Lunar Research Research Articles

SPACE RESOURCES: A NEW BALANCE OF INTERESTS FOR THE 21ST CENTURY

Purpose: the Art. II of the 1967 Outer Space Treaty (OST), which prohibits national appropriation by claim of sovereignty, use or occupation, is a jus cogens norm and has become a guarantee of peace in space, as well as the basis of international space law. In opposition, the creation of national legislations that allow the appropriation of space resources to their citizens, the advance of the private sector in space activities all over the world, the initiative to create the International Lunar Research Station (ILRS), and The Artemis Accords, put the interpretation of this fundamental principle under debate. In the 21st century, the proliferation of new space missions of public-private nature has created feasible commercial opportunities, such as space mining. All of this has deepened the debate within the United Nations Committee on the Peaceful Uses of Outer Space (UNCOPUOS) on the need (or not) for elaborate new legal definitions and a legal regime for space resources. For this, it’s necessary to generate a unified interpretation by the States based on the "balance of interests" principle. Research methods: study of specialized bibliography and national and international legislations, observation methods and analysis of information related to space activities. Results: in this article it will be evaluated whether it’s possible to establish a new global legal interpretation of Art. II of the OST of 1967 to allow the harmonization of private and public interests. Discussion: the Artemis Accords, the ILRS and the national legislations of States that currently allow their citizens to appropriate space resources are, in fact and in practice, the spearhead of NewSpace and their main characteristic is that they are initiatives carried out outside UNCOPUOS. What would happen if soon, in a hypothetical case, The Artemis Accords and the ILRS project were to have more than a hundred signatory States in total?

Lunar Regolith Geopolymer Concrete for In-Situ Construction of Lunar Bases: A Review.

The construction of lunar bases represents a fundamental challenge for deep space exploration, lunar research, and the exploitation of lunar resources. In-situ resource utilization (ISRU) technology constitutes a pivotal tool for constructing lunar bases. Using lunar regolith to create geopolymers as construction materials offers multiple advantages as an ISRU technique. This paper discusses the principle of geopolymer for lunar regolith, focusing on the reaction principle of geopolymer. It also analyzes the applicability of geopolymer under the effects of the lunar surface environment and the differences between the highland and mare lunar regolith. This paper summarizes the characteristics of existing lunar regolith simulants and the research on the mechanical properties of lunar regolith geopolymers using lunar regolith simulants. Highland lunar regolith samples contain approximately 36% amorphous substances, the content of silicon is approximately 28%, and the ratios of Si/Al and Si/Ca are approximately 1.5 and 2.6, respectively. They are more suitable as precursor materials for geopolymers than mare samples. The compressive strength of lunar regolith geopolymer is mainly in the range of 18~30 MPa. Sodium silicate is the most commonly utilized activator for lunar regolith geopolymers; alkalinity in the range of 7% to 10% and modulus in the range of 0.8 to 2.0 are suitable. A vacuum environment and multiple temperature cycles reduce the mechanical properties of geopolymers by 8% to 70%. Future research should be concentrated on the precision control of the lunar regolith's chemical properties and the alkali activation efficacy of geopolymers in the lunar environment.

Geologic History of the Amundsen Crater Region Near the Lunar South Pole: Basis for Future Exploration

We provide the first detailed 1:100,000 scale geomorphologic map of the ∼100 km Amundsen crater region, which is of high scientific relevance for future exploration, e.g., NASA’s VIPER mission, the Artemis program, and the Chinese International Lunar Research Station. We investigated the complex geological history of the region before and after the formation of Amundsen crater on the rims of the South Pole–Aitken (SPA) and Amundsen–Ganswindt basins. We present a new Amundsen crater formation age of ∼4.04 Ga, which, in contrast to previously derived ages, is based on non-light-plains terrain. The estimated maximum excavation depth for Amundsen crater is ∼8 km, and elevated concentrations of FeO near the crater suggest that Amundsen may have redistributed SPA-derived materials. Plains materials of various kinds were observed both inside and outside Amundsen crater and are estimated to be up to 350 m thick and ∼3.8 Ga old. A less cratered, tens of meters thick mantling unit indicates a resurfacing event ∼3.7 Ga ago. We highlight five potential exploration sites that satisfy technical constraints (such as shallow slopes, solar illumination, and Earth visibility), provide materials that can be sampled, and are capable of addressing multiple science objectives. Due to its accessibility and traversability, combined with its geologic diversity, proximity of permanently shadowed regions for studying volatile processes, and ability to address multiple science objectives, we confirm and reinforce the Amundsen crater region as a high-priority landing and exploration site.

Topical Trends in Outer Space Cooperation/Competition and Perspectives on Space Law

Introduction. Immanent in all history of space activities cooperation and/or competition are currently subject to significant changes. The most meaningful by impact on activity, its forms, subject content and other features, as well as on relevant international legal and national legal framework including perspectives on their development, are the two emerging international partnerships: China and Russia, primarily on the International Lunar Research Station, as well as with other cooperating states; and the United States with partners on the Artemis Program.Materials and methods. Materials – selected by criterion of fundamental impact on the ‘space law’ development examples of international legal and national legal acts underpinning one of the alternative competing international partnerships on the exploration of the Moon and deep space. Methodology includes analysis, synthesis, analogy, as well as combination of comparative law with systematic methods, and modelling with forecasting.Results. It is justified that: 1) it is reasonable to consider the combination of relevant current factors, trends, and processes (including political, economic, and technological) as the components of the branch-specific coordinate system; 2) timely identification of factors with significant impact on the industry development significantly reduces, or absolutely offsets the negative impact of transformational potential of cooperation/competition in outer space on the efficiency of the industry-related forecasting; 3) factors of fundamental impact on the development of the “space law” system are found in the international legal and national legal approaches behind the two alternative international partnerships; 4) the necessity to determine common basis for diverging legal (and other branch-specific) approaches is grounded on economic feasibility; 5) The conditions/criteria of determination of factors of fundamental impact on the system of ‘space law’ are developed; 6) The feasibility of consideration of the combination of challenging issues of space law as the components of the branch specific legal coordinate system is confirmed; 7) It is revealed that meeting priority issues leads to an updated actualization of other unsettled issues of diverse-level within the system of ‘space law’; It is justified that: 8) under actual trends actualization of branch-specific legal issues and their regulation is less dependent on relevant state’s technological potential and more on inclusion in global economic projects and political processes; 9) consideration of matches and differences under specific criteria between international legal and national legal approaches behind the two alternative international partnerships would bring forecasting on perspectives on ‘space law’ closer to real development.Discussion and conclusion. It is reasonable to consider as conceptual foundations for branch-specific risk management the creation of conditions (primarily of economic, technological and strategic character): for transformation of ‘common denominator’ of international legal and national legal approaches of the two international partnerships to the willingness of competing states to harmonize their positions on additional multilateral treaty regulation of issues subject to international space law; as well as for that competition leads to ‘balancing’ of commitment to international obligations, and inclusive development of diverse elements of the ‘space law’ system.

Higher water content observed in smaller size fraction of Chang'e-5 lunar regolith samples

Water has been detected in lunar regolith, with multiple sources identified through the analysis of individual grains. However, the primary origin of water in the bulk lunar regolith remains uncertain. This study presents spectroscopic analyses of water content in sealed Chang'e-5 samples. These samples were sieved into various size fractions (bulk, <45 μm, and 45–355 μm) inside a glovebox filled with high-purity nitrogen. Results indicate a higher water content in the fine fractions (∼87 ± 11.9 ppm) than in bulk soil (∼37 ± 4.8 ppm) and coarse fractions (∼11 ± 1.5 ppm). This suggests that water is predominantly concentrated in the outermost rims of the regolith grains, and thus exhibits dependence on the surface volume ratio (also known as surface correlation), indicating solar wind is a primary source of lunar surface water. Laboratory, in-situ, and orbital results bridge sample analysis and remote sensing, offering a cohesive understanding of lunar surface water characteristics as represented by Chang'e-5. The discovery provides statistical evidence for the origin of water in lunar soil and can be considered representative of the lunar surface conditions. The water enrichment of the finest fraction suggests the feasibility of employing size sorting of lunar soils as a potential technological approach for water resource extraction in future lunar research stations.

Lunar Rover Collaborated Path Planning with Artificial Potential Field-Based Heuristic on Deep Reinforcement Learning

The International Lunar Research Station, to be established around 2030, will equip lunar rovers with robotic arms as constructors. Construction requires lunar soil and lunar rovers, for which rovers must go toward different waypoints without encountering obstacles in a limited time due to the short day, especially near the south pole. Traditional planning methods, such as uploading instructions from the ground, can hardly handle many rovers moving on the moon simultaneously with high efficiency. Therefore, we propose a new collaborative path-planning method based on deep reinforcement learning, where the heuristics are demonstrated by both the target and the obstacles in the artificial potential field. Environments have been randomly generated where small and large obstacles and different waypoints are created to collect resources, train the deep reinforcement learning agent to propose actions, and lead the rovers to move without obstacles, finish rovers’ tasks, and reach different targets. The artificial potential field created by obstacles and other rovers in every step affects the action choice of the rover. Information from the artificial potential field would be transformed into rewards in deep reinforcement learning that helps keep distance and safety. Experiments demonstrate that our method can guide rovers moving more safely without turning into nearby large obstacles or collision with other rovers as well as consuming less energy compared with the multi-agent A-Star path-planning algorithm with improved obstacle avoidance method.

Automatic extraction of non-bifurcated lunar lineaments from Lunar Reconnaissance Orbiter (LRO) monochromatic images based on a Markov chain method

ABSTRACT Lineaments play a critical role in the study of lunar geological evolution. However, lineaments still need to be extracted manually which constrains lunar research. To fill this gap, a Markov chain-based methodology is introduced for automatically extracting vector-based non-bifurcated lineaments from images. The extracted lineaments are formed by successive nodes and line segments, where nodes denote pixel positions and line segments represented connecting lines between nodes. A modified U-net with residual shortcut connections and dilated convolutions is proposed based on the elongated shapes of lineaments to evaluate the probability of nodes belonging to lineaments. Four connection shape features are proposed to describe the connection shapes of nodes and a Gaussian Mixture Model is used to evaluate the probability of line segments belonging to the lineaments based on the proposed features. In the final stage, the lineament probabilities of both nodes and line segments to extract lineaments are considered in the Markov chain. Our method has experimented on a dataset containing 220 samples and 10-fold cross-validation was used to evaluate the performance. Both qualitative and quantitative results indicated that our method can effectively extract non-bifurcated lunar lineaments with arbitrary bending. The method sheds useful light on automatic lineament extraction.

Development Idea of Optical Surveillance System of International Lunar Research Station (Invited)

Development Idea of Optical Surveillance System of International Lunar Research Station (Invited)

Multi-scale Based Approach for Crater Detection on Lunar Surface using Clustering algorithm

This paper introduced automated detection and analysis of lunar craters are crucial for advancing lunar research and facilitating mission planning. This study introduces a comprehensive multi-scale approach for crater detection on the lunar surface using LROC (Lunar Reconnaissance Orbiter Camera) Dataset, enabling the identification of craters across varying scales and enhancing overall detection accuracy and efficiency. The process involves preprocessing lunar imagery to enhance features and delineate potential crater regions, utilizing edge detection algorithms to extract crater boundaries. Clustering techniques are then applied to group similar edge points and isolate potential crater candidates. A thresholding mechanism based on statistical edge intensity analysis refines the detection process. To address multiple detections of the same crater at slightly different diameters, radius criteria are implemented, yielding high diameter accuracy of 84% for mare Imbrium (test site 1) and 80% for mare Nubium (test site 2). Subsequently, a depth-diameter analysis validates crater-like characteristics by combining depth measurements with crater diameter estimation, showcasing the authenticity of potential craters. we compare our diameter and depth with different reference papers and we get high accuracy of R2 =0.99 then other methods. The results highlight the potential of this multi-scale approach for automated lunar crater detection, providing deeper insights into the Moon's surface morphology and history for future lunar exploration missions.

Magnetically-Induced Buoyancy for Efficient Electrochemical Oxygen Production in Microgravity

The realization of long-term space travels and the establishment of a lunar research platform are challenged by the lack of technological device architectures for efficient, stable and continuous oxygen production. One obstacle is that the near-absence of buoyancy in microgravity results in hindered gas bubble desorption at the electrode-electrolyte interface, causing lower efficiencies for electrolyser systems when operating in space.1,2 The significantly reduced buoyancy force also has severe complications for phase separation, which is a crucial process for a variety of space technologies also including heat transfer, fuel production and removal of carbon dioxide from cabin air. Multiple approaches have been studied in the past to induce phase separation such as creating centrifugal force fields. These approaches lack however efficiency, stability and robustness.3,4 In our recent work, we demonstrate the use of magnetic polarisation to artificially induce buoyancy in the electrolytic oxygen evolution half-cell reaction and thus significantly increasing the overall half-cell efficiency. This was carried out in a series of experiments at the Bremen Drop Tower, Center of Space Technology and Microgravity (ZARM, Germany), where short-term microgravity conditions of 9.2 s were achieved.5,6. Chronoamperometric and cyclic voltammetry measurements on high-activity RuOx electrocatalyst structures on a Ti substrate were undertaken with and without a magnet in close proximity to the electrode to closely monitor the electrochemical performance in a three-electrode potentiostatic arrangement and to demonstrate the magnetic effect on gas bubble desorption during free fall. It was observed that current densities for the electrodes at a potential of +1.49 V vs RHE were significantly higher in the presence of a magnet. We conclude that magnetic phase separation can be utilised in electrolytic systems in reduced gravitational environments to efficiently detach oxygen bubbles from the electrode surface and operate the devices at higher efficiencies, providing a key advancement for future oxygen production in space missions.2,3,5 References Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. (2018). Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett., 3, 1795−1800.Saravanabavan, S.; Coulthard, C. T.; Kaur, M.; Brinkert, K. (2022). Catalysis in Space Environments. doi.org/10.1002/9783527830909.ch7 In: Hessel V., Stoudemire J., H. Miyamoto & I. D. Fisk (Eds.). In Space Manufacturing and Resources.Wiley-VCH. ISBN: 978-3-527-83091-6Romero-Calvo, Á.; Akay, Ö.; Schaub, H.; Brinkert, K. (2022). Magnetic phase separation in microgravity. npj Microgravity 8 (32). doi.org/10.1038/s41526-022-00212-9.Harvey, A.; Ramshaw, C.; Reay, D. A. Process Intensification: Engineering for Efficiency. Sustainability and Flexibility. 2nd edition. Butterworth- Heinemann; 201Brinkert, K.; Richter, M. H.; Akay, Ö.; Liedtke, J.; Giersig, M.; Fountaine, K. T.; Lewerenz, H. J. (2018). Efficient solar hydrogen production in microgravity environment. Commun. 9(2527). doi: 10.1038/s41467-018-04844-yhttps://www.zarm.uni-bremen.de/fileadmin/user_upload/drop_tower/Users_Manual_0412.pdf

From Antarctic prototype to ground test demonstrator for a lunar greenhouse

The Moon has returned into the focus of human endeavors regarding human spaceflight, e.g., with NASA's Artemis program, ESA's Moon Village, and the Russian/Chinese International Lunar Research Station. In difference to the pathfinding missions of the Apollo-era, the goal for these future missions is to stay on the lunar surface for longer durations and inhabit the lunar environment (near-)permanently. This requires a different approach to be affordable, i.e., instead of resupply as mostly used on e.g., the International Space Station, resource management has to include recycling and in-situ utilization. The former especially calls for the application of so-called BLSS to allow providing essential life-support services to the crew without prohibitive resource consumption, which is economically not feasible to achieve with resupplies. Bio-regenerative-life-support systems have been researched for decades, yet the system complexity, technology advancements, and singular aspects as e.g., plant biology require more research, especially if combined as in a greenhouse. For instance, the understanding of how a microbiome develops in a closed environment and what implications the microbiome has on plant growth is still insufficient. Within the EDEN project, the German Aerospace Center built a lunar analogue greenhouse and operated it at the Neumayer-III research station in Antarctica for four years, testing the technology – which was not space hardware – and operations. Derived from this experience the next step in the project is to design and subsequently operate a ground test demonstrator for a lunar greenhouse, as close as possible to the actual space hardware and operations. This paper explains the current design and trade-offs that led to it. Furthermore, the concept of operations is shown to illustrate the demonstrator's utility for researching bioregenerative-life-support. Overall, the system presented is feasible and useful to close the gaps, currently still existing in this field of research, and thus a mission enabler for future long-duration human space exploration missions.

Space resource activities and the evolution of international space law

We are experiencing a period of renewed interest in human activities in outer space and, in particular, the return of humans to the Moon. Multiple lunar missions, including the NASA led Artemis project and the China and Russia International Lunar Research Station propose a sustained human presence on the Moon which will require in-situ resource extraction and usage. This article explores the likely impact of statements and actions by states on the future evolution of international space law in relation to space resource activities, highlighting the urgent need for greater clarity in the law regarding space resource activities. It will examine the relevant international law, domestic legislation, multilateral agreements, and recent international discussions concerning space resource activities. It concludes that the current international space law regime lacks clear laws regarding space resource activities and emphasises the importance of space law evolving in order to progress with industry and exploration.

China’s Bifurcated Space Diplomacy and Institutional Density

Abstract This article compares two central pillars of China’s space programme and observes that Chinese space diplomacy is not uniform regarding international scientific co-operation either in its approaches or in results. In the case of the Chinese Space Station programme, the China National Space Administration went through existing United Nations (UN) channels and successfully attracted international partners. However, the International Lunar Research Station has avoided UN channels and used national and bilateral platforms. This bifurcation in approaches and results offers an intriguing puzzle concerning international co-operation: practices of institutionalised multilateral co-operation and areas of state-centric bilateral co-operation co-exist in this case and further complicate the issue of space diplomacy. To propose a potential explanation, it is argued here that a crucial intermediate variable — institutional density — requires further theorising, as it seems to influence strategic choices about space diplomacy, which may lead to success or failure.

Possible sites for a Chinese International Lunar Research Station in the Lunar South Polar Region

Possible sites for a Chinese International Lunar Research Station in the Lunar South Polar Region

Promoting international cooperation on the International Lunar Research Station: Inspiration from the ITER

Promoting international cooperation on the International Lunar Research Station: Inspiration from the ITER

The International Lunar Research Station: China's New Era of Space Cooperation and Its New Role in the Space Legal Order

Tension between competition and cooperation is common in space activities. China's role in spearheading the International Lunar Research Station, in direct opposition to the US-led Artemis program, demonstrates its strong determination to upgrade its space cooperation. The configuration discourse on the International Lunar Research Station and the Artemis program is clearing a path for a new mixture structure and the interplay of competition and cooperation among space powers. The evolution of China's approach to space cooperation––from an emphasis on national independence to initiating multilateral cooperative platforms––provides insights into understanding the dynamics between techno-nationalism and external constraints, as well as the motivation to increase and an approach towards actually increasing soft power, alongside the development of hard power. This decade-lasting dynamics determines that openness and neutrality are viable means to guarantee the international prospect of China's lunar cooperation platform. Well-designed and targeted engagement measures and overall neutral strategies are needed to realise openness and promote neutrality. To compensate for China's ‘leadership deficit’ in international cooperation, a practical way forward is taking the Artemis program as a point of reference and transplanting the successful American experience while also differentiating from the Artemis program. The interplay of competition and cooperation challenges, forces compromise in, and maintains and develops the space order. Leading an international cooperation venture will accrue the soft power China needs to sustain its hard power and bring new positive energy to the space order. China's unique dual identity as a space power and a developing country could contribute to solving the dilemma of democratisation and decentralisation in the development of international space law.

Application of topography analysis based on multi-source data in the construction of the International Lunar Research Station

Application of topography analysis based on multi-source data in the construction of the International Lunar Research Station

Alternative futures for crewed space cooperation after the international space station

The International Space Station (ISS) has been the largest and most visible international space cooperation project for decades. The program began in the 1980s during the Cold War, incorporated Russia after the end of the Cold War, and has sustained permanent human occupancy in low Earth orbit for over 20 years. Although political support among the partners appears solid, it is uncertain how many more years the Station will be able to operate technically. The operating costs of the Station for the United States and international partners are considerable and have been increasing as the facility ages. These costs impose a constraint on the willingness and ability of international partners to participate in new human exploration efforts, such as Artemis, the European Space Agency (ESA) Lunar Village concept, and the International Lunar Research Station (ILRS) plans of China and Russia. Given the central importance of the ISS to international human spaceflight cooperation today, what are the possible alternative futures for crewed space missions in a post-ISS world? States and private sector actors could work to extend the ISS, participate in the Chinese Space Station, create multiple, smaller government and private human-tended platforms, focus on human missions to the Moon and Mars, or even cease participation in human space exploration altogether. This paper examines these alternatives futures, assesses the attractiveness and practicality of the alternatives to spacefaring states, and seeks to identify the key areas of technical, economic, and policy uncertainty that will shape the post-ISS era of human space exploration.

Frontier scientific questions in deep space exploration

<p indent="0mm">Deep space exploration is one of the frontiers of scientific research and driven by human instincts to explore the unknown. Since the Moon Race in the 1960s and 1970s, deep-space probes have explored many celestial bodies in our solar system, including eight planets. These explorations have yielded tremendous scientific discoveries, but they also revealed many more unsolved mysteries. The 21st century has witnessed a remarkable increase in deep-space exploration activities, with several new agencies launching their spacecraft in the past few decades. As a result of technological advancement, exploration success rates have improved significantly. A systematic review of frontier scientific questions can provide invaluable references for mission planning strategies and boost scientific discoveries and breakthroughs in the future. In general, these questions are related to three primary scientific subjects: (1) One of the primary scientific objectives of deep-space exploration is to study the origin and evolution of the solar system. From the collapse of the solar nebula to the formation of the planetary system, the solar system has undergone a complex evolutionary process. The components of the current solar system, particularly the residual presolar material, can provide critical information for understanding the initial conditions and evolution of the solar system. The Moon, being the nearest celestial body to Earth, serves as an outpost for exploring other planetary bodies and is especially important for testing deep-space exploration equipment and studying solar system evolution. (2) Another objective of deep space exploration is to better understand the evolution of planetary habitability. The Sun is an important factor affecting habitability, and eruptions of solar activities can have a substantial impact on the space environment of the solar system. The environment of the Earth and other planets has changed dramatically over billions of years of evolution. Although life has only been confirmed on Earth, other celestial bodies might also have had habitable conditions along the evolutionary process. Decoding the evolution of the habitable environment of solar system planetary bodies is of great significance for understanding the formation and evolution of habitable Earth and the origin of life on Earth. (3) Extraterrestrial life detection is a critical scientific topic in deep space exploration, with far-reaching implications for understanding the origin, evolution, and distribution of life in solar and extrasolar systems. The current scientific challenges in the detection of extraterrestrial life primarily involve four aspects: the origin and evolution of life, extraterrestrial habitable environments, life signal identification, and analogy of terrestrial extreme environments. In endeavors to answer these scientific questions, more well-planned deep-space exploration missions are required to collect evidence. In order to outpace opponents in the continuing space exploration endeavor, the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and other rising roles have made their ambitious roadmaps. NASA will encourage commercial companies to join future missions through the Artemis program, and has planned missions to explore Venus, asteroids, and icy bodies. The ESA proposed tens of missions in its white paper <italic>Voyage 2050</italic>. In the coming decades, China will lay the groundwork for the International Lunar Research Station through three Chang’e and four Russian Luna missions. Furthermore, Tianwen-2 and Tianwen-3 will return samples from asteroids and Mars, respectively; Tianwen-4 is intended to explore the Jupiter system. The success of these missions will yield unprecedented information for deciphering the mysteries of the solar system and life, as well as considerably expanding our knowledge of the universe we live in.

Size-Dependent Behaviour of Representative Volume Element for Granular Material under Gravity-Induced Stress Gradient

Gravity is the primary load source in geotechnical engineering, and its effect becomes extremely important in these cases such as lunar research station construction and in situ resource utilization. However, at the current stage, the study on the effect of gravity-induced stress gradients on the behavior of geomaterials is seriously lacking. In this work, the size-dependent behaviour of representative volume element (RVE) of granular matter under the gravity-induced stress gradient is studied. A homogenization theory is proposed firstly, which homogenizes the granular matter into stress gradient continuum through RVE. Then, numerical triaxial loading for RVEs with various heights and particle sizes is carried out under 0∼10 g gravity. The results show that the size effect and its origination appear to have no obvious influence on isotropic compression. However, the macroscopic shear strength, elastic modulus, and volumetric deformation all show size effect dependence, and the gravity-induced stress gradient leads to significant differences between different originations of size effect. The valence, orientation, and elongation degree of microscopic void cells are further analyzed, which suggests that applying of stress gradient results in the evolution of void cells demonstrates a dependence on both the size effect and its origination. The results indicate that the gravity-induced stress gradient and corresponding size effect should be paid sufficient attention to granular mechanics of terrestrial and off-earth engineering.