Journalof
Research
Geophysical
VOLUME
77
OCTOBER
10, 1972
NUMBER
29
!
SoilMechanical
Properties
attheApollo
14Site
JAMESK. MITCHELL;
• LESLIEG. BROMWELL,
2 W. DAVIDCARRIER
III, s
NICHOLAS
C. COSTES,
4 ANDRONALD1•. SCOTT
5
The Apollo14lunarlandingprovided
a greateramountof information
on the mechanical
properties of the lunar soil than previous missionsbecauseof the greater area around the
landing site that was explored and because a simple penetrometer device, a special soil
mechanicsti•ench,and the modularized equipment transporter (Met) provided data of a
type not previouslyavailable.The characteristics
of the soil at shallowdepthsvaried more
than anticipatedin both lateraJand vertical directions.While blowing dust causedless
visibility impairment during landing than on previous missions,analysisshowsthat eroded
particleswere distributedover a large area aroundthe final touchdownpoint. Measurements
on core-tubesamplesand the resultsof transportertrack anaJyses
indicatethat the average
density of the soil in the Fra Mauro region is in the range of 1.45 to 1.60 g/cm3. The soil
strength appearsto be higher in the vicinity of the site of the Apollo 14 lunar surface experiments package,and trench data suggestthat strength increaseswith depth. Lower-bound
estimatesof soil cohesiongive values of 0.03 to 0.10 kN/m 2, which are lower than values of
0.35 to 0.70 kN/m 2 estimatedfor soilsencounteredin previousmissions.The in situ modulus
of elasticity, deducedfrom the measuredseismic-wavevelocity, is compatible with that to
be expectedfor a terrestrial silty fine sandin the lunar gravitational field.
Studiesof the soil (regolith) at the Apollo 14
site have been made (1) to obtain data on the
compositional,
textural, and mechanicalproper-
formulate, verify, or modify theories of lunar
history and lunar processes;(3) to develop
information that may aid in the interpretation
ties of lunar' soils and the variations of these
of data obtained from other surface activities or
propertieswith depthandlocationat and among experiments (e.g., lunar field geology, passive
Apollo landing sites; (2) to use these data to and active seismic experiments); and (4) to
developlunar-surfacemodelsto aid in the solution of engineeringproblemsassociatedwith
• Department
of Civii Engineering,
University future lunar exploration. The in situ character-
of California, Berkeley, California 94720.
2 Department of Civil Engineering, Massachusetts Ins•.i.tute of Technology, Cambridge, Massachusetts
02139.
s GeophysicsBranch, Planetary and Earth SciencesDivision, Manned SpacecraftCenter, NASA,
Houston, Texas 77058.
4George C. Marshall Space Flight Center,
Huntsville, Alabama 35812.
5Division of Engineering and Applied Science,
California Institute of Technology, Pasadena,
California
Copyright ¸
91109.
1972 by the American GeophysicalUnion.
5641
istics of the unconsolidated lunar-surface
ma-
terials can provide an invaluable record of the
past influencesof time, stress,and environment
on •he moon.Of particularimportanceare particle size,particle shape,particle-sizedistribution, density,strength,and compressibility,
and
the variationsin thesepropertiesboth regionally and locally.
To date it hasbeennecessary
to rely heavily
on observational
data, suchas are providedby
photography,astronaut commentary,and ex-
5642
M•ITCHELL
aminationof returnedlunar samples,sincequantitative data sourcesare limited. Semiquantitative analysesare possible,however,as Costes
et al. [1969] showedfor Apollo 11 and Scott
et al. [1970] showedfor Apollo 12. Suchanalyses
are enhanced through terrestrial simulation
studies [Co.steset al., 1971; Mitchell et al.,
1971].
,
The resultsof the Apollo 11 and 12 missions
have generally confirmedthe lunar-surfacesoil
modeldevelopedb.yScott and Roberson[1968];
ET AL.
formations that are-characteristic
of the Fra
Mauro formation.This extensivegeologicalunit
is distributed in approximately radially symmetric
fashion around the Sea of Rains over
much of the near side of the moon and is con-
sideredto be older than the Apollo 11 and 12
mare sites.It is believed,also,that someof the
material
accessible at the site has come from
depthsof tens of kilometerswithin the original
lunar crust. Bright ray material from Copernicus Crater covers much of the landing site.
that is, the lunar s0il is similar to a silty fine ConeCrater, a young,blocky,crater of Copernisand, is generally gray-brown in color, and can age, 340 meters in diameter, penetratesthe
exhibits a slight cohesion.Evidence of both soil layer east of the landingpoint. The Apollo
compressibleand incompressibledeformation 14 traversesand a map of the major geologic
has been observed. The lunar soil erodes under
featuresare shownin Figure 1.
the action of the exhaust of the lunar module
descentengine during lunar landing, kicks up
easilyunder foot, and tends to adhereto most
objects with which it comesinto contact. The
value (or range of values) of the in situ bulk
density of the lunar soil remainsuncertain,although Apollo 11 and 12 core-tube data and
core-tube simulations [Carrier et ed., 1971]
suggesta rangeof 1.5 to 1.9 g/cms for the upper
few tens of centimeters
of the lunar surface.
Observationsat five Surveyor landing sites
and the Apollo11 and 12 landingsitesindicated
relatively little variation in surface soil conditions with location.Core-tubesamplesfrom the
Apollo 12 missionexhibiteda greatervariation
in grain-sizedistributionwith depth than had
been found for Apollo 11 core-tubesamples,
however.
The Apollo 14 mission provided a greater
amount of soil mechanics data than either of the
previous missionsfor two reasons.The crew
covereda much greater distanceduring the extravehicularactivity periods,and the Fra Mauro
landing site representeda topographicallyand
geologicallydifferent region of the moon. In
addition, three featuresof particular interest to
soil mechanicswere new to the Apollo 14 mission: the Apollo simple penetrometer,the soil
mechanicstrench, and the modularizedequipment transporter (Met). Each of thesehas been
used to shed new light on lunar soil character-
DESCENT AND LANDXNC
Blowing dust. The astronauts commented
that blowingdustwas first observedat an altitude of approximately33 meters and that the
quantity of dust from that altitude downto the
surface seemed less than had been encountered
duringthe Apollo 11 and 12 landings.The crew
estimatedthe thicknessof blowing dust to be
less than 15 cm; rocks were readily visible
throughit. The sunangleat landingwashigher
for the Apollo 14 missionthan it had beenfor
the Apollo 12 landing. The appearanceof the
blowinglunar-surfacematerial in motion pictures taken during the Apollo 14 descentseems
qualitativelysimilarto that observedduringthe
Apollo 11 landing. Dust was first observedat
altitudesof 24, 33, and 33 metersfor the Apollo
11, 12, and 14 landings,respectively.Because
of the effect of sun angle and spacecraftorientation, however,the appearanceof the dust in
the motion picturesmay not be a reliableindicationof the quantity of material removedfrom
the surface.
Surface erosion. The astronauts reported
that the lunar surface gave evidence of the
greatest erosion in an area approximately 1
meter southeastof the regionbelow the engine
nozzle,where as much as 10 cm of surfacematerial may have been removed during the landing. A distinct erosionalpattern is visible in
istics.
Figure 2. Except for a disturbedarea in the left
middle distance,the surface gives the appearArOLLO 14 LANmNG Sx•
ance of having been swept by engine gasesin
The Apollo14 landingsitein the Fra Mauro the same way as on previous missions.The
region of the moon containsexposedgeological disturbedarea may have developedas a conse-
So•, AT APo•,Lo14 S•
5643
Crater
I
100
illIll
0
I
I
I
I
I
500
Fig.1. Apollo14sitemap[from$wannetal., 1971]:Cc•,materials
of ConeCrater;Is,
smoothterrainmatedhal
of the Fra Mauro formation;I•r, ridgematedhal
of the Fra Mauro
formation;
opencircle,panorama
station;opentriangle,
stationwithoutpanorama.
Contact:
longdashedlinesindicateapproximate
locations,and shortdashedlinesindicatethat loca-
tionis inferred
withoutlocalevidence.
Footo• scarp:the linebounds
a smallmesa,and
thesolidtriangles
pointdownslope;
theshortdashed
linesindicate
inferred
location.
Edge
o.•hill: longdashedlinesindicateapproximate
locations,
and shortdashedlinesindicate
inferredlocation.The opentrianglespoint downslope.
Traverseroutesfor first and second
periodsof extravehicular
activityaremarkedby solidlineswith arrows.
quenceof grazingcontactof the q-Y footpad ejected radially from the surface below the
contactprobeduringthe landing.
spacecraftat predominantlylow anglesto the
In the Apollo14 descent
motionpictures,it horizontal.
Thereis thus,duringthe descent,
a
is evident that the lunar surface remains indis- regionfrom whichsoil is beingremoved,
and
tinct for a number of secondsafter descent- an adjacentregion,kilometers
in lateral extent,
engineshutdown.This event was probably on which the ejected particles descend.Since
causedby venting from the soil of the exhaust the.spacecrafttraverseslaterallyover the surgas stored in the voids of the lunar material face at a decreasing
altitude, erosionin some
duringthe final stagesof descent.The outflow- regions
will be followedby deposition
of partiing gas carrieswith it fine soil particlesthat cles removed at later times' from other ar•a.s.
obscure the surface.
The entireregionin the vicinityof the landed
Implications o[ blowing dust. The lunar spacecraft to a radius of several hundred me-
soil removedby the engineexhaustgas is
tersisparticularly
subject
to thisprocess,
which'
½-
Fig. 2. Area below the descent-engine nozzle showing erosional features caused by the
exhaust gas. The --Y footpad can be seenin the distance (AS14-66-9262).
may have someimplicationsin the analysesof
the soil and rock samplescollected.The special
environmental sample obtained from material
in the bottom of the trench dug at station G
(Figure 1) may be used as an example.It is
likely that this soil sample included granular
fragmentsboth from below the surfaceand at
the surface, since material fell into the trench
as it was being excavated.
During descentto its landed position, the
lunar module followed a track approximately
W22øN, going slightly south of the center of
North Crater. The spacecraftwas about 60 me-
ters (200 feet) southof station G at its point of
closestapproach,'
and at this point its altitude
abovethe lunar surfacewas55 meters (180 feet).
Station G is slightly south of due east of the
landing site at a distanceof about 230 meters
(750 feet). In the descentmovie, the first signs
of blowing dust are visible as the spacecraft
passedover North Crater. Consequently,a small
amount of erosiontook place at station G as the
spacecraftpassedby during descent.This erosion is probably not significantto the analysis
of the specialenvironmentalsample.However,
the amount
of material
removed
from the sur-
SOIL AT APOLLO 14 SITE
face increasesgreatly as the spacecraft descends,and major quantities are eroded from
the landingsite.
The concentrationof particlesarriving at station G and originatingfrom the landingsite can
be estimated by comparisonwith the observations of the Apollo 12 mission.The Apollo 12
lunar module landed 155 meters (510 feet)
from the Surveyor 3 spacecraft,which at the
Station
5645
G
is about
230 meters
from
the
landedpositionof the Apollo 14 lunar module.
If it is assumedthat the Apollo 12 and Apollo
14 vehicleserodedidentical quantitiesof lunar
soil in the final stagesof touchdownand that
the emitted particle cloud expandsspherically,
the density of the particle cloud at station G
would be (155/230) 8 -- 0.3 of that at the Surveyor 3 location.Consequently,it would appear
time had been on the lunar surface 31 months.
that each squarecentimeterof surfaceat right
Detailed study of the Surveyor 3 camera re- anglesto the unobstructedline joiningstation G
vealed a distinct shadowpattern on the paint, to the landing site would receiveof the order oi
and this pattern was shown [Ja#e, 1971] to 10• impacts of particles,a few micronsor more
arise from a lunar soil sand blasting. It was in diameter, ejected from the landing site. To
demonstrated,moreover,that the sand-blasting reach station G, the particle velocitieswould
particlescame from the Apollo 12 landing site needto be of the order of 100 m/sec or greater,
rather than from a sequenceof points along its dependingon their ejectionangle.
Footpad-surfaceinteraction. 'The -- Y (Figlanding track. The particles must have had a
velocity greater than about 70 meters per sec- ure 2) and --Z footpadspenetratedthe surface
ond with a shallow angle trajectory to have only to a depth of 2 to 4 cm, whereasthe +Y
reachedthe Surveyor spacecraftand must have and +Z footpadspenetratedto a depth of 1'5
arrived at a fairly high concentrationto have to 20 cm. The +Y footpad penetration mechachieved the sharpnessof shadow effect ob- anism is clearly visible in Figure 3; the footserved. The abrasion appears to be uniform, pad contacted and plowed into the rim of a
and there is no indicationof individual impacts. 2-meter-diameter crater. The motion of the footTherefore, the surface or surface coating has pad through the soil caused a buildup of a
been struck by so many particles that their mound of soil on the north side of the pad. The
impact areas overlap. It will be assumedthat +Z footpad also landed on the rim of a small
the majority of particles reaching Surveyor crater, and its appearanceand penetrationwere
were of micrometer size or larger and that the similar to the appearanceand penetration of
average diameter of each impact might be of the + Y footpad.
the order of 10 •m. If it is further assumedthat
The responseof the soil to the landing (which
the area of impacts just saturatesthe surface occurred with little or no shock-absorber strok(conservative), it appears that each square ing) and the appearanceof the soil in the footcentimeter of the abraded area was subjected pad photographssuggestthat the mechanical
to impact by about 106 particles. One of us propertiesare similar to the mechanicalproper(Scott) has examinedthe Surveyor 3 surface ties of the lunar material on which the Apollo
sampler (also exposedto blowingdust) in de- 11 and 12 lunar moduleslanded. The penetratail at about 100 magnification,at which he tion of the +Z and +Y footpads causedthe
shouldcertainly have been able to seeany im- lunar module to tilt 1ø to 1.5ø in the westerly
and northerly directions.Consequently,at the
pact marks in the size range of order 100 •m,
but there were none.Therefore, it can be tenta- landing site, the strike of the lunar surface
tively concludedthat eachsquarecentimeterof slopeis approximatelyW16øN, and the dip is
the Surveyorcamera saw at least 10• particle approximately5.5ø in the direction N16øF,.
impacts from the material eroded by the descentengine.
As a check,it is foundthat thesenumberscorrespondto removal of the lunar soil to a depth
of 3 or 4 cm over a diameter of 5 meters from
the lunar surfacebelow the descentenginenozzle. This is compatiblewith astronaut observations.
SOIL CONDITIONS AT TIlE LANDING SiTE
The behavior of the surfacesoil was in many
respectssimilar to that observedduring earlier
missions.The color was gray-brown and appearedto changewith sun angle.The soil could
be kicked up easily during walking, but would
also compressunder foot. Footprints ranged in
5646
MITCHELL
ET AL.
..
-..
:"
•..?•::.
---.....
-,'•-..;;':"?:.•
'•":
r"..==========
.... .
.
...:.
' -'-•:::
'...'.i";:."%."
;?":'
.-;•:-::......
..
.-..::•::
.... •::-:..::,!{;•...
,' .::•;::......
-'-"*':•''"}'
ß,,,--.½--.-.?:...,..•.::
:-:
,;•.;;•:?;:.......:
:
.%.,';"•
....
':.:.
'*$<.'
.......
:'"'::::::1.4;;!!,.....
--
...,..,:,
Fig. 3. The '-I'-Y footpad embedded in the lunar soil. The gold foil surrounding the landing
, leg is probably the protective wrapping on the transporter (AS14-66-9234).
depth from 1 to 2 cm on level groundto 10 cm with blocky ejecta [Swarm et al., 1971] lead to
on the rims of fresh, small craters. The trans- estimates of thicknessof the soil layer in the
porter tracks averagedi cm and rangedup to Fra. Mauro regionrangingfrom 10 to 20 meters.
8 cm in depth.
Data from the active seismic experiment
Patterned ground ('raindrop' pattern) was [Ko.vach et al., 1971] indicatean unconsolidated
fairly general, except near the top of Cone surficiallayer 8.5 meters thick at the site of the
Crater. The reasonfor the developmentof this Apollo lunar surface experiments package
surfacetexture is not yet known, althoughit is (Alsep).
probablyrelatedto the impact of smallparticles
Surfacecharacteristicsare markedly different
on the lunar surface.
within a zone extending outward about 350
Crater morphologies
rangefrom freshthrough meters from the rim of Cone Crater from what
subduedto almost completelyobliterated. De- they are elsewhere.In this region, the soil is
terminations of minimum crater sizes associated
coarsegrained, and rock fragments are abun-
SoIL AT APOLLO 14 SiT•
5647
disturbedareas.Smoothedand compressed
areas
(e.•., transporter tracks and astronaut footsurface.
prints) are brighter at some sun angles,as is
The crew reported a zone of firm, compact shown in Figure 4. In some instances,it was
soil betweenstationsB1 and B2 (Figure 1) but difficult for the astronauts to distinguishbesaid that this was • small, isolated patch in a tween small, dust-coveredrocks and clumps or
generally powdery surface. Downslope move- clods of soil. The tops of many of the large
dant; elsewherethe soil is fine grained, and
rock fragmentsare sparselydistributed on the
ment
of loose soil has obliterated
some small
rockswere free of dust, althoughfillets of soil
were
craters on slopes.
As has been observedduring previous missions,disturbed areas appear darker than un-
..-- •
.........
.
•
:.½..:....--•.:.,:.,
•
%•
•..'•,,
•-•--%
..:'......
..
,• ß •.
. •,•:
.......•;..-...
.;;%.'•-,?:.. ?-'
.,.• •.½..:,....
......
":'"'•"
'"'•-":2;-;•;..?...:•
..• * • ....
;;.,.•'* .
.•.-.',%..• .....'..,
½•"'•'•'•••:'?;•..
.
•
•.:•-•--.•
......
-:;.•.
::.
':'.".........
...
•.,.•..••.".';•".-.:,...
•.•
.= '? .... • •'½.. •.•
...
...•,, ... :..,•
'-•"•-,;.;-'
"'-' -
:
,
-'
common
around
the bottom.
The
astro-
nauts commentedthat the maior part of most
large rocks a.ppearedto be buried beneath the
•,
•"
•':".
.•.?.,..::-•
• '"•....
...•..
?
•.
.•....?,...:
.....
.....•:•
• '...
•
•-.
' .
•..,
,
.½,;
•.•,'•.•'.:
.... :';':"/;•..-•'•;•
...... "-:....
. .'"•,.½.
,.: '.':•.,.....,.,..
- .. ';.%'
...........
ß....-"--,.•'-.•.."-'•.......5-:.•%"-.' '
.• • .,.:'.•,;•'..?...•f.-.•¾:•;•'%...•::.-...•:,•..•,...•.•..;•.,•?..-.
, . ,,:.:?,.:..:.:..,..---...•
... •.:?-'-",•;%.•;½
.......
. ......
... ?---..?-:?
-, .•;:•*;
..............
•.; • •..•.....
Fig. 4. The transpo•er tracks on relatively level, firm soil in the vicinity o• station
du•ng the secondextravehicular activity (AS14•-9058).
MITCHELL
5648
ET AL.
surroundinglunar surface. They also saw no
obvious evidence of natural soil-slope failures
hesion reflects coarser grain-size distribution,
on crater walls.
exposureto atmosphereis not yet known.
different
composition,
ortheeffects
ofprolonged
Grain-size distribution.
CHARACTERISTICSOF RETURNED SOIL SAMPLES
Grain-size distribu-
tions for severalsamplesare shownin Figure 5.
in distribution
for Apollo12 samples
are
Nearly 14 kg of fines (soil) were returned Ranges
It can be seenthat the
from the lunar surface.In addition, two single shownfor comparison.
of surfaceand
and onedoublecore-tubesampleswere obtained. bulk sample,which is composed
Some observationsrelative to the physicaland near-surfacematerial, and the surface layer
compositionalcharacteristics
of these materi-
from the trenchhaveparticlesizescomparable
als are useful for subsequentinterpretation of
the mechanicalpropertiesof the soil in situ.
to thoseof the Apollo 12 samples.However,material
that
fell out of the Cone Cr;•ter
core
sample and the material from the lower layer
of the trench are considerablycoarser,although
still somewhatfiner than the coarselayer found
Physical characteristics.The soil from the
Fra Mauro site is generallylighter in colorand
showsmore colorvariability than that from the
Apollo11 and 12 sites.The coloris morebrown in the Apollo 12 doublecore tube [Lindsay,
than black, probably becausethere is more 1971]. Median grain sizesfor the samplesshown
plagioclase
in the Apollo14 soilthan in earlier in Figure 5 are summarizedin Table 1.
Core-tubesamples.A doublecore-tubesamsamples.Thus far, a significantproportionof
glassbeadshas not been found,althoughthe ple was obtainedat stationA (Figure 1). Near
the rim of Cone Crater at station C', an attempt
content of angular glassfragmentsis high.
The Apollo 14 bulk sampleappearsto be less to take a singlecoretube in a rock-strewnarea
cohesivethan the soil from previous Apollo failed when the soil sample (some or all) fell
sites,evenafter removalof the fractioncoarser out of the coretube. At Triplet Crater (station
than I mm by sieving.Whether this lesserco- G), two attemptsto recovera triple core sam-
120
FSurfoce
Ioyer
of
trench
(19
N)/,
rttdouble
Apol
i0core
12tubesomples
/ Comprehensive
somple7
/'
(11somples}
lOt'J_ •
, ,,',,'
•ond
surfoce
somples
(6somples)
•=, I trench
(mixture}
(2IN)
'• 801••• ax/Cone
Croter
••
.
Bulk- sorepie
' • .
-•
4o-
'-
_
-
,,._
•orse Ioyer
intheApollo
12
doublecoretube
//
• somple
somple
, II i I I I I
010
--..
I
I•;• • • I I
I
I
I177+•
OJ
•
•
I
0.01
Groinsize, mm
[1971]. T•e •o]• p•t• of •e •po]]o 1• cu;•e• •;e to; ]•;ge
•e •e•
p•;• •e to; •mM] (•l-mm) •b•mple•
(0.036 to 0.113 g;•m) of t•e ]•;ge
•mp]e;.
Soir, ,T APor,r,o 14 Sing
5649
TABLE 1.. Median Grain Sizes oœ Apollo 14 Soil Samples
Median Grain
8amp
le
Location
a
Size,
mm
i.
Comprehensive
Bulk sample, bag 2
Cone crater
Lunar module, upper 1 ½m
Lunar module
core
Surœace oœ trench
Middle
oœ trench
Bottom o• trench
0.050
0.055
Station
C'
0.735
Station
Station
Station
G
G
G
0.087
0.068
0.410
asee Figure 1.
ple failed, and two single core-tube samples cm thick. Grain sizes appear to increasefrom
were obtainedinstead.In the first attempt for top to bottom.
a singlecore tube, the astronaut reported that
2. The singlecore-tubesamplefrom Triplet
he had struck
rock and had been unable to
penetrate deeperthan one core tube. Examina-
Crater containsat leastthreelayers,rangingin
thickness from 0.5 to 5 cm.
tion of the stainless steel bit from the core tube
3. The calculatedbulk densitiesin the upper
and lower halvesof the doublecore tube (1.73
burred.A secondattempt was made approxi= and 1.75 g/cm•) are significantlylessthan the
mately 9 meters north at a site that appeared values of 1.98 and 1.96 g/cm• determinedby
similar. The coarseness
of the soil at depth in Carrieret al. [1971]for the Apollo12 double
this area, as revealedby the sample from the core-tubesamples.Similarly, the bulk density
trench bottom, could account for the driving for the singlecoretube of 1.60 g/cm is lessthan
difficulties. Data on the returned core-tube samthe value of 1.74 g/cm• obtainedfor the Apollo
plesare summarizedin Table 2.
12 singlecoretube. Accordingto Carrier et al.
Data on the core tubes thus far available
[1972], the Apollo 14 core-tube densities cor[Mitchell et al., 1971; Lunar Sample Prelimi. respond to in situ densitieson the lunar surnary Examination Team, 1971; Carrier et al., face of 1.45 to 1.60 g/cmL These smallervalues
1972] indicate:
of density are possiblydue to a lower effective
1. The double core-tube sample contains specificgravity for the Apollo 14 soil particles,
from sevento nine layersrangingfrom I to 13 which consist of more microbreccias and fewer
confi•rmed
this, becauseit wasvisibly dentedand
TABLE2.
Data on Apollo !4 Core-Tube Samples
Core
Sample #eight,
Location
StationAb
grams
Sample Length,
cm
39.S
Density in Tube,
7.Sf
Station
.• 169.7 $1.if
f
Station
80.7
16
Station Ge
76.0
12
g/ca3
1,73
1.75
1.60
Recovery,
Estieated
Density
In Situ,
63
.60
%
>49
½a•/e•et al. [1972].
oUpper
part of doublecoretube.
dLower
part oœdoublecoretube.
Single core tube near Triplet
crater.
inglecoretubenearTripletcrater;looseandundisturbed
sample.
etermined by Heik•n [1971] by X radiography.
g/c=•
.45
5650
MITCHELL
ET AL.
crystalline fragments than the Apollo 12 soil
particles.
4. Some of the sample from Cone Crater
may have mixed with the samplefrom Triplet
Crater, becausethe sametube was usedat both
1. From less than 10% to 75% of the different samplesare composedof glass.
2. There are abundant glassand lithic fragments in the coarserpart of the • l-ram fraction, but the finer part is dominatedby mineral
locations.
particles.
3. Plagioclaseand pyroxene are the most
Soil composition. The results of studies by abundant, the plagioclaseto pyroxene ratios
the Lunar Sample Preliminary Examination varying from 16:1 to 1:2. This ratio varies
Team [1971] have establishedthat the Apollo both from sample to sample and from one size
14 soil samplesvary in composition,as well as fraction to another.Someolivine is present.
in grain size.The fractionscontainingparticles
4. Highly angular glassfragmentsfiner than
finer than i mm have the followingcharacteris- 62.5/•m in diameter are present,and there are
tics:
only a few glassspheres.
.
...
.
.
Maximum depth in lunarsoil
with one hand
Test number
Depth,cm
5a
50
la
2:0
44
42
Fig. 6. Apollo simple penetrometer.The pilot pushedthe penetrometerinto the lunar
soil three times. The arrowsindicate,the maximum depthshe achievedwhile pushingwith
one hand. The full black-and-whitestrips are each 2 cm long (S-70-34922).
So• •T A•oœ•o14 S•TE
Lithic fragments,similar to the lithic clasts
found in the fragmentalrock samples,comprise
70 to 100% of the 1- to 2-mm fraction of the
soil. Most of the rocksand soil samplesclosely
resembleeach other chemically.
ADHESIVE AND COHESIVE CHARACTERISTICS
Objects brought into contact with the lunar
surface or with flying dust tended to become
coated, although the layer generally remained
thin. The equipmenttransporter sprayed dust
around; however, thick layers did not form on
any of the component parts, and no 'rooster
tail' dust plume was noted behind the transporter wheels.
Although dust adhered readily to the astro•tauts' suits, it could be brushedoff easily, except for that part that had been rubbed into
the fabric. Dust sprinkledonto the thermal degradation sample array was easily brushedoff,
althoughthat part of the dust that filled in the
recessednumber depressionsof the sample
tendedto cohere.In this case,the dust formed
into the pattern of the numbers; then, when
the samplewas tapped, the dust remainedintact and bouncedout of the depression,retaining the shapeof the depression.
That
the surface soil at the Fra Mauro
site
possessesa small cohesionis demonstrated in
several ways; for example:
1. Clumping of soil in the vicinity of the
lunar module footpadsand elsewhere.
2. The numbersformedby dust on the thermal degradationsample.
3. Retention of deformedshapesand nearvertical slopes in astronaut footprints and
transporter tracks.
5651
penetrationsinto the lunar surface,thus providing information on soil property variations
with depth.The lunar modulepilot first pushed
the penetrometer into the lunar soil as far as
possiblewith one hand, calledout the penetration depth, and then pushedit deeperwith both
hands.This procedurewas repeatedtwice more
at points approximately4 meters apart.
A photographof the penetrometeris shown
in Figure 6. It can alsobe seenin the left middle of Figure 7, where it was later used as anchor for the geophonecable of the active seismic experiment. The penetrometer consistsof
a 68-cmdongaluminumshaft, which is 0.95 cm
in diameterand has a 30ø (apex angle) cone
tip. The penetrationdepthsare given in Figure
6 and Table 3. When the pilot pushedwith both
hands, no difficulty was experiencedin penetrating deeperinto the lunar surface,which indicatesthat the penetrometerhad not struck a
rock during the one-handpart of each test. The
forces associated with the one-handed and twohanded modes shown in Table 3 were estimated
from simulationsperformedby an experimenter
in an astronaut's space suit flying in a 1/6-g
trajectory in a KC 135 airplane.
The resistanceof the lunar soil to penetration by the penetrometerconsistsof two components,point resistanceand skin friction along
the shaft:
F=
F•,q--F.
(1)
where F is the lunar soil resistance,F• is the
point resistance,and F s is the skin friction.
Both components are functions of soil
strength, density, and compressibilityand of
frictional characteristics at the penetrometersoil interface. Of the two types of resistance,
point resistanceis by far the more sensitiveto
variationsin soil properties.
A general expressionfor skin friction is
It appears, however, that the cohesionis less
than would be anticipated from the results of
previousmissions,as is notedsubsequently.
The
sourceof lunar soil cohesionhas not yet been
L =
(2)
determined; however, there are several possibilities: e.g., primary valencebondingbetween whereL is the unit skin friction, D is the shaft
freshlycleavedsurfaces,electrostaticattraction, diameter, and z is the penetrationdepth.
The unit skin friction is commonlycalculated
and bondingthroughabsorbedlayers.
by usingthe equation
PENETRATION RESISTANCE OF THE
LUNAR SURFACE
• = '7zK tan a
(3)
At the Alsepsite in Figure 1, the Apollosim- where 7 is the unit weight of the soil, K is the
ple penetrometerwasusedto obtain 3 two-stage coefficientof lateral earth pressure,and tan a
5652
MITCHELL
ET AL.
Fig. 7. The penetrometer, in position as anchor for the geophone cable, is shown in the
left middle ground. Approximately 14 cm. of the penetrometer shaft are visible above the
lunar surface. The deep footprints and slope failures in the vicinity of the small crater in
the foreground suggestsofter soil in this area (AS14-67-9376).
is the coefficient
of friction
between
the shaft
friction. However, the value of F, has also been
investigatedby laboratory simulationsand has
Analysis of data by Vesi• [1963] for co- been found to be negligiblecomparedwith the
hesionless soils shows that actual unit-skin-fricuncertainty associatedwith the forces exerted
tion values can be as much as five times greater by the astronaut.Thus, in the followinganalysis,
than values calculatedby using equation3 and values have been determined two ways: (1)
that • ratio of unit skin friction to unit point F, -- 0, and (2) •,/(F•,/A) -- 0.3%, where A
bearingof 0.3% is the leastthat can reasonably is the penetrometer cross-sectional
area.
be assumed.If this were the casefor the peneBefore Apollo 14, the strength of lunar soil
trometer measurements,a significantpart of the had beendeterminedquantitativelyby direct inapplied force would have been carried by shaft place force-deformationmeasurementsonly by
and the soil.
SOIL AT APOLLO 14 S•
TAB'LE 3.
5653
ApolloSimplePenetrometer
Depths
Unit
Deptha,
cm
Test
)/odeof Force
App
1i cation
i
ForceF,
N
i•
of Penetration
Resistance
F/Aa,
N/cm
2
L
ß
la
lb
44
62
One-Handed
T•o-handed
71 to 134
134 to 223
100 to 188
188 to 314
2a
2b
42
68
One-handed
T•o-handed
71 to 134
<134 to 223
100 to 188
<188 to 314
3a
3b
50
68
One-handed
Two-handed
71 to 134
<134 to 223
100 to 188
<188 to 314
i
aCross-sectional
areaof the penetrometer,
0.71 cm2;all forceis assumed
to be carried in point bearing(F• = 0).
means of the soil mechanicssurface sampler
experimenton Surveyors3 and 7. On the basis
of bearing-capacitytheory for a shallow fiat
plate, Scott and Roberson[1968] calculatedthe
followingvalues.
Friction angle•
37ø-35ø
Cohesion
c
Bulk densityp
0.35-0.70 kN/m •'
1.5 g/cm8
'.o
F
_•
- •.
No skin friction
------ Unit
skin
friction
=0.3percent
oftheunit-point
bearing
•X•
2.0
resistance
-%•XN
% /•For
F/A
.-188
N/cm2
X -N •.-' /
Z:42•(t.t 20)
While the bearing-capacitytheory for an in•
strument like the Apollo simple penetrometer
is not as well developedas that for a shallow • •.o
fiat plate, theory can be usedto gain somein• 0.$
sight into the shear-strengthparametersof the
soil in situ at the Apollo 14 landing site.
The point-bearingcapacity of the penetrom0.4
eter is given by
._
r/.a
=
+
Surveyors
(4)
whereNc and Nq are functionsof friction angle
•, p is the soildensity,and g is the acceleration
of gravity.
• and
•
O.2
F•F/A:
I• N/•
z: 50cm(t•t
A uniqueset of valuesof • and c cannotbe
obtained from this equation,becausethere are
three unknowns(c, •, p). However,if a value
of p is assumed,
then for eachvalueof • there
is only one value of c, and a curveof • as a
function of c can be calculated. This has been
done in Figure 8 using a value of p -- 1.8
g/cms (variationsin p have little effecton the
calculations)and values for Nc and Nq (incam-
35
4O
45
Friction angle,{,deg.
Fig. 8. Combinationsof cohesionand friction
angle to accountfor penetrometerresistance.The
values of • and c at the Apollo 14 Alsep site
probably lie within the indicated bands.The parameters were calculated from the penetrometer
data by means of the bearing-capacity theory.
5654
i•ITCHELL
ET AL.
pressible
deformation
assumed)
fromMeyerho• The site chosenfor trenchingwasthe western
[1961, 1963]. The two cases(Fs -- 0 and rim of a smallcrater approximately6 metersin
)•s/(Fp/A) -- 0.3%) notedaboveare shown.
The bandsof valuesfor • as a functionof c
result from the range of depthsfor the three
diameter and 0.75 meter deep. A trenching
tool consisting
of a 21.6-cm-long(8.5-inch)by
15.2-cm-wide(6-inch) blade orientedat 90ø to
one-handedtests and from the uncertainty asso- a handlewas used like a backhoeby the astrociated with the force appliedby the astronauts naut. A view of the completedtrench from the
(Table3). The setof valuesfor • and c for the
in situ soil at the Apollo 14 Alsepsite must lie
somewherewithin these bands. Also plotted in
northeastis presentedin Figure9, whichshows
the steepest
trenchwall. AstronautShepardreportedthat diggingwaseasyand estimated
his
first cut to be to a depth of approximately15
Figure8 'is• as a functionof c for data.from
the Surveyor3 and 7 sites.The band for the cm with sidewallsat an angle of 70ø to 80ø.
penetrometer
parameters,
if no skinfrictionis With the next cut, the walls were steepenedto
assumed,falls above and to the right of the 80ø to 85ø, but at that point, they started
Surveyorparameters,which implies that the caving.It is not clear.fromthe varioustranjust howsteepthe walls
values of the soil-strengthparametersat the scriptsand debriefings
Apollo 14 landingsite may be greater than remainedover-all, but it appearsthat, for the
those at the two Surveyorsites. However, if rest of the excavation,slopessteeperthan ap60ø couldnot be maintained.
skin friction is a significantfactor, the param- proximately
The excavationpassedthroughthree distinct
eter values at the different landing sites may
be approximately
the same.The former situa- layers.The upper 3 to 5 cm were dark brown
tion is not unreasonable,however,becausethe and fine grained.Next, a very thin layer (0.5
Surveyordata are from surfacetests,whereas cm thick or less) of black glassyparticleswas
the penetrometerdata are from subsurface encountered.Beneath this layer was a material
tests, and in generalthe lunar soil strength muchlighter coloredand coarsergrained.This
stratificationis not readily visible in Figure 9;
probablyincreases
with depth.
The detailed variation of unit penetration however,the differencein grain sizeis marked,
resistance
with depthin the lunar soil is prob- as is seenby the distributioncurvesin Figure 5
ably very complex,as a result of layers and for samplesfrom the upper and lowerlayers.
On the basisof previousestimatesof lunar
pocketsin the soilwith varyingbulk density,
at other sites,
grain-sizedistribution,cohesion,
and other fac- soil friction angleand cohesion
that trenchingto a depthof
tors. An infinite number of possibledistribu- it wasanticipated
tions of unit penetrationresistanceas a func- 60 cm with near-vertical sidewalls should be
withoutdifficulty.If oneassumes
contion of depth would fit the penetrometerdata possible
soil with a density
obtainedduringthe Apollo 14 mission,because servativelya homogeneous
of 0.35kN/cm•, anda
only end-pointvaluesare known,althoughthe of 1.9 g/cm•, a cohesion
friction
angle
of
35
ø,
then
a stabilityanalysis
pilot did indicatethat penetrationresistance
increasedwith depth. Thus the data are in- indicatesthat a vertical cut shouldbe possible
sufficientlysensitiveto reveal lateral inhomo- to a depthof 85 cm beforesidewall
failurede-
geneities
at depthamongthe three penetration velops.Analysisof the Apollo14 photographs
sites.
showsa maximumtrench depth in the rangeof
25 to 36 cm for sidewallslopeanglesincreasing
SOIL MECHANICS TRENCH
from 60 ø to 80 ø.
At station G (Figure 1), Astronaut Shepard
attempted to excavatea 60-cm-deeptrench
with one vertical sidewall to: (1) exposethe
in situ stratigraphy; (2) providea meansfor
estimationof soil-strengthparametersby using
stability analyses;(3) providedata on variations in soil texture and consistency;(4) enable
samplingat depth,and (5) determinethe ease
Combinationsof cohesionc, density7, slope
heightH, slopeangle•, and frictionangle•
that correspond
to incipientfailure for homogeneous
slopesare shownin Figure 10. These
curvesare basedon the assumption
that curved
(circular) failure surfacespassthroughthe toe
of the slopeand that soil is homogeneous.
A
with which surface material could be excavated.
band of values that probably encompasses
the
Sore •T APollo
14 S•
5655
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Fig. 9. View of the soil mechanicstrench from the northeast. The steepestsidewalls
a•re estimated to be 65ø to 80ø. A pile of excavated material is at the end of the trench
at the left in the photograph (AS14-64-9161).
conditionsin the Apollo 14 trench is shownin
Figure 10.
For the slopeanglesand corresponding
trench
depthsnotedabove,an assumedunit weightof
3 kN/m 8 in the lunar gravity field, and an assumed friction angle of 35ø, the required cohe-
sion values for stability fall in the range of
approximately0.03 to 0.10 kN/m •. Thesevalues
requiredwouldbe evenless,as can be seenfrom
Figure 10. It shouldbe noted,however,that the
computed values of cohesionare lower-bound
estimates;that is, they are the minimumvalues
requiredto maintain stability for the conditions
shown.Nonetheless,it doesappear that the soil
beneaththe surfaceat the locationof the Apollo
14 trench is less cohesive than would have been
are considerablyless than the values estimated anticipatedon the basisof previousdata. This
usingpreviousluna.
r-surfacedata. For higher lower value for cohesionis compatiblewith the
values of friction angle, the values of cohesion coarser grain sizes encountered.
5656
1VIITCHELL ET AL.
probewascalculated.The influenceareais given
by
0.20
=
(
+
D•) Ni
where At is the total influence area of all particles larger than i•, i• is the smallestparticle
size that will stop penetration,D, is the diam-
.15
eter of particle i, D•, is the diameterof the
probe,and N, is the numberof particlesin the
size range i (850 size rangeswere used to describe the Apollo 12 size-versus-frequency
dis•
.IO
tribution curve).
.o5
samplearea representsthe probabilityof striking a critical particle.in a unit volume.The
analysisis conductedto any desireddepth,and
the cumulativeprobability of not encountering
The ratio of this influence area to the total
Probable
conditions for
Apollo14
trench
a critical obstructingparticle is calculated.
The probability analysiswas used to deter-
minethe likelihoodthat the three penetrometer
penetrationscouldbe madeto the full depth of
04O
•o
60
7o
8o
the instrument(0.95 cm). The prQbabilityfor
Slopeangle,/1,deg
Fig. lO. Stability
numbers for
slopes.
homogeneous
IN SITU PARTICLE-SIZE DISTRIBUTION
A preliminaryanalysishasbeenmadeto compare behavior during surfacepenetrationand
trenching with that predicted on the basis of
the Apollo 11 and 12 particle-sizedistribution
curvesgiven by Shoemakeret al. [1969, 1970].
For the analysis,the assumptionshave been
made that particle-sizedistribution does not
vary with depth and that the volumetric ratio
of variouslysizedparticlesequalsthe area ratio
observedat the surface,or
,4,/,4
the instrument (68 cm) without encounteringa
rock equal to or larger than the diameter of
= v,/v
whereA, is the area of particlesof sizei in the
the three events was calculatedto be 51%.
The four core-tubeevents were analyzedin a
similar manner. The combined probability of
the four tubes being driven to their respective
depths without encounteringa fragment equal
to or greaterthan the core-tubediameter (1.95
cm) was 67%. In fact, one of the core tubes
(tube 2022) hit an obstruction.The probability
that one or more core tubes would hit an ob-
structionwas only 33'%.
The Apollo 11 and 12 sizedistributioncurves
were also used to predict the number of particlosof a givensizethat shouldbe foundin the
soil mechanics trench. The results indicate that
only a few particleslarger than 1 cm would
have been expectedin the excavatedsoil. How-
ever,manyrocksin the 1- to 2-cmrangewere
reported,and numerousparticlesof this size
surfacesample,A• is the total surfaceareao.• can be observed in the photographsof the
the sample,V, is the volumeof particlesof size
i in the sample,and V• is the total volume of
the sample. Sufficientdata are not available at
presentto determinereliablythe probablevariations in size-versus-frequency
distribution with
depth.
The total area of influence of all particles
large enough to interfere with a penetrating
trench.Furthermore,the grain-sizedistributions
determinedin the Lunar ReceivingLaboratory
for material
from
the bottom
of the trench
(Figure 5) showed more than 20% of the
sampleto be coarserthan I cm. This is strong
evidence that the determinationsof particle
size versus frequency made at the surface of
the Apollo 11 and 12 sites are •notgoodindi-
SO•L AT APOLLO 14 S•T•
5657
esses,as would be indicatedby nearly random
local variations and near homogeneityon a
regionalbasis.
LATERAL VARIATION OF SOIL PROPERTIES
Variations in crater morphologyat the Apollo
Study of the variability of soilpropertieswith 14 landing site indicate the following age selateral position is important in order to ascer- quence of craters along the geologictraverses
tain both the magnitudeand randomnessof the (Figure 1), beginningwith the oldest,according
variations that may be encountered It is of to interpretations by Eggleton and Offield
interest to know whether variations can be
[1970] and by Trask [1966] as reported by
associateddirectly with differencesin geomor- Swannet a/. [1971].
phologicalfeaturesor whetherthe influenceson
1. Highly subduedcraters characterizedby
the soil of original geomorphologymay have
at (a) the landing site
been obliteratedby random lunar-surfaceproc- very gentle depressiol•s
cators of the size distributionslikely to occur
with depth at other sites.
Fig. 11. The transporter in use during the second extravehicular activity (AS14-68-9404).
5658
MITCHELL
ET AL.
of the lunar module; (b) west of the lunar
moduleat the Alsep deploymentsite; and (c)
soils.With this quantity known and the results
of tests on lunar soil simulants,it has been
north of station A.
possibleto estimate the in-place void ratio
2. Moderately subduedcratersat (a) North densitypL, and friction angle•L for the soil on
Triplet Crater; (b) the 50-meter-diameter the moon. This procedureis describedin the
crater east of station F; and (c) the 10-meter Appendix.
crater at station A.
The results of the analysisare presentedin
3. Cone Crater and the sharp 45-meter Tabl• 4. From the indicatedrangesin lunar
crater at station E.
soil properties, it can be seen that within re4. The sharp 300-meter crater at station C
gionsof comparablegeologic
agethereis a small
and a small 10-meter crater next to which a
but distinct difference between average propfootball-sizerock was collectedduring the first erties of soil located in intercrater areas on
extravehicularactivity.
firm, level ground and soil located in soft
pockets and fresh crater rims and slopes.No
Information relating to soil properties asso- appreciabledifferencesin lunar soil properties
ciated with these features can be obtained by between regions of different geologicage are
inference
from the observed interaction
of
discernible,however, on the basis of the comvarious objectsof known geometryand physi- puted values.
cal characteristics with the lunar surface. One
It appears,therefore,that, within the upper
suchtype•of interaction
that has'beenuseful few centime•tersof the lunar surfaceat the
for studyof soilpropertiesis that of the wheels Fra Mauro landing site, the initial consistency
of the modularizedequipmenttransporterwith of the lunar soil resultingfrom a major geologic
the surface.
event of short duration, such as the primary
The transporter(Figure 11) is a two-wheeled, impactof a largemeteorold
or th• impactof
ricksha-type vehicle equipped with pneumatic ejecta,hasbeenmodifiedby long-durationproctires that was used to carry equipmentduring essesthat have createda steady-statecondition.
both periods of extravehicular activity. The Such processes
may includethe continuousflux
maximum weight of the vehicle plus payload of micrometeoroidsand the steady downslope
in lunar gravity was 128 N (28.9 lb), including movementof material, superposedon the cona maximum weight of rock and soil samplesof tinuous formation and extinction of small and
38.7 N (8.7 lb). During the secondperiod of medium-sizeimpact craters.The continuousinextravehicular activity, the transporter trateraction of such events with the lunar surface
verseda distanceof approximately2.8 km, and is further evidencedby the followingobservathe total weight fluctuated between89 and 120 tions: (1) formation of raindrop patterns on
N, dependingon the amountof samplecollected. relatively level surficial fine-grainedmaterial,
The transporter tracks in the vicinity of but low frequency of occurrenceof such patstation B can be seen in Figure 4. The astro- terns on steeper slopes; (2) rounding 6ff of
nauts estimatedthe track depths to vary be- boulders and development of fillet material
tween 0.5 and 2 cm and noted that the largest against outward-slopingrock surfaces;and (3)
sinkagesoccurred around soft crater rims. A the presenceof small fine-grainedhummocksa
closeupphotograph of a transporter track in few centimetersin size. Irrespective of age,
the immediate vicinity of station A is shownin however,the lunar soil at the surfaceappears
Figure 12. The estimatedtrack depth at this in generalto be firmer at level intercraterareas
location is 1.0 cm.
The facts that the transporterwas equipped
with pneumatic tires of known geometry and
load-deflection characteristics and that the lunar
than at small fresh crater rims and slopes.
LUNAR
SOIL MODULUS
The results of the Apollo 14 active seismic
soil at the Apollo 14 site is predominantlygran- experiment,reportedby Kovach et al. [1971],
ular make possiblethe estimationof the average indicatedthat the seismicvelocity (P wave) in
changein the penetrationresistance
with depth, the upper 8.5 meters of the Fra Mauro site is
designatedby GL, on the basis of the analysis 104 m/sec. Accordingto elastic theory for a
developedby Freitag [1965] for cohesionless homogeneous,isotropic medium, the P-wave
5659
So•, •T A•o•,•,o 14 S•T•
.-
.
.
.
Fig. 12. Close-upphotographof transportertrack in the vicinity of station A. The
transporterhas evidentlyrolled acrossslightly firmer soil, becausethe track narrows,indicating less sinkage.A small white rock has been pushedinto the soil by the tire in
the lower left-hand comer of the photograph.The indention at the upper left-hand comer
of the photograph
wasprobablyproducedby the edgeof the camerawhile it wasbeingpisitioned (AS14-77-10358).
velocityv• is relatedto Young'smodulusE by
E(1
--•)2•,)
11/2(7)
v,'- p(1
-•-•,)(1wherep is density,and v is Poisson's
ratio.
known that Young' modulusat small deformation is related to confiningpressure(rsaccording to
E = Kp,(as/p,)"
(8)
Combinationsof modulus,density,and Pois- whereK and n are parametersdependenton
presson'sratio corresponding
to a velocity of 104 soiltype,andpais terrestrialatmospheric
m/sec are shownin Figure 13. From a number sure in the same units as (rs. Kulhawy et al.
of studies on cohesionless
terrestrial soils, it is
[1969] have shownthat for well-graded
sands
MITCHELL
5660
ET AL.
TABLE4. Variation in Lunar Soil Properties with
Lateral Position as Dete•ined
From Transporter Tracks
Terrain
Type
Geology Traverse
Stations a
G.L,
N/cm
a
• .,b
g/Zc•a
eL
$L'
deg
Highly SubduedCraters
Lunar
module
site
Level
0.83
to
0.47
0.68
0.73
to 0.27
O. 30
0.74
to
0.70
1.85
0.69
to 0.75
O. 74
1.79
to
1.81
43.4
to
41.0
1.84
to 1.77
1.78
39.6
42.8
to
39.0
38.5
Alsep site,
sta.
A, B
Firm
Lunar module site
Soft spots
Crater
0.34
rims
Moderately SubduedCraters
A, G, B2
Level
2.02 to 0.47
0.62 to 0.71
1.91 to 1.81
0.91
0.68
1.85
43.2
0.47 to 0.20
0.71 to 0.77
1.81 to 1.75
41.0 to 37.2
0.30
0.75
1.77
38.5
1.85 to 1.79
43.4 to 39.6
Firm
A, B1, B2
Soft spots
Crater
B3, C'
rims
Sharp Craters
0.83 to 0.34
0.68 to 0.74
Level
Firm
C'
Soft spots
Crater
Values of lunar soil
0.65
0.70
1.83
42.1
0.47 to 0.20
0.71 to 0.77
1.81 to 1.75
41.0 to 37.4
0.34
0.74
1.78
39.4
rims
properties
47.1 to 41.0
given as either
ranges or averages.
Accordingto descendinggeologic age.
Thesevaluesof densityare somewhat
higherthanthoseestimated
on the basisof core-
tube data (Table 2).
They can be attributed
in part to the value of 3.1 assumed for
specific gravity in the analysis.
The results of recent (1972, unpublished) tests on two
Apollo 14 soil samples suggest that a value of 2.95 is more appropriate.
typical values of K and n are 300 and 0.50,
respectively.From test data given by Namiq
[1970] for a lunar soil simulant, n. can be de-
ducedto be 0.51, and K increases
from 34 at a
void ratio of 0.8 to 130 at a void ratio of 0.6.
From these values,one can computeaverage
2.4 --
2.0
moduli corresponding
to confinementat a depth
of 4.25 meters on the lunar surface (half the
depth of the estimatedthicknessof the unconsolidatedlayer). If the lateral pressureis taken
as one-half the vertical pressure,probably a
reasonablelower bound, the resulting modulus
values are in the range of about 0.1 X 10' to
0.75 X 10' kN/m •'. It can be seen that these
values fall within the range, but at the lower
end, of those shown on Figure 13. There is
reasonto believethat, had the terrestrial measurements been carried out under high-vacuum
conditions,the modulusvalueswould have been
somewhathigher owing to the absenceof adsorbedfilms betweenparticles.
The importanceof theseresultsis that special
or unusualsoil propertiesneed not be assumed
in order to account for the low seismic veloci-
o 0.8
ties. On the moon, the low-gravity conditions
result in low confinement,which in turn gives
a low seismicvelocity.
0.4
CONCLUSIONS
0
1.0
I
1.2
I
1.4
I
1.6
I
1.8
I
2.0
I
2.2
Density
- (;i/cm
$)
Fig. 13. Young's modulus as a function of density and Poisson's ratio for a seismic velocity of
104 m/sec.
The results of the Apollo 14 missionhave
provided new insightsinto the propertiesand
behavior of the soil at the surface of the moon.
Some of the more significantfindingsresulting
from the analysesherein are as follows:
So•. AT A•o•o
1. A greater variation exists in the characteristics of the soil at shallow depths (a few
centimeters)in both lateral and vertical directions than had previouslybeen supposed,and
soil with a particle-size distribution in the
medium- to coarse-sandrange may be encountered at shallowdepths.
2. There is evidencethat venting from the
soil voids of descent-engine
exhaust gas after
engineshutdowncausedfine particle movement.
3. Particleserodedby the exhaustplume of
the lunar moduledescentengineduring landing
were distributed over a large area around the
final touchdownpoint.
4. As had beenthe casein previousmissions,
dust was easilykicked up and tendedto adhere
to any surfacecontacted.
5. Soil density determined from a double
core-tubesampletaken at station A was about
1.60 g/cm'. These values are lower than those
obtainedfrom the Apollo 12 core tubes,but are
well within the range thought to be characteristic of the moon over-all.
6. The resultsof simplepenetrometermeasurementssuggestthat the soil in the vicinity of
the Apollo 14 Alsepmay be somewhatstronger
than soil at the landing sites of Surveyor 3
and 7, and comparisonof the results of analysesusing penetrometer,transporter-track,and
trench data suggeststhat soil strengthincreases
with depth.
7. Computationsof soil cohesionat depth at
the site of the soil mechanics trench result in
lower-boundestimatesconsiderablylower than
were expectedfrom the resultsof previousmissions (0.03 to 0.10 kN/m' as opposedto 0.35
to 0.70 kN/m').
8. The determinationsfrom the Apollo 11
and 12 sites of surfaceparticle size versusfrequencyare not goodindicatorsof the size distributions likely to occur with depth at other
14 S•T•
5661
pected for a terrestrial silty fine sand layer in
the lunar gravitationalfield.
APPENDIX
DETERMINATION
FROM MET
OF SOIL PROPERTIES
TRACK DATA
It is assumedthat the functional relationships
describingthe tire-soil interaction under lunar
environmental
conditions and low velocities are
essentiallythe sameas the relationshipsdescribing the correspondingtire-soil interaction on
earth. On this basis, the normalized sinkage
z/d, in which z is the penetrationdepth of the
tire and d is the tire diameter,is relatedto the
dimensionless
quantity
Ns•= G(bd)S/•
w •
(9)
in which G is the averagechangeof conepenetration resistancewith depth, b is the tire section width, W is the tire load, $ is the tire deflection,and h is the tire sectionheight,by the
samefunctionrelationshipas the corresponding
quantities are related to sand under terrestrial
environmental
conditions.
The dimensionless
quantity Ns•, termed the
sand-mobility number by Freitag [1965], expressesthe soil strength and density characteristicsthroughthe quantity G. The averagepenetration-resistancegradients GL of the lunar
soil at various locationsalong the geological
traverse were determined using transportertrack depth data (estimatedfrom Hasselblad
camera stereo-pairs), known tire characteristics, and the z/d versusNs• diagram shownin
Figure 14. The solid cur•e representsthe best
fit for terrestrial
wheel-soil interaction
test data
(180 tests) reported by Green [1967]. These
tests were performed with pneumatic tires of
differentgeometriesand sizeon a uniform dune
sand from the Arizona desert, designatedYuma
sites.
sand.Figure 14 alsoshowswheel-soilinteraction
9. Analysis of transporter-track depths has data obtainedfrom tests performedwith transindicatedanglesof internal frictionin the range porter tires inflated to the same pressureand
of 40ø to 45ø (if a cohesionless
soil is assumed) subjectedto the same load range as those on
for soil near the surface,and that the soil is the lunar surface. A lunar soil simulant consistlessdense,more compressible,
and weakerat the ing of a crushed basalt, Waterways Experirims of small craters than in level intercrater
mental Station (WES) mix, was used for these
tests.
regions.
10. The magnitudeof the in situ modulusof
Valuesof GL (Table4) determined
from Figelasticity, based on the measuredseismic-wave ure 14 were convertedto correspondingvalues
velocity, is compatible with that to be ex- in the earth'sgravitationalfield G• by assuming
MITCHELL
5662
that penetration resistancevaries in direct proportion to gravity level; i.e., Gr -- 6 G•,, in
accordancewith the analysis of Costes et al.
[1971].
The behavior
of the lunar
soil simulant
and
that of the actual lunar soil were assumed to
be the same if both are at the same void ratio.
Accordingly, G• values were used to estimate
the in-place lunar soil void ratios e•, (Table 4)
at corresponding
locationsfrom the diagram of
ET AL.
of gravity, B is the least dimensionof foundation base (dimensionof foundation), z is the
vertical
distance from the surface to the base
of the foundation,No, N7, Nq are bearing capacity factors that dependon the angle of internal friction • of the soil, and so,•, s• are
factorsthat dependon the shapeof the foundation.
If it is assumed
that c, N•, p, N7, and N• are
independentof depth, then from equation 10
G versus void ratio for the lunar soil simulant
dq,/dz = S•pgN,
(WES. mix) shownin Figure 15. Corresponding
values of soil density have been determinedas- But, from the definitionof G,
suming that the specificgravity of the lunar
aq,/azsoil particlesaverages3.1.
Values of friction angle for different values sothat Nq can be found from
of G and density were estimated as follows
Nq- G/S•pg
(11)
[Cosreset al., 1971]: The resistance
to penetration of the lunar soil can be expressed
by the
Values of friction angle were determined,
Terzafthi [1943] simplified bearing-capacity using equation11 and N, versus• diagrams
equation for foundations:
developedby Meyerho/[1961] for shallowconical-shapedfoundationsand are listed.in Table
q, -- sccNc-[- -•s
2 • pgBN• -[- sqpgzNq
(10)
4.
where qz is the bearing capacity of the foundaIt couldbe arguedthat the bearing-capacity
tion (i.e., the resistanceto penetration of the factorsN•, N, and N, will increasewith depth,
lunar soil at depth z), c is the soil cohesion, sincethe density of the soil will tend to increase
p is the bulk densityof soft,g is the acceleration with depth, and both the cohesionc and •
0.15
0.10
0.05
oo
O0
I0
20
30
SandMobility
No.Nsu=
40
50
G(bd
•/= •
W
h
Fig. 14. Tire sinkage versus sand mobility number for two soils. The curve represents
the best fit to first-passtests on Yuma sand at Towed Point [Green, 1967]. The points represent first-passtests with transporter tire on lunar soil simulant (WES mix) at Towed Point.
8OIL AT APOLLO 14
RelativeDensity,Dr' (percent)
300
ø
I0
I
•
I
30
,10
5o
Go
I
I
I
I
70
- $0.0
200-
I00 80•.
14)0
ß
60 -
40-
20-
'•/•' o
'•// o
io86-
- I0.0 co
-
5.0:•
-
2.0
•
.•,
5663
suchthat p and e are not sensitiveto the variations in Gr values that result by multiplying
the corresponding
G• valuesby G, rangingbetween3 and 6. The relationshipbetweenNq and
•b is suchthat • is not sensitiveto the resulting
variationsin Nq given by equation11. The •
valuesassociated
with G• rangingbetween3
and 4 are lower than the .• values resulting
from taking G• = 6 by I to 1• deg. Corresponding variations in density are of the
order of 2%.
4
Acknowle.dgments. Professor W. N. Houston,
Dr. H. J. Hovland, and •. T. Durgunoglu assisted
in simulation studies and data analyses at the
University of California, Berkeley. Dr. Stewart W.
0.8
Johnson, Richard A. Werner, and Ralf Schmitt
0.6
participated in phases of the work carried out at
0.4
the Manned Spacecraft Center, Houston. G. T.
I
I
I
I
I
I
o.3 I
Cohron assisted in the transporter-track analyses,
0.4
4).1
1.0 0.9
0.8
0.7
0.6
O.S
which were done at the Marshall Space Flight
Void Ratio - (e)
Center, Huntsville, Ala. The assistanceof these
Fig. 15. Variation of penetration resistance individuals is gratefully acknowledged.
gradient of lunar soil simulant (WES mix) with
The studies reported herein were supported, in
soil consistencyand packing characteristics.The
part, by NASA contract NAS 9-11266, principal
moisture content • is 0.9% for the solid curve
investigator support for. soil mechanics experiand 1.8% for the dashed curve.
ment (S-200), and NASA contract NAS 9-11454,
co-investigatorsupport for soil mechanicsexperiment (S-200).
o
._
- 0o5•
1.0
I
o.
increasewith incressingdensity, and because
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dq,/dz = G = A -•- (B -•- S•pN•)g
(12)
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of q and are obtainedby differentiatingc, p, q•,
No, N7, and N• with respectto z. Thus the
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gradient
of the soil at 1-g gravity level to that for the
same soil at 1/6-g gravity level is lessthan 6.
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the strength.
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