Lunar surface mechanical properties

.•OURNALOF GEOPHYSICALRESEARCH

VOL. 74, No. 25, NOVEMBER15, 1969

Lunar Surface Mechanical Properties R. CHOATE, • S. A. BATTERSON? E. M. CHRISTENSEN, 1 R. E. I-IUTTON, 3 L. D. JAFFE, 1 R. H. JONES, 4 H. Y. Ko, 5 l•. L. SPENCER, 1 ANDF. B. SPERLING • Although the lunar surface at the Surveyor 7 highland landing site is somewhat rougher than the surface at previous mare landing sites, many of the physical properties of the soil at the sites are similar. The soil is primarily fine-grained, compressible,and slightly cohesive; only 2.8% of the surfaceis coveredby rocks larger than 5 cm in diameter. The averagesoil static bearing strength is 0.2 N/cm • at 0.2-cm depth and 3.4 N/cm • at 4-cm depth.

The Surveyor7 landingsite, a highlandregionnearthe rim of the craterTycho,provided an opportunityto investigatethe lunar surface mechanicalpropertiesof an area thoughtto be quite different from the previousmare landing sites. The influenceof the greater rock populationon the mechanical propertiesat this

against the resistanceof shock absorbers.Following initial impact, the shock absorbersre-

extended,returning the legsto their pre-touchdownpositions.Additionalcapabilityfor energy dissipationwas provided by crushablehoneycomb blocks mounted

on the underside of the

spaceframe,inboard of each leg, and by crushlanding site, comparedwith that of previous able foot pads. An assessment of the Surveyor 7 lunar landSurveyor landings, is presented, along with propertiesderived from telemetry data and ing, based on the touchdown telemetry data from studiesof pictures of surfacedisturbances and on the post-landingattitude determination, causedby the landingimpact.Analysesand lab- showsthat the spacecraftattitude and landing oratory simulationswere performedto assistin velocitiesat touchdownwere closeto the optimum designvalues. the interpretations. Figure 2 showsthe axial loading historiesof In this report, centimeter-gramunits are used. To convert to foot-pound units, the following the three shockabsorbers,as measuredby strain gages,throughoutthe landingphase.Peak loadfactors apply: 1 meter -- 3.28 feet; 1 cm -0.394 inch; i N (newton) -- 10• dynes-- 0.225 ings and times of initial footpad contact are given in Table 1. An evaluationof these data lb; 1 N/cm • -- 1.45 lb/inch•. and other engineeringtelemetry has resultedin the following reconstructionof events during SPACECRAFTLANDING the final descent and landing sequence.The Description 3.1-m/sec velocity was generatedabout 7.4 sec The basic configuration(Figure 1) and land-

before touchdown at an altitude

of 13 __+ 1

ing mechanismfor Surveyor7 were essentially meters. Immediately following this mark, the the sameas for the previousSurveyors[Chris- spacecraftwas slowedto a constantdescent tensenet al., 1967, 1968a,b, c]. During landing velocity of 1.6 ñ 0.2 m/sec, which was maintained until an altitude of 3.6 ñ 0.3 meters was impact, the three landing legs rotated upward reached about 1.3 sec before touchdown. At this

x Jet Propulsion Laboratory, California Institute of Technology, Pasadena,California 91103. 2 Dynamic Loads Division, NASA Langley Research Center, Hampton, Virginia 23361. s TRW Systems,Inc., Redondo, California 90278. 4 Hughes Aircraft Co., E1 Segundo, California 90045.

5University of Colorado, Boulder, Colorado 80302.

Copyright ¸ 1969by the AmericanGeophysical Union.

time, all three vernier engineswere shut off, resultingin a free-fall period until groundcontact; during this period the vertical velocity increasedto 3.8 ---+0.2 m/sec. Changesin angular orientationbetweenthe spacecraftattitude at the 3.1-m/sec mark (spacecraftattitude durdescent,just beforeenginecutoff,was constant) and the spacecraftattitude after settling were -3.1 ø ñ 0.1ø in pitch, -1.7 ø ñ 0.1ø in yaw, and 0.0 ø ---+1ø in roll.

6149

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6150

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-Z AXIS

CLOCKWISEROTATIONS ARE

• • ROLL

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POSITIVE WHEN VIEWED IN THE POSITIVE DIRECTION OF THE AXIS

I••-THERMAL

\\\\ •.v•y-•;•v,s,o. • II CAMERA%

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COMPARTMENT A•

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COMPARTMENT B

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OMNIDIRECTIO ANTENNA B

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Fig. 19. Surface-samplerscoop after i[itial attempt to pick up rock A. The rock is approximately the •me size • the 5-cm width of the scoop (January 12, 1968, 02h 29m 00s GMT).

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.,

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SURVEYOR

7--LUNAR

SURFACE

MECHANICAL

PROPERTIES

6169

'...,.

Fig. 21. Enlarged view of rocks visible above electroniccompartmentB (January 13, 1968; catalog 7-SE-19).

a horizontal lunar surface. It is known that the

surfaceis not horizontal; it slopesdown to the north and up to the south.Thoughtheseslopes tend to be self-compensating, the amount-of error introducedby this assumptionis not yet known.It is noted,however,that all five points, representedby the size rangesin Table 2, lie on a straightline when plotted log-log(Figure 22). If error is presentin the data, it would seemto be systematic,not random; additional work on rock countsis being conducted. These data, as well as the observationsde• scribedin the previous paragraphs(and in

reportsby Christensen et al. [1967,1968a,b, c] indicate that the lunar soil is composedof a

fine-grainedmatrix in which a smallpercentage of rock fragmentsis suspended.Tests of terrestrial soils indicate that comparable small percentages of coarsefragmentssuspended in a

fine-grained soilnormally do not significantly, affect mechanicalproperties such as bearing strength.It is not surprising,therefore,that, even thoughmany more rocks can be seenon the surfaceat the Surveyor7 highlandlanding

site,the mechanical propertiesof the soilat this site are not significantlydifferent from the mechanicalpropertiesdeterminedat the previous mare sites.

Rock hardness. Although quantitative data are scarce,it is desirableto draw possiblecon-

TABLE 2. SizeDistributionand Lunar SurfaceArea Coveredby Rock Fragments(Basedon Figure 20) Lunar Surface

Rock

Width, cm

Number

Cumulative

of Rocks Number per 1000 ms per 1000 ms

80 20 to 80

1' 67*

I 68

15 to 20 10 to 15

106' 347*

174 521

5 to 10

51001

5621

Cumulative

Cumulative

Average Area Covered Area Covered Percentage Percentage of Area Diameter, per 1000 ms, per 1000 ms, of Area ms

ms

Covered

Covered

80 33

0.5 5.7

0.5 6.2

0.05 0.6

0.05 0.6

16• 11•:

2.2 3.5

8.4 11.9

0.2 0.4

0.8 1.2

15.6

27.5

1.6

2.8

cm

6-•$

* Value determinedby actual countof rocksin the 1000ms of lunar surfacevisiblein the panoramaof Figure20, below -- 5ø cameraelevation.

l Valueextrapolated from745rockscounted in 146msof lunarsurface visiblebetween-10 øand -35 ø

cameraelevationfor six of the ten segments of the panoramain Figure20, i.e., for thosesegmentswhere view of the lunar surfaceis not partly blockedby the spaceframe

$ Averagediameter,for rocksnotmeasured directly,wastakenasthe quarterpointin the sizerange.

CHOATE

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AL.

clusions on the hardness of the largefragments (rocks)observed by the televisioncamera(Figure 20) and of large fragmentsencounteredin previousSurveyormissions.It is possibleto concludethat large fragmentsare hard, not weak; i.e., they would be resistantto crushing if impactedby a landingspacecraftor moving vehicle.

105

0

During the Surveyor 3 mission,the surface sampler exerted a pressureof about 2 X 107 dynes/cm• on a 1.2-cm-diameterrock fragment, without breakingit [Christensenet al., 1968a]. This pressure is sufficient to crush weak terrestrial rocks such as some tuffs, siltstones, claystones,and friable sandstones[Christensen et al., 1968a]. During the Surveyor7 mission,the rock used in the drop test (Figure 19) was also squeezed by the surfacesampler,and it did not break. During landing,footpad 3 struck a rock, which made a hole in both parts of the footpadaluminum honeycomb.The lower sectionhas a crushing strength of 6.9 N/cm'; the upper section has a crushingstrengthof 13.8 N/cm •. At least someof the larger rock fragmentscanbe broken by a sharp blow, however,as indicated during Surveyor7 operationswhenthe surfacesampler was allowed to fall from a. height of 35 to 40 cm onto a 5-cm-diameterrock (rock E), which

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w

w

o o

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w

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broke on impact (seeFigure V-30 of Scott a•d

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Roberson [1968] ). It can be demonstratedthat most large rock fragmentsare hard, as is shownby their resistance to erosion. The rocks above the leg 3 shock absorber in Figures 20 and 23, for example, appear to have undergoneextensiveerosion.The planar surfacesof the rocksare almost certainly fracture surfacesthat have been modified by erosion.The roundededgesof the rocks and the nonvesicular,but pitted, surfacesindi-

I01

cate a substantial

I00 I

I0

I00

ROCK FRAGMENT DIAMETER, cm

Fig. 22. ,Size-frequency distribution of rock fragments within 18 meters of Surveyor 7. The graph represents the data in Table 2. The number of rocks larger than any specificsize per unit area can be estimated from this graph. For example, there are approximately 1200 rocks larger than 7 cm in diameter per 1000m• of lunar surface.

amount

of erosion from im-

pact by small fragments. Fragments that are formed by agglomerated, fine soil particlesejectedby footpadsand crushable blocksduring Surveyor landingsare weak and do not exceed a few centimeters in diameter.

Rock fractures. Of special interest are the fracturesin the large rock aboveleg 3 shockabsorber in Figures 20 and 23. The fractures could have been causedby impact of another rock. It seemsunlikely, however,that the ira-

SURVEYOR

?--LUNAR

SURFACE

pacting rock would have had just enoughenergy to fracture this rock, without any excessenergy that would have dislodgedthe resulting fragments.It may be morelikely that thesefractures were causedby expansionand contractionduring lunar day and night temperature cycling.

MECHANICAL

PROPERTIES

6171

crops,and the generalsmoothness of the terrain indicate that the visible lunar surface is almost

entirely formedby soil comprisinga fine-grained matrix with a small percentageof coarserock fragments. l:)RELIMINARY

The width of the fractures in the rock indicates

CONCLUSIONS

that the fragmentshave separatedby several

First evaluations of television and telemetry data, made with the aid of analytical and labsimilarlywell developedfracturesis besidecom- oratory simulations,have provided us with the partment B (Figure• 20 and 24). following conclusionson the mechanical propTerrain characteristics. As shown in Figure erties of the soil at the Surveyor 7 high]and 20, most of the surface visible to the camera landing site. has gentle slopesless than 10ø. The steepest The soil is predominantlyfine-grained,granuslope on the flank of the ridge north of the lar, and s]ighfiy cohesive; it is similar to the spacecraft,just below the horizon, is 34ø. The soil found at the previous Surveyor landing angleof reposeof most loosematerial is 35ø to sites. Not only is the soil cohesive,but soil 37 ø. particles, once disturbed,tend to readhere. Soil ejected by the footpadsis darker than The slopes,the lack of distinct bedrockout-

millimeters

.

and are loose. Another

."

rock with

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ß

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Fig. 23. View of rocksshowingfractures,whichare the best developedfracturesobserved duringthe Surveyormissions(January13, 1968; catalog7-SE-28).

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Fig. 24. Fractures in rock beside electronic compartment B (January 13, 1968, 14h 45m 18s GMT).

SURVEYOR 7--LUNAR SURFACE MECHANICAL

undisturbed soil on the surface, possibly becauserecently disturbedsoil has a roughersurface and a larger effective grain size than undisturbedsoil and thereforereflectslesslight. Imprints of footpads and crushable blocks indicate that the soil is compressible,at least in its upper few centimeters. Static bearing strength of the lunar soil increaseswith depth.Bearingstrengthscalculated for penetratorsvarying in size are as follows: (1) in approximatelythe upper millimeter, less than 0.1 N/cm • (from imprints of small rolling fragments); (2) at a depth of I to 2 ram, 0.2 N/cm • (from imprints of the sensorhead of the a-scattering instrument); (3) at a depth of about 2 cm: 1.8 N/cm • (from Surveyor 6 and 7 imprintsof crushableblocks); (4) at a depth of 5 cm, 5.5 N/cm • (from Surveyor I footpad penetration). An average 3.4-N/cm • lunar surface static bearing strength, determined from Surveyor 7 footpad penetration into an assumedcompressible soil model, is similar to that observedin the Surveyors1, 3, and 6 landings. Dynamic bearing stressdevelopedon crushable block 2 exceeded2.4 N/cm 'øduring penetration to a depth of 3 cm, as evidencedby the mound of soil in the center of the imprint; the mound indicates the aluminum

sheet on the

bottom of the crushable block was ruptured during landing. The depressioncaused by a rock dropped from the surface sampler provided an upper bound estimate of 3.1 N/cm • for the dynamic bearingstrengthof the top 0.8 cm of the soil; static bearing strength would be less. During landing,lunar soil was thrown against an auxiliary mirror and adhered to it. During previousmissions,lunar soil adheredto the spacecraftprincipally when the soil impacted with substantialvelocity,primarily duringfiring of the vernier engineswhile the spacecraftwas on the ground.

Only 0.6% of the area at the Surveyor 7 site (within an 18-meter radius of the camera) is coveredby rocks larger than 20 cm in diameter, 1.2% by rocks larger than 20 cm, and

2.8% by rockslarger than 5 cm (seeTable 2). In summary,soil at this highlandsite is generally similar in its mechanicalproperties to that at the mare landingsitesof previousSurveyorsexceptthat there is a higherrock popu-

PROPERTIES

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lationwithin the soiland on the surface.Though individual large rocks and clusters of small

rockswill substantially increase bearingstrength locally, the higher rock population does not, in general, increase appreciably the bearing strength of soil at the Surveyor 7 site. Acknowledgments. We thank Dr. Ronald F. Scott, California Institute of Technology, for providing data on surface-sampleroperations,and Mr. F. I. Roberson, Jet Propulsion Laboratory, for his help in their interpretation; Mr. Charles Goldsmith and Mr. William Peer, also of JPL, for constructing the mosaics that appear in this paper, as well as for their support in mission operations and laboratory simulations; Mr. Lloyd Starks, JPL, for supporting mission operations and assisting in laboratory simulations. We also thank Dr. George Sutton, University of Hawaii, for his continuing work on analysis and interpretation of lunar soil elastic properties from shockabsorber strain-gage data; and Dave Conway, Margaret Dove, and John Hinchey, Hughes Aircraft Co., for help in the landing dynamic simulations.

I•EFERENCES

Choate, R., S. A. Batterson, E. M. Christensen, R. E. Hutton, L. D. Jaffe, R. H. Jones, H. Y. Ko, R. L. Spencer, and F. B. Sperling, Lunar surface mechanical properties, in Surveyor 7 Mission Report, Part 2, Science Res•lls, Jet Propul. Lab. Tech. Rep. 32-1264,77-134, Pasadena, Calif., March 15, 1968. Christensen,E. M., S. A. Batterson, H. E. Benson, C. E. Chandler, R. F. Jones, R. F. Scott, E. N. Shipley, F. B. Sperling, and G. H. Sutton, Lunar surface mechanical properties--Surveyor 1, J. Geophys. Res., 72, 801-813, 1967. (Also in Surveyor I Mission Report, Part 2, Scientific Data and Results, Jet Propul. Lab. Tech. Rep. 32-1023, 69-85, Pasadena, Calif. September 10, 1966.)

Christensen,E. M., S. A. Batterson, H. E. Benson, R. Choate, L. D. Jaffe, R. H. Jones,H. Y. Ko, R. L. Spencer, F. B. Sperling, and G. H. Sutton, Lunar surface mechanical properlies at the landing site of Surveyor 3, J. Geophys.Res., 73, 4081-4094, 1968a. (Also in Surveyor 3 Mission Report, Part 2, Scientific Results, Jet

Propul. Lab. Tech. Rep. 32-1177,111-153,Pasadena, Calif., June 1, 1967.) Christensen,E. M., S. A. Batterson,H. E. Benson, R. Choate, R. E. Hutton, L. D. Jaffe, R. H. Jones,H. Y. Ko, F. N. Schmidt,R. F. Scott, R. L. Spencer,and G. H. Sutton, Lunar surface mechanical properties, J. Geophys. Res., 73, 7169-7192, 1968b. (Also in Surveyor 5 Mission Report, Part 2, Science Results, Jet Propul. Lab. Tech.Rep. 32-1246,43-88,Pasadena,Calif., November 1, 1967.)

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Christensen, E. M., S. A. Batterson, H. E. Benson, R. Choate, R. E. Hutton, L. D. Jaffe, R. H. Jones, H. Y. Ko, F. N. Schmidt, R. F. Scott, R. L. Spencer,F. B. Sperling, and G. H. Sutton, Lunar surface mechanical properties, in Surveyor 6 Mission Report, Part 2, Science Results, Jet Propul. Lab. Tech. Rep. 32-1262, 47108, Pasadena, Calif., January 10, 1968c. Scott, R. F., and F. I. Rob•rson, Soil mechanics surface sampler, in Surveyor 7 Mission Report, Part 2, Science Results, Jet Propul. Lab. Tech. Rep. 32-1264, 135-185, Pasadena, Calif., March 5, 1968.

Shoemaker, E. M., R. M. Batson, H. E. Holt, E. C. Morris, J. J. Rennilson, and E. A. Whitaker, Television observations from Surveyor 5, in

ET AL.

Surveyor 5 Mission Report, Part 2, Science Results, Jet Propul. Lab. Tech. Rep. 32-1246, 7-45, Pasadena, Calif., November 1, 1967. Shoemaker, E. M., R. M. Batson, H. E. Holt, C. Morris, J. J. Rennilson, and E. A. Whitaker, Television observations from Surveryor 7, in Surveyor 7 Mission Report, Part 2, Science Results, Jet Propul. Lab. Tech. Rep. 32-1264, 9-76, Pasadena, Calif., March 5, 1968. Spencer, R. L., Determination of footpad penetration depth from Surveyor spacecraft shadows, Jet Propul. Lab. Tech. Rep. 32-1180, Pasadena, Calif., October 1967. (Received July 7, 1969.)