Applicability of Simulants in Developing Lunar Systems and Infrastructure: Geotechnical Measurements of Lunar Highlands Simulant LHS-1

Abstract

Landing sites for the upcoming NASA Artemis program are in south polar region highlands, near permanently shadowed craters (PSCs) thought to contain water ice. The presence of water ice will alter the geotechnical properties of the regolith, which could impact hardware assets and other mission requirements. Geotechnical experiments performed on dry lunar regolith simulants may be able to inform in situ measurements that indicate volatiles, including water ice. Exolith Lab's lunar highlands simulant, LHS-1, was examined before and after drying to measure density-related properties than may indicate volatile-bearing lunar regolith. Density (minimum, uncompressed bulk, maximum, and relative) and specific gravity were measured for oven-dried simulant and are compared to that of undried, water-bearing simulant stored in a high ambient moisture environment. LHS-1 simulant was placed in an oven at 70°C for 24 hours and was kept in a sealed container after drying to minimize the amount of re-adsorbed moisture. Minimum density (ASTM D4254) was calculated from the average of three samples of known mass placed in a graduated cylinder and gently inverted. Minimum densities are 1.27 g/cm 3 and 1.24 g/cm 3 for undried and dry, respectively. Uncompressed bulk densities (ASTM D7481) are 1.39 g/cm 3 and 1.58 g/cm 3 for undried and dry, respectively, and was calculated as the mean of 10 samples of known mass placed in a graduated cylinder and gently agitated to encourage natural settling. The maximum density (ASTM D4253) was measured by taking the mean of three samples of known mass in a graduated

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Applicability of Simulants in Developing Lunar Systems and Infrastructure:
Geotechnical Measurements of Lunar Highlands Simulant LHS-1
Jared Long-Fox1, Michael P. Lucas2, Zoe Landsman1, Catherine Millwater1, Daniel
Britt1, and Clive Neal2
1University of Central Florida, Department of Physics, 4000 Central Florida
Boulevard, Orlando, FL 32816; email: jared.long-fox@ucf.edu,
zoe.landsman@ucf.edu, cmillwater@knights.ucf.edu, dbritt@ucf.edu
2Department of Civil & Environmental Engineering & Earth Sciences, University of
Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, mlucas25@nd.edu,
cneal@nd.edu
Abstract
Lunar exploration activities and infrastructure development demand well-
constrained information about the geotechnical properties of lunar regolith. Since
returned lunar regolith is too scientifically precious to allocate for large-scale
geotechnical studies, terrestrial materials must be used to create regolith simulants
that reproduce the properties of lunar regolith. Studies that use inappropriate
simulants to develop lunar systems are invalid and give misleading results, but as
shown here, the lunar highlands simulant produced by Exolith Lab, called LHS-1,
maintains high geotechnical fidelity to highlands regolith samples. This study
compares the particle size distribution, specific gravity, density, and shear strength of
LHS-1 simulant with two different preparation methods: ambient storage and oven
dried.
Introduction
The upcoming NASA Artemis program aims to return humans to the Moon to begin
the process of establishing permanent lunar infrastructure for sustained human
residence and activity, with plans to achieve this at the lunar south pole (NASA, 2020).
Lunar south polar region highlands are home to permanently shadowed regions (PSRs),
which are thought to contain significant amounts of water ice and other volatiles. Since
it is not economically feasible to deliver all raw materials from Earth, many of the
necessary resources required for a sustained human presence on the lunar surface (e.g.,
water, rocket fuel, building materials) will be obtained through in situ resource
utilization (ISRU). The efficiency and feasibility of various ISRU and exploration
activities will be affected by the composition and physical state of the regolith,
including the presence (or absence) of volatiles, thus the need to adapt to a wide range
of possible regolith properties. The presence of volatiles is expected to alter the
geotechnical properties of lunar regolith (Gertsch et al., 2006) and these altered
properties will impact hardware requirements for base camp infrastructure and other
mission logistics, such as ISRU activities and vehicle mobility. These logistical
requirements necessitate a thorough understanding of the geotechnical properties of

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lunar regolith as well as the sensitivity of these properties to perturbations in
composition and physical state.
The dominant composition of the lunar surface around south polar region PSRs is
anorthositic (Lemelin et al., 2021; Lemelin et al., 2022), similar to the highlands
material returned by the Apollo 16 mission. Therefore, a mineralogically accurate lunar
highlands regolith simulant is the appropriate analog for the geotechnical
characterization of potential south pole landing sites. Reported here are the
geotechnical properties of the Exolith Lab lunar highlands simulant, LHS-1, an analog
material designed to approximate the mineralogy and mechanical behavior of average
lunar highlands (see LHS-1 Fact Sheet; Exolith Lab, 2021). This work compares the
geotechnical properties of undried versus oven-dried LHS-1 to establish baseline
geotechnical data for dry lunar regolith (outside of PSRs) and demonstrate variations
in these properties due to adsorbed volatile content. This study illustrates that the
geotechnical properties of LHS-1 are consistent with those measured in situ at the
Apollo 16 highlands landing site (Mitchell et al., 1972).
LHS-1 Simulant and Laboratory Tests
Exolith Lab lunar highlands simulant, LHS-1, was designed to optimize simulant
fidelity relative to the typical mineralogy and particle size and shape distributions of
returned Apollo highland regolith samples (Cannon and Britt, 2019). Undried (ambient
storage) and oven-dried LHS-1 were used in each of the geotechnical laboratory tests.
Undried LHS-1 was stored in plastic buckets in ambient laboratory conditions and,
based on gravimetric measurements before and after drying (see below), has an
adsorbed water content of 0.095 wt.%. Dried LHS-1 was taken from the same source
but was dried in a forced convection sediment oven at 70°C for 24 hours. Tests were
performed immediately after removal from the oven to minimize re-adsorbed moisture.
If the dried simulant was exposed to ambient conditions for more than 4 hours, it was
not used for further testing. Geotechnical properties of both undried and dried LHS-1
were tested to quantify their respective particle size distribution, specific gravity,
minimum (ρmin), uncompressed bulk (ρ), maximum (ρmax), and relative densities (ρR),
as well as shear strength. Results for these properties are given in Table 1.
Particle Size Distribution
Particle size distribution is one of the most important requirements for lunar
simulants for geotechnical studies. The lunar science, exploration, and engineering
communities need to understand the geotechnical properties of regolith in potential
landing sites at the lunar south pole in order to prepare the site for a permanent habitat
and manipulate regolith for ISRU purposes. Therefore, reproducing the particle size
distribution of Apollo lunar regolith samples with LHS-1 lunar highland simulant is a
critical step in establishing a high-fidelity analog for geotechnical studies of the south
polar region of the Moon. Target particle size distributions are based on both highlands
and mare reference samples from the Lunar Soils Grain Size Catalog (Graf, 1993). To
achieve lunar-like particle size distributions in LHS-1, a maximum allowable grain size
of 1 mm is set, and the simulant is percussively crushed to a power law size distribution

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(mean particle size of 60 µm) below the 1 mm maximum grain size, then simulant
particle sizes are measured with a CILAS 1190 particle size analyzer (Figure 1).
Figure 1. Particle size distributions of LHS-1 and Apollo lunar highlands
samples. Apollo sample particle size data were obtained from Graf (1993). Plot
from Exolith Lab (2021).
Specific Gravity
Specific gravity is defined by ASTM D4439 (ASTM, 2020) as the ratio of density
of the material in question to the density of a reference substance at specified conditions
of temperature and pressure (in this case pure water at room temperature). Specific
gravity of dried and undried LHS-1 was calculated as the average of three samples with
known masses placed in a graduated cylinder and mixed with a known volume of
deionized water (ρ = 1.0 g/cm3) to reach a pre-determined combined volume from
which to calculate the specific gravity. The average specific gravity of LHS-1 was
found to be 2.35 for undried and 3.11 for dried. The dry value of 3.11 is consistent with
the recommended assumed value of 3.1 used to calculate bulk and relative density of
lunar regolith (Houston et al., 1974; Mitchell et al., 1974; Carrier et al., 1991).
Minimum, Uncompressed Bulk, Maximum, and Relative Densities
The minimum density of a granular material is the density of the material when the
grains are packed in the loosest possible stable arrangement, and hence, coincides with
maximum porosity. Since the minimum density of a granular material is typically
observed on the free surface, this property is especially useful in characterizing the
uppermost layers of in situ lunar regolith. The uncompressed bulk density is the density
of a material that is typically observed in storage and processing equipment such as
small bins and hoppers. Uncompressed bulk density is generally regarded as the

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nominal density for granular materials and therefore a key parameter for lunar ISRU
processes that involve the transport and storage of regolith. Maximum density is
achieved when the simulant grains are in the densest possible arrangement (porosity is
at a minimum), while still an unlithified, granular material.
The minimum density of undried and dried LHS-1 was measured using a procedure
that conforms to ASTM D4254 (ASTM, 2016) by measuring the volume of three
samples of known masses after gentle inversion in a graduated cylinder. Our reported
values (Table 1) are the mean of these three measurements. The uncompressed bulk
density of LHS-1 was measured following ASTM D7263 (ASTM, 2021). In this
procedure, uncompressed bulk density is taken as the mean of 10 dried and undried
volume measurements of samples of known mass after gentle agitation in a graduated
cylinder to encourage natural packing of the simulant grains. Ten replicate tests each
for undried and dried LHS-1 were conducted to define the limits of reproducibility. A
procedure that approximates ASTM D4253 (ASTM, 2016) was followed to measure
the maximum density of dried and undried LHS-1. For this test, maximum density is
taken as the mean of three volume measurements of samples of known mass after the
sample is mechanically agitated in a graduated cylinder. Mechanical agitation was
achieved using three 3V DC coin-style vibration motors for 10 minutes for each
sample, after which the final volume was recorded. Using the measured minimum and
maximum densities, it is possible to calculate the relative density, which represents the
nominal degree of particle packing experienced by regolith and is one of the key
properties in determining the strength and stiffness of regolith (Zeng et al., 2010).
Obtaining constraints on the relative density of lunar regolith is critical to provide
useful information for future ISRU and construction activities on the lunar surface.
Furthermore, differences in densities observed for undried and dried LHS-1 (Table 1)
are most likely due to adsorbed water content and may be used as a proxy for volatile
content for lunar regolith. Estimates of relative density (ρR) for dried and undried LHS-
1 (uncompressed bulk – Table 1) were calculated using the Equation 1 (Zeng et al.,
2010):
ρR=(ρmax
ρ× ρ − ρmin
ρmax − ρmin)×100% (1)
where ρmin = minimum bulk density, ρmax = maximum bulk density, and ρ = bulk density
of the sample. The relationship between bulk density and relative density as determined
using Equation 1 is shown in Figure 2.

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Table 1. Results of geotechnical measurements for undried and dried LHS-1
compared to values for lunar regolith. Some of the mean values for undried and
dried LHS-1 properties are presented here without uncertainties due to small
sample size.
Simulant Property
Undried
Dried
Lunar Regolith
Specific Gravity
2.35
3.11
2.90 – 3.24 (avg. 3.1)a
Minimum Density (g/cm3)
1.27
1.24
0.9 – 1.36
(A11,12,14,15)a
Bulk Density (g/cm3)
1.39
1.58
1.4 – 1.8 (Apollo 16)b
Maximum Density (g/cm3)
1.65
1.95
1.51 – 1.93
(A11,12,14,15)a
Uncompressed Relative
Density (%)
36.5
59.3
65±3 (0-15 cm)c
74±3 (0-30 cm)c
92±3 (30-60 cm)c
Cohesion (kPa)
0.311±0.014
0.299±0.018
0.25 – 0.60b
Angle of Internal Friction (°)
31.49±1.82
31.67±2.36
46.5 – 50b
a data from Carrier et al. (1991)
b Apollo 16 data from Mitchell et al. (1972)
c typical average values for lunar intercrater areas from Carrier et al. (1991)
Figure 2. Bulk density versus relative density for dried LHS-1 simulant.
Shear Strength
Shear strength, which includes its Mohr-Coulomb parameters of cohesion and
angle of internal friction, is frequently used to characterize the strength of geologic
materials. To measure the shear strength and estimate cohesion and angle of internal

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friction for undried and dried LHS-1, a procedure that conforms to ASTM D3080-11
(ASTM, 2011) was developed using custom, in-house direct shear testing equipment.
First, a known mass of simulant is lightly compacted in a direct shear box of known
volume and a variable mass is placed on top of the simulant sample to deliver the
normal load for the direct shear test (0.098, 0.193, 0.288, 0.383, 0.478, 0.573, and 0.668
kPa). Once the direct shear box is filled with simulant and the desired normal load is
applied, the box is displaced parallel to the plane of failure by an HP-500 force gauge
that is mounted to a microcontroller-driven linear actuator. The maximum force
observed is recorded for five replicate samples at each normal load for a total of 35
tests for both undried and dried LHS-1. Direct shear data is analyzed via linear
regression, per the Mohr-Coulomb assumption of linear proportionality between shear
stress at failure and applied normal stress. This regression yields cohesion as the
vertical (shear stress) axis intercept and the angle of internal friction as the arctangent
of the slope of the best-fit line, as well as their respective 1-sigma uncertainties (Figures
3 and 4; Table 1).
Direct shear testing and subsequent linear regression analysis of undried and dried
LHS-1 direct shear data gives cohesion values and 1-sigma uncertainties of 0.311 ±
0.014 kPa and 0.299 ± 0.018 kPa, respectively. Resulting angle of internal friction
estimates were 31.49° ± 1.82° and 31.67° ± 2.36° for undried and dried LHS-1,
respectively. Plots of direct shear results for undried and dried LHS-1 are given in
Figures 3 and 4, respectively.
Figure 3. Direct shear data and best linear fit for undried LHS-1 simulant. Light
dotted line shows 1-sigma uncertainties. The red line on the vertical axis
indicates the range of cohesion values for lunar highlands from Mitchell et al.
(1972).

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Figure 4. Direct shear data and best linear fit for dried LHS-1 simulant. Light
dotted line shows 1-sigma uncertainties. The red line on the vertical axis
indicates the range of cohesion values for lunar highlands from Mitchell et al.
(1972).
Discussion
The specific gravity of geologic materials is inversely proportional to the volatile
content, and thus has potential to serve as an indicator of the presence or absence of
volatile-enriched lunar regolith at a given location on the Moon’s surface. The specific
gravity of the dried simulant is higher than that of the undried simulant, which
conforms to the expected relationship. The minimum densities of undried and dried
LHS-1 are similar in magnitude, with the dried simulant being 0.03 g/cm3 less than that
of the undried simulant. The similarity of these minimum densities could be driven by
particle geometry (particle size and shape). For example, the relatively small forces
that the simulant experiences during gentle inversion may not exceed cohesive forces
(which are dominated by particle geometry), and therefore are less sensitive to effects
introduced with varying adsorbed moisture content. The small difference between
these minimum densities is attributed insufficient resolution of the volume
measurements. The uncompressed bulk density and maximum densities of the undried
and dried simulant show that the dried simulant takes on more dense grain packing
arrangements. These increased packing densities are attributed to a reduction of
cohesion in the dried simulant allowing grains to settle into more dense arrangements.
Conversely, uncompressed and maximum densities of undried simulant are lower due
to adsorbed moisture increasing grain-to-grain cohesive forces, making it more
difficult for grains to slide past one another into more dense arrangements. Carrier et

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al. (1991) propose a relationship between lunar regolith density and depth below the
lunar surface based on data collected during the Apollo missions, and this relationship
suggests an asymptotic approach to a maximum density of 1.92 g/cm3. This suggested
maximum density is 0.03 g/cm3 less than the measured maximum density of oven-dried
LHS-1, a small difference attributed to insufficient resolution of volume
measurements.
The estimated value of cohesion for undried LHS-1 is higher than that of dried
LHS-1, but the respective 1-sigma confidence intervals for these estimates overlap, and
the angle of internal friction estimates are similar with confidence intervals that also
overlap. Differences in shear strength are attributed to the variable amounts of adsorbed
moisture in the undried and dried simulant samples, as mineralogy and particle
geometry are invariable in these experiments. The similarity of the angle of internal
friction measured for undried and dried LHS-1 indicates that the angle of internal
friction is relatively insensitive to adsorbed moisture content. However, the angle of
internal friction derived for LHS-1 is lower than the range measured for highland lunar
regolith (Table 1). This difference could be caused by differences in the particle size
distributions in the larger grain sizes (>60 µm; Figure 1), particle shapes, or the highly
irregular agglutinate particles found in lunar regolith that are not contained in general
purpose LHS-1 simulant (cf., Exolith Lab LHS-1-25A Lunar Highlands Agglutinated
Simulant). All of these factors could contribute to the orientation and magnitude of
stresses within the simulant during shearing, which directly manifest as the angle of
internal friction. Work is ongoing to characterize the shear strength of LHS-1 with
varying amounts simulated agglutinates included. The direct shear test results provided
here indicate that cohesion could be a candidate indicator of the presence of volatile-
laden regolith on the lunar surface, but further study is needed to quantify these
relationships. However, it should be noted that the adsorbed water content in lunar
regolith is likely to only range from 10 to 1000 ppm (Clark, 2009), a much lower
magnitude than in ambient terrestrial conditions, but there is a possibility of high
concentrations of ice in the regolith in PSRs. This means that locality must be taken
into account when using physical properties to infer the state and composition of the
regolith.
The particle size distribution of LHS-1 matches that of lunar highlands samples
particularly well at and below the mean particle size of 60 µm (Figure 1). The more
notable discrepancy between Apollo samples and LHS-1 above this mean value of
particle size does not appear to affect the geotechnical fidelity of LHS-1 relative to
Apollo highlands samples, as the only property that is markedly different from Apollo
regolith samples is the angle of internal friction. The specific gravity of dried LHS-1
matches the values for Apollo 16 (highlands) regolith samples (Carrier et al., 1991),
and since density (and therefore specific gravity) varies with composition, this result
validates the high mineralogical fidelity of LHS-1. The noted discrepancy between the
specific gravity of undried LHS-1 and intercrater lunar regolith is expected, as the
intercrater highlands regions sampled in the Apollo missions are lacking in adsorbed
volatiles that would alter the specific gravity of LHS-1, which was created and stored
in the high humidity of the ambient conditions of Florida. Though different from each
other, the minimum, uncompressed bulk, maximum, and relative densities of undried
and dried LHS-1 are in general agreement with the range of respective values for lunar

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regolith (Carrier et al., 1991). The relative densities of undried and dried LHS-1 are
lower than that of Apollo 16 samples (Mitchell et al., 1972), reflecting a tendency for
lunar regolith to achieve densities nearer to the maximum density. The estimated
cohesion of undried and dried LHS-1 is within bounds of the cohesion range suggested
by Mitchell et al. (1972). The differences in angle of internal friction for both undried
and dried LHS-1 as compared to Apollo regolith samples (Table 1) may be a function
of differences in agglutinate content, particle sizes (Figure 1), and particle morphology,
but values are consistent regardless of adsorbed moisture content.
The various similarities and differences regarding the geotechnical properties of
lunar regolith and lunar regolith simulants directly affect terrestrial studies that aim to
characterize or predict lunar regolith mechanical behavior. Such discrepancies must be
minimized and taken into consideration for development of lunar infrastructure
development, ISRU and construction activities, and vehicle mobility studies to
decrease risk associated with achieving a sustained human presence on the Moon
(National Space Council, 2020). This study shows that adsorbed moisture alters the
physical properties of the high-fidelity lunar highlands simulant, LHS-1, demonstrating
that the geotechnical properties of lunar regolith are potential indicators of volatile
content which can be used to map resources, travel routes, and suitable locations to
develop infrastructure on the lunar surface.
Conclusions
Laboratory experiments were conducted to measure the geotechnical properties of
the Exolith Lab LHS-1 lunar highlands simulant. These measurements establish
baseline values for use in engineering studies that aim to reproduce the properties of
lunar highlands regolith. Knowledge of the geotechnical properties of a high-fidelity
lunar regolith standard reference material are essential to plans for future lunar
infrastructure development, ISRU and construction activities, and vehicle mobility at
lunar highlands sites, and are especially critical in light of NASA’s current plan to
establish lunar infrastructure at the Moon’s south pole. The particle size distribution of
LHS-1 simulant is comparable to Apollo 16 highlands regolith for particle sizes less
than 60 µm, though there is an observed deficit of particles greater than 60 µm relative
to Apollo highland samples. The average specific gravity of LHS-1 was found to be
2.35 and 3.11 for undried and dried, respectively, with the dry value providing good
agreement with the recommended value of 3.1 for general scientific and engineering
analyses of lunar regolith (Carrier et al., 1991). Minimum densities were found to be
essentially identical for undried and dried (1.27 and 1.24 g/cm3, respectively), while
values of uncompressed bulk, maximum, and relative densities for undried LHS-1 are
less than those of dried LHS-1 (Table 1). These differences are attributed to adsorbed
moisture content of the undried simulant. Cohesion for undried and dried LHS-1 was
found to be 0.311±0.014 kPa and 0.299±0.018 kPa, respectively, while angle of
internal friction for undried and dried simulant was found to be 31.49±1.82° and
31.67±2.36°, respectively. Angle of internal friction values are significantly less than
those found for lunar regolith and are likely due to differences in particle size
distributions and particle shapes (i.e., the presence of highly irregular agglutinate
particles typically found in lunar regolith). Based on the results of the measurements

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described here, it is shown that the geotechnical properties of LHS-1 are consistent
with those measured in situ at the Apollo 16 highlands landing site and provide a
suitable analog for Earth-based engineering studies to understand the regolith
properties at the south pole of the Moon.
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