Lunar Regolith

What do we know about engineering properties of lunar regolith?

Nearly the entire moon surface is covered with regolith, a layer of loose, heterogeneous material, composed of mostly dust and rock fragments. Note that the term, lunar regolith, is often interchangeably used with lunar soil. However, the term, regolith, is more pertinent than the term, lunar soil, since the soil term is used generally for earth material that has organic content, whereas the moon has none. The lunar regolith is a complex mixture of five basic ingredients: crystalline rock fragments, mineral fragments, breccias, agglutinates, and glasses. The relative proportion of each type varies from one place to another and is dependent on the mineralogy of the source rocks and the geologic processes that the rock has undergone (Heiken et al. 1991).

The exploration program of the lunar surface began with a variety of information collection methods including visual, photographs, thermal, radio and radar measurements. The first estimates of the lunar surface properties were obtained from the landing of unmanned US Surveyor spacecraft series (Costes et al. 1970). Lunar regolith (soil) samples have been returned by US Apollo manned missions and by the Soviet unmanned Luna flights. Apollo 11 lunar landing provided the first opportunity for collection of data related to physical properties and strength characteristics of the lunar regolith (Costes et al. 1970). The six Apollo landing missions provided a huge amount of scientific data in form of astronaut observations, photographs, samples, and in situ measurements. Apollo missions brought back to earth about 381.7 kg of rock and regolith samples from six different landing sites (Carrier et al. 1973).

Due to the value of the collected lunar rocks/ regolith only small quantities of the lunar regolith was made available to a few researchers (2-3 g/ researcher) which eliminated the opportunity of determining its geotechnical properties using conventional methods that require relatively large quantities of the regolith. As a result, a few simulants were developed over the years to closely match the composition, mineralogy, particle size, cohesion and friction of the lunar regolith. Johnson Space Center Number One (JSC-1) lunar soil simulant was developed and characterized under the auspices of National Aeronautics and Space Administration (NASA), Johnson Space Center (McKay et al. 1994). It has been widely used by researchers for many applications (e.g., Willam et al. 1995; Perkins and Madson 1996; Klosky et al. 2000). NASA awarded a contract to Orbitec company to produce JSC-1A with the objective to match the composition and particle size distribution of the original JSC-1 simulant.

Why do we need to learn the engineering behavior of Lunar regolith?

Future exploration missions to the moon will involve establishing large habitats, processing plants, and vehicle landing/ launch pads that require massive regolith excavation and hauling activities. For example, lunar regolith can be used as a shield against radiation to protect crew living quarters. The gravitational acceleration on the moon surface is 1.63 m/s² (g/6). For example, assuming an in situ mass density of 1.96 g/cm³ (D_r = 0.95) for the lunar regolith at a depth of 3 m. Then the vertical stress will be only 9.58 kPa and the lateral confining stress (σ₃) is approximately 3.5 kPa. The predicted Φ_p and ψ according to Alshibli and Hasan (2009) are 61.7° and 40.9°, respectively. On the other hand, at a depth of 0.5 m and assuming a very loose packing with a bulk density of 1.57 g/cm³ (Dr = 0.05), σ₃ will be only 0.47 kPa and the predicted Φ_p and ψ are 40.1° and 0.8°, respectively. Mitchell et al. (1972) reported that in situ density can rapidly increase to as high as 1.96 g/cm³ at a depth of only 0.5 m which gives Φ_p and ψ of 61.6° and 46°, respectively. The high Φ_p will impact many construction activities. Space construction equipment such as excavators and bulldozers will likely have lightweight to reduce the cost of launch and delivery to the moon surface. The weight of such equipment will further drop on the moon surface due to the reduced gravity environment, which will result in less traction between the equipment wheels/ track and the regolith. As a result, a bulldozer or an excavator will be less efficient as it operates on the moon surface compared to terrestrial environment. Furthermore, the regolith will exhibit higher Φ_p and ψ on the moon surface due to the reduced gravity environment which will further reduce the efficiency of the construction equipment. Also, the unusually high ψ cannot be ignored since the process of moving regolith by a bulldozer plate will result in a substantially higher volume at relatively loose density. Unfortunately, in situ strength measurements of the lunar regolith relied on limited cone penetration measurements and analyses of images of astronaut footprints, interaction of Apollo footpad with lunar surface, etc. Such estimates do not provide accurate measurements of shear strength and deformation characteristics of lunar regolith. This paper shows that the peak friction angle can vary over a wide range depending on regolith density and stress conditions. The unusually high dilatancy angle should be incorporated in any model used to better understand the constitutive behavior of lunar regolith.

Physical properties and morphology of JSC-1A JSC-1A approximates a low-titanium mare regolith and contains a high percentage of glass. It is mined from a volcanic ash deposit in a commercial cinder quarry (the same site as JSC-1) located in the San Francisco volcano field near the Merriam Crater outside of Flagstaff, Arizona (Orbitec, 2007). According to Orbitec (2007), JSC-1A contains major crystalline silicate phases of plagioclase, pyroxene and olivine, with minor oxide phases of Ilmenite and Chromite, and traces of clay. We measured the specific gravity of solids (G_s) of JSC-1A using water pycnometer (ASTM-D854) and found it to be 2.92. McKay et al. (1994) reported G_s value of 2.90 for JSC-1. G_s of Apollo regolith and rock fragment samples are in the range of 2.9 to 3.4 (Carrier et al., 1991, p. 482). The higher Gs values were for rock fragments. The variation of Gs is attributed to different mineralogy of samples collected from different sites on the moon surface. The maximum index density of JSC-1A was measured according to Method A of ASTM-D4253 standard and found to be 2.016 g/cm³ which yields a minimum void ratio (e_min) of 0.448 or a minimum porosity n_min of 0.309. The minimum index density was also determined using ASTM-D4254 standard procedure and found to be 1.556 g/cm³ which is equivalent to a maximum void ratio (e_max) of 0.877 or a maximum porosity n_max of 0.467. Mitchell et al. (1972) estimated the change of in situ bulk density of lunar regolith from Apollo 15 core tube samples with an average of 1.35 g/cm³ for the top 25 to 35 cm. The bulk density increased rapidly with depth from 1.35 g/cm³ to 1.85 g/cm³ at a depth of 30 to 60 cm. Mitchell et al. (1972) also reported in situ bulk densities in the range of 1.92 to 2.01 g/cm³ at the soil mechanics trench (Station 8, near the ALSEP site). Costes et al. (1970) gave upper bound estimates of the density at two sites based on penetration resistance data from Apollo 11 and 12 landing sites; they are 1.81 to 1.94 g/cm³ for Apollo 11 and 1.81 to 1.84 g/cm³ for Apollo 12. Carrier et al. (1972) reported in situ densities of 1.45 to 1.6 g/cm³ for the Apollo 14 core tube samples. Mitchell et al. (1972) reported an excellent literature review of estimates of lunar regolith densities from different missions/ estimate sources. Figure 1 shows the results of sieve analysis combined with the hydrometer analysis (ASTM-D422) of JSC-1A in comparison with the particle size distribution of some of the Apollo samples. Scanning Electron Microscopy (SEM) analysis was performed on few samples of JSC-1A (Figure 2) which reveal very angular shape with sharp corners and crevices on the surface.

Hasan and Alshibli (2010) proposed the following model to predict the peak state friction angle of the lunar regolith:

where p’_cs is the mean effective stress at the critical state in kPa. Φ_p and Φ_cs are the peak and critical state friction angles, respectively. D_r is the relative density of the material.

Related Publications:

1. Hasan, A. and Alshibli, K. A. (2010) “Discrete Element Simulation of Strength Properties of Johnson Space Center (JSC-1A) Lunar Regolith Simulant”, ASCE, Journal of Aerospace Engineering, Vol. 23, No. 3, pp. 157-165, DOI: http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000020
2. Alshibli, K. A. and Hasan, A. (2009) “Strength Properties of JSC-1A Lunar Regolith Simulant”, ASCE, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 135, No. 5, pp. 673-679, DOI: http://dx.doi.org/10.1061/(ASCE)GT.1943-5606.0000068