosion from the engines. Several laboratory tests were carried out prior to the mission with a descent engine to determine what surface alterations might occur during a Mars landing. On sol 2 the shroud, which protected the surface sampler collector head from biological contamination, was ejected onto the surface. Later a cylindrical pin which dropped from the boom housing of the surface sampler during the modified unlatching sequence produced a crater (the second Mars penetrometer experiment). These two experiments provided further insight into the physical properties of the martian surface.
A flight test project evaluating an integrated differential GPS (DGPS)/inertial navigation system as an approach/landing aid has recently been completed. The test objectives were to acquire a system performance database, and to demonstrate automatic landings using the integrated DGPS/inertial system augmented with barometric and radar altimeters. The airborne and ground components of the DGPS/inertial system are built around standard transport aircraft avionics: a global positioning/inertial reference unit and two GPS sensor units. Flight tests were conducted using a specially modified Boeing 737-100. Performance of the DGPS/inertial system was evaluated in real time by comparison against Microwave Landing System (MLS) position. DGPS/inertial, MLS, and laser tracking system position data were recorded during the tests for postflight analysis. Over 120 landings were flown during the project, 36 of which were fully automatic DGPS/inertial landings. The paper describes the objectives of the project, the system implementation, and the flight tests conducted. Laboratory test results and preliminary flight test results are also presented.
Three permanent magnet arrays are aboard the Viking lander. By sol 35, one array, fixed on a photometric reference test chart on top of the lander, has clearly attracted magnetic particles from airborne dust; two other magnet arrays, one strong and one weak, incorporated in the backhoe of the surface sampler, have both extracted considerable magnetic mineral from the surface as a result of nine insertions associated with sample acquisition. The loose martian surface material around the landing site is judged to contain 3 to 7 percent highly magnetic mineral which, pending spectrophotometric study, is thought to be mainly magnetite.
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.
