Regolith Simulant Research Articles

(Invited) Zirconia-Based Hollow Anode Development for Molten Regolith Electrolysis

Molten regolith electrolysis (MRE) is a promising in-situ resource utilization technique for producing oxygen and molten metal alloys from lunar regolith. However, MRE requires operating temperatures at or above 1600°C to melt lunar regolith and dissociate the molten oxides to enable electrolysis. Anode degradation in this corrosive and oxidizing environment is a major concern for long-term operation on the lunar surface and can be mitigated by a novel hollow anode design with a solid electrolyte shell and porous metallic core as proposed here. The metallic anode will be shielded from the regolith melt by an oxygen ion-conducting zirconia-based ceramic, allowing electron transfer to occur in the interior of the shell. Additionally, bubble formation at a traditional anode surface is avoided, thereby eliminating the concerns of molten regolith splatter and increased ohmic resistance. This work focuses on corrosion studies of dense stabilized zirconia in contact with molten lunar mare and highlands regolith simulants at 1600°C to support hollow anode development. The interactions between zirconia and molten regolith are characterized using SEM/EDS, with an emphasis on elemental analysis to assess reactivity and degradation of zirconia.

Soil flowing characteristics during flexible tube coring for lunar exploration: Modeling and verification

Obtaining planetary soil samples and returning them to the Earth is the most effective solution to detect the distribution and reserves of planetary resources. In order to maintain the stratification and a high coring ratio of soil samples during the sampling process, China National Space Administration adopted a flexible tube coring (FTC) method for the upcoming Chang’e-6 mission. In this paper, the lunar regolith FTC characteristics, coupled with its spiral debris discharge were modeled. Orthogonal experiments were carried out with different drilling parameters using the prepared high-density lunar regolith simulant. The results show that the coring height and coring ratio are greatly determined by the penetration per revolution (PPR) index. The lunar regolith simulant’s coring ratio was as high as 90% when the PPR was optimized. The physical and mechanical parameters of artificial lunar regolith were characterized with respect to the PPR index as well as the dynamic flowing of the internal core, with a resulting accurately tuned coring ratio prediction model. Furthermore, the drilling load analysis can be applied to the optimization of drilling parameters employed by Chang’e-6 when collecting lunar regolith.

Thermal absorptivity of a metallic surface contaminated by EAC-1A regolith simulant

The aim of this study is to estimate the impact of EAC-1A lunar regolith simulant contamination on the thermal absorption properties of a metallic surface. Masses ranging from 1.0 mg to 1016 mg of the European Astronaut Centre lunar regolith simulant 1 (EAC-1A) were placed on a chemically polished aluminium disc and the total hemispherical absorptivity (αTH) was measured at temperatures TA of the disc itself between 5 K and 100 K. Temperature TR of the thermal radiation source ranged from 20 to 280 K. Even a small amount of EAC-1A (several μg/mm2) increased the original αTH of the bare surface up to thirty times and with increasing volumes the effect was even more prominent. However, αTH significantly decreased at higher T R in case of higher simulant volumes. The results suggest that grains of EAC-1A acted as a thermal shield, preventing the incident thermal radiation to reach the substrate surface. Our findings may have a practical relevance for the thermal management of spacecrafts for exploration missions on Moon and Mars surface.

Regolith Inhibits Salt and Ice Crystallization in Mg(ClO4)2 Brine, Implying More Persistent and Potentially Habitable Brines on Mars

On Mars, liquid water may form in regolith when perchlorate salts absorb water vapor and dissolve into brine, or when ice-salt mixtures reach their melting temperature and thaw. Brines created in this way can chemically react with minerals, alter the mechanical properties of regolith, mobilize salts in the soil, and potentially create habitable environments. Although Martian brines would exist in contact with regolith, few studies have investigated how regolith alters the formation and stability of brines at Mars-relevant conditions. To fill this gap, we studied magnesium perchlorate brine in a Martian regolith simulant at salt concentrations up to 5.8 wt.%. We measured the water mass fraction and water activity between 3 and 98% relative humidity at 25 °C using the isopiestic method, and monitored salt and ice crystallization between −150 °C and 20 °C with differential scanning calorimetry. Results show that regolith inhibits salt and ice crystallization, allowing water to form and persist at much colder and drier conditions than pure brine. Remarkably, in several samples, neither salt nor ice crystallized at any conditions. These results suggest that brines could exist in regolith for longer periods of the Martian year than previously thought, and could persist indefinitely under certain conditions. By retaining water, inhibiting salt and ice crystallization, and maintaining habitable water activity, briny regolith may be a more favorable environment for life than pure brine alone. These findings indicate the critical importance of brine–regolith interactions for understanding the properties, evolution, and potential habitability of Mars’s surface.

Estimation of the mass fraction and distribution of ice in a lunar regolith simulant from seismic velocity

The exploration of ice distribution on the Moon and in other extraterrestrial environments has recently accelerated because of the growing interest in the distribution of ice in, and its acquisition from, the Moon's polar regions. Understanding of the relationship between the mass fraction of ice and the velocities of P-wave and S-wave is expected to allow ice amounts to be estimated from seismic exploration results. To clarify this relationship, we numerically investigated the effect of ice on elastic wave velocities using a digital rock physics approach for regolith with heterogeneous pore geometries. We digitized a lunar regolith simulant and modeled ice distribution in its pore spaces. Numerical simulations employing a finite element method were then performed to calculate the P-wave and S-wave velocities of the regolith simulant while varying the ice particle size and the mass fraction of ice. The results demonstrate that the characteristics of the velocity changes with increases in the mass fraction of ice differ depending on the particle size distribution of the ice. For the same mass fraction of ice, P-wave and S-wave velocities are higher for smaller ice clusters (up to 1.75 times for P-wave velocity and 1.76 times for S-wave velocity). Using the numerical simulation results, we succeeded in developing an empirical equation for estimating the mass fraction of ice from P-wave and S-wave velocities. This equation should be useful for quantifying ice content from lunar seismic survey data.

Development of waterless extra-terrestrial concrete using Martian regolith

Human colonization on Martian land is gaining significant attention in space exploration activities that demand in-situ resource utilization in the development of construction and building materials for human habitation. This research explores the utilization of Martian regolith simulant and sulfur to create extra-terrestrial concrete (ETC) with a property suitable for constructing human habitat on Mars. The primary objective of the study is to maximize the utilization of Martian regolith simulant to achieve the desired compressive strength of 25 MPa (average compressive strength specified for concrete used in residential buildings on Earth). Mechanical properties, phase transition, and microstructural characteristics of Martian regolith based-ETC under varied temperature conditions (0 °C, 40 °C, and 50 °C) on Mars were investigated. The optimal mixture proportion of ETC had 70% (by wt.) of Martian regolith and exhibited an average compressive strength of 27 MPa. The formulated ETC could retain up to 25 MPa of compressive strength at 40 ℃ and 50 ℃, and could reach up to 35 MPa at 0 ℃ temperature conditions. The change in compressive strength was attributed to the sulfur sublimation and pore closure brought about by freezing at extreme temperatures, respectively.

Performance of chemically modified reduced graphene oxide (CMrGO) in lunar electrodynamic dust shield (EDS) applications

Electrodynamic Dust Shield (EDS) technology is a dust mitigation strategy that is commonly studied for rigid applications such as photovoltaics or thermal radiators where soiling of the surfaces can reduce performance. The goal of the current work was to test the performance of a flexible, patterned nanocomposite EDS system produced through spray-coating and melt infiltration of chemically modified reduced graphene oxide (CMrGO) traces with thermoplastic high-density polyethylene (HDPE). The EDS performance was tested for a dusting of lunar regolith simulant under high vacuum conditions (∼10−6 Torr) using both 2-phase and 3-phase device configurations. Uncapped (bare) devices showed efficient dust removal at moderate voltages (1000–3000 V) for both 2-phase and 3-phase designs. Further tests carried out while illuminating the dust surface with a UV excimer lamp showed that the EDS voltage needed to reach the maximum cleanliness was reduced by almost 50% for the 2-phase devices (500 V minimum for rough and 1000 V for smooth), while the 3-phase devices were unaffected by the application of UV. However, the performance of the uncapped devices degraded after several sequential tests due to erosion of the traces caused by electric discharges and dielectric breakdown. Capping the CMrGO traces with low-density polyethylene (LDPE) eliminated breakdown of the materials and device degradation, but larger voltages (3000 V) coupled with UV illumination were required to remove the grains from the capped devices.

Tribological behavior of Al 6061 and Ti6Al4V alloys against lunar regolith simulants at extreme temperatures

Spacecraft landings on Earth's moon are highly susceptible to lunar regolith particle impingement leading to abrasion and premature failure of structural components. Herein, abrasive wear of Al 6061 and Ti6Al4V, critical aerospace alloys in two lunar simulants environments viz. JSC-1A and Greenland Anorthosite (GA) under extreme temperature conditions were examined. The highest wear loss was observed for softer Al 6061 without simulant conditions at all temperature ranges, while harder Ti6Al4V has the highest wear loss when sliding with JSC-1A. Wear loss increased under high temperature (HT) conditions due to thermal softening in alloys. The synergy of material removal and addition in three-body wear showed that wear volume quantification via the conventional method is improbable. A novel technique to quantify “true wear volume” incorporating embedding fraction (Ef) is developed. The results showed that JSC-1A regolith, owing to sharper morphology than GA, creates more embedding at RT and HT by 12% and 15% on Al 6061 and 23% and 29% on Ti6Al4V. A non-dimensional embedding index (Ei) is postulated, which can potentially serve as a tool to establish abrasion caused by lunar dust for future explorations.

Characterization of volatiles evolved during vacuum sintering of lunar regolith simulants

This work describes an experimental vacuum setup, designed for sintering of lunar regolith simulant materials, and its application to real-time characterization and identification of volatile species, resulting from in vacuo decomposition of the non-lunar components in the materials. We carried out a thorough experimental system characterization, highlighted critical aspects (temperature, pressure) with respect to regolith sintering, identified the volatile release / sample environment correlations, and postulated the need for high-vacuum capabilities in light of the high gas loads, generated by the simulants. We detailed the identification process of specific released gas species (H2O, NO, H2S, CO2, SO2, SO3, HCl, HF, etc.) using mass spectrometry, and advanced the use of them as markers for thermally activated processes, such as dehydration of hydrates, decomposition of major non-lunar phases (carbonates and sulfates) and lesser-represented components (nitrates, fluorides, chlorites, etc.). Quantification of individual decomposing phases is feasible within reason despite the inherent limitations of mass spectrometry. The methods, detailed in this paper, are intended for use in developing a better understanding of lunar regolith simulants with respect to sintering applications.

Influence of gravity on the micromechanical properties of portland cement and lunar regolith simulant composites

Concrete materials utilizing in-situ planetary resources are a promising option for construction on the Moon and Mars but the influence of variables, such as reduced gravity, needs to be understood. In this paper, we investigate the influence of gravity on the micromechanical properties of portland cement pastes that contain the lunar regolith simulant JSC-1A. Samples were prepared by astronauts onboard the International Space Station (ISS) and hydrated in microgravity (μg), lunar gravity (0.17 g), Martian gravity (0.38 g), and a statistical point at 0.7 g. Results were compared to a control sample hydrated on Earth (1.0 g). Specimens were analyzed by statistical nanoindentation and results for the paste suggest that gravity does not influence the in-situ mechanical properties of the hydrated phases. Results for the simulant give insight into the potential performance of lunar regolith as a component in a composite material. These results are critical to the continued development of lunar concrete technology.

The importance of Phobos simulants: a review on our current knowledge

Phobos, a satellite of Mars, was successfully studied by flyby, orbiter, and landing missions to the Red Planet, but several questions remain about its origin, composition, and relationship to Mars. It is suggested that Phobos is either a captured body from the asteroid belt or the outer Solar System (capture scenario), or a consequence of re-accreted ejecta from Mars (in situ formation/giant impact). So far, Phobos has been characterized by its two spectral units - blue and red - with different compositional restrains. The red unit represents most of the surface, while the blue unit is focused on the Stickney crater and surroundings. In the absence of samples returned from this satellite, simulant regolith must be studied to infer various proprieties, and complement in situ studies. To date, there are three simulants of this satellite: Phobos-1C, Phobos Captured Asteroid-1 (PCA-1), and Phobos Giant Impact-1 (PGI-1). Since Phobos may have a Mars-like composition, terrestrial analogues of Mars should also be analysed. The data retrieved from the various assays performed with these planetary field analogues may be used as a database to complement future space missions to Phobos, but, ultimately, the composition of Phobos will have to be analysed by a sample-return mission.

Additive manufacturing by laser powder bed fusion and thermal post-treatment of the lunar-regolith-based glass-ceramics for in-situ resource utilization

Powder bed fusion is one of the most promising additive manufacturing approaches for in-situ resource utilization (ISRU) in the following lunar exploitation missions. However, the harsh conditions for energy supply and equipment operation on the lunar surface and the amorphization of materials due to rapid heat-cooling cycles could hinder the efforts to produce high-quality lunar regolith-based products. Laser powder bed fusion (LPBF) with large laser spot sizes and low scan speeds were used to print the glassy lunar regolith simulant (LRS) precursors. Then the thermal post-treatment was carried on to transform the LRS precursors into glass–ceramic products. The precursors and glass–ceramic samples' microstructure, mechanical properties, and thermophysical properties were investigated. The results indicated that the acceptable range of volumetric energy density (VED) for the LPBF process of lunar regolith was from 3.5 to 4.3 J/mm3. The porous structure resulting from the large molten pools leads to low thermal conductivities and an acceptable strength compromise. The glassy precursors formed augite phase-dominated LRS glass-ceramics at 823 °C during thermal post-treatment and augite-plagioclase polycrystalline LRS glass-ceramics with better mechanical properties at 1100 °C. The increase of the augite phases precipitation contributed to the strength enhancement, and the grain coarsening caused by the prolonged crystallization process at 1100 °C would reduce the strength. The average compressive strength, fracture toughness, and Vickers hardness of the LRS glass-ceramics could reach 50.71 MPa, 1.49 MPa·m1/2, and 897.91 HV1/15, respectively. This multi-process method of laser powder bed fusion of glass-ceramics can maximize the application potential of the ubiquitous lunar regolith in future in-situ additive manufacturing to produce lunar regolith-based building materials for different stages of future lunar exploitation missions.

Additive manufacturing of high solid content lunar regolith simulant paste based on vat photopolymerization and the effect of water addition on paste retention properties

Additive manufacturing of high solid content lunar regolith simulant paste based on vat photopolymerization and the effect of water addition on paste retention properties

Sampling of lunar regolith with a deep fluted auger: Experimental study on sampling performance

Currently, sampling and water detection of lunar regolith in the permanently shadowed regions (PSRs) of lunar polar regions is a hotspot in lunar exploration. Using deep fluted augers (DFAs) for lunar regolith sampling was proposed long ago. However, a comprehensive study on how to effectively capture regolith using a DFA has not been reported so far. In this study, based on the principles of regolith capturing with a DFA (RCDFA), we analyzed the factors possibly affecting the mass of the captured sample in terms of lunar soil conditions, drilling operating parameters, and structural design. Using the CUG-1A lunar regolith simulant (LRS), an experimental study was conducted on the influence of drilling operating parameters and the structure of the auger flutes on the mass of the captured sample during sampling operation. The experimental results revealed that the regolith sample might not be captured at all with unreasonable operating parameters; however, when using operating parameters within the feasible range, the mass of the captured sample was positively correlated with the cut per revolution (CPR) at the sampling and retracting stages and with the length and depth at the sampling stage. Among all factors, the CPR at the sampling stage had the most significant impact on the mass of the captured sample. Moreover, the width-to-depth ratio (WDR) of the auger flutes should be neither too small nor too large to obtain a good trade-off between the mass of the captured sample and the stability of the brushing efficiency. This study lays the foundation for further research on the mechanism of how DFAs capture lunar regolith and the optimal design of drilling tools.

3D shape analysis of lunar regolith simulants

Lunar regolith simulants are instrumental in the demonstration and validation of several space technologies, and the geometry properties are essential in understanding their mechanical and geotechnical behaviors. Although some geometric parameters (diameters, sizes, volumes, etc.) are measured, the morphology of regolith particles has not been well studied in the past decades. The main purpose of this research is thus to get insights into the particle shape and analyze their geometric properties in 3D space. In this work, micro X-ray computed tomography (micro-CT)-based reconstruction is used to extract slices of simulant samples (LHS-1). Then, image processing-based segmentation methods are applied to the slices to reconstruct individual particles. Finally, 3D spherical harmonic shape descriptors are used to analyze the surface geometry of particles and their statistics. This study proposes a general pipeline for the reconstruction, modeling, and shape analysis of simulant samples, which will be instrumental for future efficient discrete element simulation.

Preparation and characterization of elevated and cryogenic temperature-resistant regolith-based epoxy resin composites

The construction of lunar bases, with their crucial role in future deep space exploration and extraterrestrial resource harvesting, requires the integration of multiple manufacturing techniques. Selecting appropriate solutions to meet the feature requirements is of particular importance. Extreme environments on lunar surfaces pose uphill challenges for construction, and transportation between Earth-to-Moon involves costs and risks. In-situ utilization of lunar regolith for fabrication has been proven feasible, among which polymer forming possesses promising prospects due to its sample manufacturing process, excellent mechanical properties, and low energy consumption. To investigate the feasibility of regolith-based polymer composites, specimens with varying proportions were prepared based on the Taguchi method. The lunar regolith simulants (LRS) and resulting polymer were characterized by laser particle analyzer and X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and scanning electron microscopy (SEM). Their elevated (180°C) and cryogenic (-196°C) temperature resistance properties were investigated by compressive and flexural strength tests after shifting freeze–thaw cycle tests. The results revealed that curing temperatures and mass ratio of curing agents to the hybrid epoxy matrix significantly affect the compressive strength and flexural strength, respectively. The maximum 24 h compressive and flexural strength of regolith-based polymer composites was obtained as 156.22 MPa and 103.40 MPa, respectively. And remained stable over 160 MPa and 70 MPa after ten freeze–thaw cycles. Microstructural results imply variations in the curing agent’s and aggregate’s ratio, which tentatively explain the reaction mechanism of regolith-based polymer composites. This study demonstrates the feasibility of preparing polymer utilizing LRS that accommodates immense temperature fluctuation on the Moon, with undemanding fabrication, excellent mechanical properties, and low energy consumption, providing potential applications for future lunar construction, especially for rapid repair and reinforcement of launch pads and runways.

A laboratory study of the phase ratio imagery method

As a new planetary remote sensing tool, the phase ratio imagery calculates the ratio of images taken at different phase angles and may suppress surface albedo variations and enhance surface texture features. This technique has been used in the study of surface structure of airless bodies such as the Moon and Mercury. To understand the effectiveness of the method, we carried out laboratory phase ratio measurements on eight planetary analog materials including four pure minerals olivine, orthopyroxene, labradorite, ilmenite and four mixtures, the lunar regolith simulant JSC-1A, the lunar highland simulant, the Martian soil simulant, and the CI asteroid simulant, all in two size distributions, 0–45 μm and 90–105 μm. For each sample, the phase ratio A(α1)/A(α2) is obtained by measuring the reflectance at two phase angles α1 and α2 with α1<α2 at two radiation wavelengths, 633 nm and 905 nm. The results show that: (1) The particle size distributions can be differentiated by measuring the phase ratio A(α1)/A(α2), and in order to increase the discriminative power of the particle size distribution, the value of (α1-α2) should be as large as possible. (2) For pure minerals, larger grains have smaller phase ratio values, because larger grains of pure minerals are more forward scattering, leading to larger A(α2) and thus smaller phase ratio. For mixtures with simulated agglutinates that hold minerals together as composite particles, larger grains have higher phase ratios because they are less forward scattering due to multiple internal reflections and hence more absorptions. Since real planetary regoliths are likely dominated by composite particles with agglutinates, it is expected that larger grains would have larger phase ratio values.

Improving the Quality of Lunar Regolith Simulant Soil for Future Food Security

This research explores the possibilities of using lunar soil resources in agriculture. The effectiveness of humic micro-encapsulated was investigated for physical and chemical quality improvement for lunar culture applications by using TLS-01 (Thailand artificial lunar regolith Simulant-01) as a test substance. Sunflower (Helianthus annuus L.) was used as the test plant. TLS-01 particles were evenly distributed, uncluttered, and consisted of 54.55% polygons and 45.45% rods, without having high water-holding capacity. The plants were grown in TLS-01, but their physical characteristics were unacceptable. Sunflower seedlings grown in TLS-01 soil showed a lower percentage of germination, and root and stem length compared to those grown in commercial planting material. After an improvement in physical properties and nutrients, from the study of the relationship between the soil used in the trial for planting and the humic acid concentration, no interaction between soil and humic acid was found. Therefore, while the soil types were significantly different, the humic was not. The results showed that a 1:1 mixture of TLS-01: coconut coir with the addition of 2 times the recommended dose of humic acid could enhance sunflower seedling relative to commercial planting material (p≤0.05). The germination percentage and growth indexes were 85.00% and 6.62, respectively. However, in order to stimulate actual usage for interplanetary application, coconut fiber-like materials and the humic substances were applied as in microcapsules. Additionally, the humic release test showed that after 6 h, the release of humic substance by Fickian diffusion was 73.45% and it then slowed down. The encapsulation efficiency was 90.37%.

In-situ visualization of powder wrapping behavior in millimeter-scale-beam lunar regolith powder bed fusion

Future lunar base additive manufacturing is accelerated by ground-based technology. Naturally, loosely packed lunar regolith must be directly treated into a solid surface on the first layer of lunar base construction. Solar powder bed fusion (Solar PBF), with huge spot sizes and low energy densities, is the most efficient energy and material usage method on the moon. However, the powder bed's low heat conductivity hinders energy penetration. This investigation used a millimeter-scale laser beam to recreate the solar PBF spot. The beam was directed at the powder bed without substrate. In-situ observation revealed a wrapping mechanism works for multiple parameters. The wrapping process was primarily observed with lunar regolith simulant CUG-1A, and SS316L was used for comparison under a similar spot and no substrate. The periodic wrapping helps to form deeper melt tracks. The wrapping process has the potential to produce a substrate on a loose powder bed directly.

Outgassing behavior and heat treatment optimization of JSC-1A lunar regolith simulant

As NASA strives towards a long duration presence on the Moon, it has become increasingly important to learn how to better utilize resources from the lunar surface for everything from habitats, vehicle infrastructure, and chemical extraction. To that end, a variety of lunar simulants have been sourced from terrestrially available volcanic minerals and glass as Apollo regolith is unavailable for experimentation needing large masses. However, while mineralogy and chemical composition can approach that of lunar material in these simulants, there are still distinct non-lunar phases such as hydrates, carbonates, sulfates, and clays that can cause simulants to behave distinctly non-lunar in a variety of processing conditions that maybe applied in-situ to lunar material. Notably, severe glassy bubbling has been documented in a variety of vacuum sintering experiments on JSC-1A lunar mare simulant heated via microwaves. The origins of this outgassing have not been well understood but are normally attributed to the decomposition of non-lunar contaminates intrinsic to virtually all terrestrially sourced simulants. As such, a series of controlled environmental tests were performed to ascertain the origins of the high temperature outgassing and to develop heat treatments that can drive JSC-1A closer to lunar composition and behavior. It was found that in JSC-1A at elevated temperatures distinct gas evolutions of water, carbon dioxide, and sulfur dioxide occur in both inert gas and vacuum. Additionally, the presence of hydrogen during heat treatments was shown to dramatically change gas evolutions, leading to distinctly more lunar-like composition and behavior from JSC-1A simulant.