The Lunar Dust Puzzle
The Lunar Dust Puzzle
- Alexander V. ZakharovAlexander V. ZakharovSpace Research Institute, Moscow, Russia
The Moon was the first extraterrestrial body to attract the attention of space pioneers. It has been about half a century since an active lunar exploration campaign was carried out. At that time, a series of Russian and American automatic landing vehicles and the American manned Apollo Program carried out an unprecedented program of lunar exploration in terms of its saturation and volume. Unique breakthrough data on the lunar regolith and plasma environment were obtained, a large number of experiments were carried out using automated and manned expeditions, and more than 300 kg of lunar regolith and rock samples were delivered to Earth for laboratory research. A wealth of experience has been accumulated by performing direct human activities on the lunar surface. At the same time, the most unexpected result of the studies was the detection of a glow above the surface, recorded by television cameras installed on several lunar landers. The interpretation of this phenomenon led to the conclusion that sunlight is scattered by dust particles levitating above the surface of the Moon. When the Apollo manned lunar exploration program was being prepared, this fact was already known, and it was taken into account when developing a program for astronauts’ extravehicular activities on the lunar surface, conducting scientific research, and ground tests. However, despite preparations for possible problems associated with lunar dust, according to American astronauts working on the lunar surface, the lunar dust factor turned out to be the most unpleasant in terms of the degree of impact on the lander and its systems, on the activities of astronauts on the surface, and on their health.
Over the past decades, theoretical and experimental model studies have been carried out aimed at understanding the nature of the lunar horizon glow. It turned out that this phenomenon is associated with the complex effect of external factors on the lunar regolith, as a result of which there are a constant processing and grinding of the lunar regolith to particles of micron and even submicron sizes. Particles of lunar regolith that are less than a millimeter in size are commonly called lunar dust. As a result of the influence of external factors, the upper surface of the regolith acquires an electric charge, and a cloud of photoelectrons and a double layer are formed above the illuminated surface. Coulomb forces in the electric field of this layer, acting on microparticles of lunar dust, under certain conditions are capable of tearing microparticles from the surface of the regolith. These dust particles, near-surface plasma, and electrostatic fields form the near-surface dusty plasma exosphere of the Moon. The processes leading to the formation of regolith and microparticles on the Moon, their separation from the surface, and further dynamics above the surface include many external factors affecting the Moon and physical processes on the surface and near-surface dusty plasma exosphere. As a result of the research carried out, a lot has been understood, but many unsolved problems remain. Recently, since the space agencies of the leading space powers have been turning their attention to intensive research and subsequent exploration of the Moon, interest in the processes associated with the dynamics of lunar dust and its influence on landing vehicles and their engineering systems is increasing, and significant attention is being paid to reducing and mitigating the negative impact of lunar dust on the activities of astronauts and their health.
- Planetary Surfaces
- Small Bodies
The Sea of Thirst was a novelty. … It was a sea of dust, not of water, and therefore it was alien to all the experience of men—therefore, also, it fascinated and attracted them. Fine as talcum powder, drier in this vacuum than the parched sands of the Sahara, it flowed as easily and effortlessly as any liquid. A heavy object dropped into it would disappear instantly, without a splash, leaving no scar to mark its passage.Arthur C. Clarke (1961)
Particles ranging in size from tens of nanometers to tens of micrometers, called dust particles, are one of the most common components of the Universe. Our knowledge suggests that the dust that is observed in the Universe and the solar system did not form immediately after the Big Bang. Modern cosmic dust is a product of the evolution of the Universe, particularly the supernova explosions, which throw out huge amounts of dust, gas, and heavy elements. This material forms the basic building blocks of new generations of stars and stellar systems. Although the properties of the early solar system are little known, it is highly likely that, in addition to dust particles, a significant portion of the nebula was ionized gas. Therefore, the early stellar nebula can be viewed as dusty plasma, in which, in addition to gravity, electric forces played a significant role. The formation of the solar system occurred from a dusty stellar nebula through planetesimals and further evolution to its current form as a system of planets, satellites, small bodies (asteroids and comets), and part of the unused cosmic dust of the stellar nebula.
However, at present, the main population of cosmic dust that can be observed in the solar system is the result of the erosion of previously formed non-atmospheric bodies: planets, their satellites, asteroids, and comets. Besides the endogenous activity of some giant planets and their satellites, degassing or decay of comets contributes to the dust population in the solar system. Erosion, or space weathering, is a term used to describe the interaction of the surface material of airless bodies in the solar system, also known as regolith, with micrometeorites and radiation. Lunar dust, which represents a significant part of the lunar regolith, is precisely the result of the space weathering that has occurred on the Moon since its formation (Pieters & Noble, 2016). Under conditions of direct impact on the lunar regolith of high-speed meteor showers, solar wind, hard radiation, and cosmic rays, lunar dust is a key link in the formation of the near-surface dusty plasma exosphere. This environment is extremely interesting as a natural plasma laboratory. It is formed under vacuum conditions that are much deeper than what is usually reproduced in the laboratory. The Moon does not have a magnetic field, but there are local magnetic anomalies on its surface (Dyal et al., 1974). Time variations associated with the solar wind fluxes that vary with the level of solar activity, changes in the conditions of exposure to the Sun, and the geomagnetic tail plasma associated with the rotation of the Moon around the Earth have a great influence on the dusty-plasma exosphere (Mishra & Misra, 2018; Vaverka et al., 2016). The formation of the finely dispersed component of the lunar regolith and the environment in which it is located makes lunar dust a special substance that has no analogs on Earth.
Lunar dust raised from the surface of the regolith as a result of external influences scatters sunlight, which may be observed in the area of the lunar terminator, the so-called lunar horizon glow (LHG). American astronauts who were on the lunar surface during the manned Apollo Program found that lunar dust suspended above the surface could adversely affect the engineering systems of the lander, and equipment clogged and mechanisms jammed in every Apollo mission. The affected systems included the equipment conveyor, lock buttons, camera equipment, and even the vacuum cleaner designed to clean off the dust (Gaier, 2005). Lunar dust made it difficult for astronauts to work outside the lander and adversely affected their health (Gaier, 2005). These manifestations of lunar dust have led to the development of a new scientific direction, theoretical research, and numerical and laboratory modeling aimed at explaining the dynamics of lunar dust, reducing and mitigating the consequences of its adverse influence on spacecraft systems and on humans. At present, when interest to the Moon has resumed and new manned missions are planned with the aim of investigation and exploration of the Moon, the characteristics of lunar dust that American astronauts encountered on the Moon during the Apollo Program in 1969–1972 are again gaining attention. At the same time, lunar dust is receiving attention not only for its negative effects on engineering systems and humans but also as the main source material for the construction of an outpost and infrastructure on the lunar surface (Benaroya & Bernold, 2008).
Lunar Regolith Formation
The surface of the Moon, like the surface of airless bodies in the solar system, is subject to external factors—constant bombardment by micrometeoroids, the effects of solar radiation, fluxes of solar wind, cosmic rays, and other factors of outer space. These processes are known as space weathering (Pieters et al., 2016). Space weathering is what leads to the formation of the regolith of these bodies, its chemical evolution, and the dispersion of the surface material; what controls the electrical properties of the surface and near-surface exosphere; what affects the composition of the lunar exosphere; and what largely leads to a change in the surface relief (Kallio et al., 2019).
As a result of the impacts of high-velocity micrometeoroids over billions of years, the silicate base of the Moon’s surface has disintegrated, turning into particles with a wide size distribution. Considering the explosive nature of their formation, the particles are characterized by an extremely irregular shape (Liu et al., 2008) with sharp edges, or are conglomerates sintered at high temperatures, or are drops that are close to spherical. Repetitive impacts by micrometeorites of different sizes mix and process preformed particles of regolith at different depths during geological epochs. It is known that the micrometeoroids of cometary and asteroid origin bombarding the Moon have a highly organized, nonisotropic structure (see Jones & Brown, 1993). The total mass of micrometeoroids bombarding the Moon is roughly estimated at 106 kg per year (Grün et al., 2011; Zook, 1975). The density of meteoroid particles is usually close to 2.5 g/cm3 (Grün et al., 1985). Most of these particles have sizes that range from 10 nm to 1 mm, and the collision velocity is in the range of 10 to 72 km/s (Grün et al., 1985). In a high-velocity impact, an explosion occurs, forming a crater on the surface. In this case, regolith material is ejected from the crater, and the mass of ejected material can exceed the mass of the “impactor” by a thousand times (Brownlee et al., 1972; Katzan & Edwards, 1991). A significant fraction of the micron and submicron size material ejected from the lunar surface due to a high-velocity impact (so called secondary particles) returns to the lunar surface (Grün et al., 2011), forming a regolith layer. Particles of this kind have an ejection velocity not exceeding the first cosmic velocity (v1 = 1.6 km/s). Particles with velocities exceeding v1 = 1.6 km/s form a bound dust cloud associated with the Moon (Grün et al., 2011; Spahn et al., 2019). At an ejection with the second cosmic velocity v2 ≥ 2.4 km/s, secondary particles leave the Moon forever. The deposition of dust particles on the lunar surface during its bombardment with micrometeorites and the resulting secondary particles are estimated at 800 particles/m2 per year for particles larger than 1 μm (Brownlee et al., 1972).
On the illuminated side of the Moon, the upper regolith layer is exposed to solar electromagnetic radiation and solar wind. Solar UV radiation plays the main role (Nitter et al., 1998; Poppe & Horanyi, 2010) in the photoemission of the upper regolith layer and the formation of a dusty plasma exosphere on the illuminated surface of the Moon. The solar wind outflowing from the solar corona is a stream of electrons, protons, helium nuclei, and other nuclei. The solar wind and micrometeorites constitute the source of rare elements in the lunar regolith (Ganapathy et al., 1970; Snelling & Rush, 1993). The average speed of particles in a calm solar wind is ~ 400 km/s, and it can vary significantly depending on the activity of the Sun. The flux of solar wind ions corresponds on average to 4.5 × 1012 ions m–2 s–1 and is extremely variable in time (Wurz et al., 2007). Considering that ~ 95% of ions are protons, this flux corresponds to 8.5 × 10–15 kg m–2 s–1. Thus, the total amount of solar matter implanted in the lunar regolith is 4.3 × 1025 ions s–1, or 0.081 kg s−1, which is approximately four times greater than the flow of micrometeorites to the Moon. It should be noted that the scattering of heavy elements from solar wind (carbon, iron, and higher) by the regolith occurs more efficiently than the scattering of hydrogen and helium; therefore, the upper layer of the regolith is enriched with these elements. The results of studies of the interaction of solar wind with the Moon carried out by the Chandrayaan-1, Kaguya, Chang’E-1, ARTEMIS, and IBEX spacecraft have shown that during the interaction of solar wind plasma with lunar regolith, the bulk of the ions are absorbed by the regolith (Bhardwaj et al., 2015). A significant percentage of solar ions, ~ 10% to 20%, capture electrons and scatter as neutral atoms (Wieser et al., 2009). However, a small part of them (~ 0.1%–1.0%) are scattered from the surface, retaining a positive charge (Saito et al., 2008).
During its orbital motion around the Earth, the Moon crosses the extended magnetospheric tail of the Earth, and thus during approximately 30% of the lunar day (~ 9 Earth days), the Moon is under the influence of the magnetospheric plasma. The structure of the Earth’s magnetosphere is quite complex. It is extremely dynamic and depends a lot on the parameters of solar wind and the activity of the Sun. The outer zones of the solar wind flow around Earth are largely determined by the parameters of the solar wind, the density of which is about 10 cm–3 (Parker, 1965). When crossing the magnetosphere–magnetopause boundary, the Moon enters the magnetosphere tail plasma and can be either in one of the tail lobes or in a low-latitude boundary layer or plasma sheet (Tsurutani et al., 1984a, 1984b). In the northern and southern lobes of the tail, the plasma density is very low, 10–3 to 10–2 cm–3 (Peterson et al., 1984). In the plasma layer of the magnetospheric tail, which divides the northern and southern lobes, the plasma density is 0.05 to 0.2 cm–3 but is characterized by a high ion temperature (1–5 keV) and a highly variable velocity (10–1,000 km/s; Frank, 1985). Plasma instruments of the Apollo 12 and 15 missions provided important information on the effect of magnetospheric plasma on the lunar surface (Clay et al., 1975; Rich et al., 1973).
The main information about the lunar regolith was obtained through studies of samples delivered to Earth by the Apollo Program and by the Luna-16, -20, and -24 missions. A systematized catalog of lunar soil samples delivered during all Apollo missions is presented in (Graff, 1993). Considering that the regolith is the result of impact metamorphism, most of it consists of small and unconsolidated fragments of underlying crystalline rocks that cover the entire lunar surface. More than a quarter (by mass) of lunar regolith particles are glass-bounded aggregates (McKay et al., 1991) and breccias. Breccia is a coarse rock resulting from impact fragmentation, and it consists of rock fragments held together by a fine-grained matrix. The thickness of this layer is usually 4 to 5 m in the region of the lunar seas and 10 to 15 m in mountainous regions (McKay et al., 1991).
The samples of the lunar regolith delivered by the Apollo missions were obtained mainly (80%–90%) from the surface and to a depth of 30 to 60 cm and, in terms of particle size distribution, are largely identical (Carrier, 1973). Particles < 1 mm in size make up more than 95% of the regolith mass. The finest regolith component, less than 100 μm, is defined as lunar dust. Consequently, the overwhelming part of the lunar regolith (by mass) is lunar dust. The average particle size is in the range from 40 to 100 microns. Such particles account for about half the weight of the lunar regolith, with most of it ranging in size from 45 to 80 microns (Carrier, 1973; McKay et al., 1991). For nonspherical particles, the “size” of a particle is usually understood as the diameter of an equivalent sphere or circle (the same definition of particle size is adopted in this article).
The particle size distribution was studied by various methods, which included a calibrated sieve, laser analysis, aerosol diagnostics, image analysis obtained by a scanning electron microscope (SEM), etc. Figure 1 shows one example of the size distribution of dust particles obtained in a sample delivered to Earth by the Apollo 17 mission (Liu & Taylor, 2011). Data on the size distribution of microparticles are important for studies of the conditions of their dynamics under the surface and for the design of cleaning systems for the human environment, as well as for studies of the toxicological effects of dust on the respiratory function of humans.
The upper regolith layer (several mm) is extremely porous (> 80%; Hapke & Sato, 2016). The density of the regolith increases with depth, and infrared measurements show that the characteristics of the upper ~ 10 cm of regolith are approximately the same over the entire surface of the Moon, except for recently formed impact craters (Hayne et al., 2017). The bulk density of the regolith is from 1.04 to 1.90 g cm–3 for various samples (Carrier et al., 1991; Leontovich et al., 1974).
The shape of dust particles, as a rule, is extremely irregular, with pronounced pointed edges, and they are very different from their terrestrial analogs. The density of individual particles is usually taken as 2.7 to 3.0 g cm–3 (Carrier et al., 1991). A rich collection of images of lunar regolith particles is presented in (Liu et al., 2008). For example, Figure 2 presents from this collection of images a typical sample of agglutinate—porous glass with fused rock and iron particles. All dust particles can be morphologically classified into four types: (1) spherical, (2) irregularly shaped blocks with sharp corners, (3) shards (flakes) of glass, and (4) irregular (porous, “Swiss cheese”). As a rule, particles of irregular shape have sharp corners (Park, Liu, Kihm, & Taylor, 2006). The shape of the particles is mainly oblong, which leads to preferential adhesion of individual particles along their longitudinal axes. The consequence of this feature of the fine fraction of the regolith is the anisotropy of physical properties (Mahmood et al., 1974).
A lunar rock usually consists of pyroxene, plagioclase, ilmenite, olivine, and a small amount of many other minerals (Agrell et al., 1970). Moreover, chemical analysis of lunar dust shows that, with a decrease in particle size, the proportion of clearly defined minerals decreases, but the proportion of glassy material increases. The general chemical composition of lunar dust varies over the entire lunar surface but is about 50% SiO2, 15% Al2O3, 10% CaO, 10% MgO, 5% TiO2, and 5% to 15% FeO (Loftus et al., 2010). A key feature of lunar regolith and lunar dust is the presence of more nanophase metallic iron (np-FeO; Keller & McKay, 1993). This component arises when micrometeorites (< 1 mm) impact the lunar soil at typical velocity of ~15 km/s (Grun et al., 2011). During this process, micrometeorites release enough energy to vaporize volatiles and the FeO (among other species) dissociates into Fe and O (Keller & McKay, 1993). The vaporized material penetrates the soil and condenses to form amorphous rings, in some cases containing np-FeO species that range in size from 10 nm up (McKay et al., 2015).
The electrical properties of dust particles and the bulk of silicates of the lunar regolith are characterized by the extremely low electrical conductivity, on the order of 10–14 S/m (for regolith) and 10–9 S/m (for lunar rock) at zero illumination (Carrier et al., 1991; Vaniman et al., 1991). As the temperature rises, the conductivity of the regolith and rock increases, and under solar UV, the electrical conductivity increases approximately 106 times (Vaniman et al., 1991). The relative permittivity (or dielectric constant) is determined by the density of the regolith, ρ, and is approximately 1.9ρ, where ρ is in g/cm3 (Carrier et al., 1991), and depends on the mineralogical composition of the regolith. The extremely low electrical conductivity and dielectric constant of the regolith indicate that the lunar regolith weakly absorbs electromagnetic energy and is characterized by an effective photoemission. These characteristics of the lunar regolith indirectly indicate the absence of water in the material of the studied samples. According to data from the Lunar Reconnaissance Orbiter (LRO), in some polar regions of the Moon hydrogen is recorded with a concentration corresponding to the calculated content of water ice from 0.5% to 4.0% by mass, depending on the depth (Chin et al., 2007; Mitrofanov et al., 2010).
Manifestations of Lunar Dust
The first indications of the presence of dust particles above the lunar surface were obtained with the Surveyor-5, -6, and -7 landers in the early stages of lunar exploration (Rennilson & Criswell, 1974). Immediately after sunset, the TV cameras of these landers registered a glow above the terminator. This effect, the LHG, was interpreted by the authors as the scattering of light by microparticles levitating above the surface at a height of < 1 m under the action of electrostatic forces. In the same work, assuming that these particles were spherical, the authors estimated their radius as ~ 5–6 microns and the concentration in the column as ~ 50 particles cm–2. Figure 3 shows one of several LHG images taken by Surveyor-6 (a fragment of Figure 4 from Colwell et al., 2007).
In addition to near-surface glow at heights of about 1 m, several observations also indicated glow at high altitudes. In particular, the astrophotometer of Lunokhod-2 (Severny et al., 1975), directed to the zenith while it was behind the terminator, detected light scattering on a cloud of dust particles, which, as shown by estimates, was at an altitude of ~ 260 m from the lunar surface. Besides, the astronauts of the Apollo 17 orbital module observed, and then the stellar sensor of the Clementine lunar orbiter acquired images of, the glow above the lunar horizon (Zook et al., 1995). In all these observations, the glow occurred at much higher altitudes, ~ 10 to 20 km.
Registration of dust particle dynamics above the lunar surface was first performed using the Lunar Ejecta and Meteorite (LEAM) experiment deployed on the lunar surface by astronauts of the Apollo 17 mission (Berg et al., 1976). The LEAM instrument had several detectors for recording high-speed (1 < v < 25 km/s–1) micrometeorites. However, one of the detectors was able to register low-velocity particles. The data from this detector were the data that gave unexpected results about rather high fluxes of low-velocity (v ~ 100–1000 m/s–1) particles, the charge of which was usually Q > 10–12 C (or > 6 × 106 electron charges). During the operation of the instrument, the maximum counting rate of the detectors was observed in the region of the terminator. Figure 4 shows the number of registrations of dust particles in a 3-hour interval (counting rate), summed up over 22 lunar days. This graph shows an increase in the count rate of dust grains for several hours before and after crossing the terminator, and the most significant increase in the count rate was in the region of sunrise. However, subsequent detailed analysis of the data cast doubt on this conclusion (Grün & Horanyi, 2013).
Lunar dust turned out to be the most unexpected and unpleasant phenomenon faced by the American astronauts when landing on the surface of the Moon. This was noted in the astronauts’ reports and in numerous scientific publications discussing the effect of lunar dust on the systems and equipment of landing vehicles, on the conditions of stay, and on activities of astronauts on the lunar surface (see Christoffersen et al., 2009; Gaier, 2005; Katzan & Edwards, 1991; Linnarsson et al., 2012; Murphy et al., 2010; Stubbs et al., 2007a). Moreover, not only should the dynamics of lunar dust be considered a consequence of the effects of the rise and levitation of dust particles caused by the influence of external natural factors on the regolith, but also, as the experience of the Apollo Program has shown, the dynamics of lunar dust near the surface occur most actively as a result of the anthropogenic impact on the near-surface environment (Gaier, 2005).
Dust rise from the surface occurs during the operation of jet engines during landing and takeoff of landing vehicles, the operation of the mechanical systems of the vehicle (for example, a drilling setup) on the lunar surface, the activity of astronauts, and their movement on the surface (Gaier, 2005). Some lander systems, such as thermal control systems and optical surfaces, are sensitive to dust deposition. In addition, a lander and an astronaut on the lunar surface cause disturbances in the system of naturally formed electrostatic fields, which obey the dynamics of charged dust particles, which also changes the picture of the dynamics of dust. All these anthropogenic factors cause a change in the natural dynamics of dust particles levitating above the surface of the regolith and lead to a more active influence of dust on the systems of the lander, on human activity, and on human health.
The effects of dust on optical surfaces have been studied for several decades since the pioneering exploration of the Moon. Such long-term studies were made possible by the fact that several reflective systems for laser ranging were deployed on the lunar surface. Laser reflectors were installed on Lunokhod-2 (Severny et al., 1975) and on the lunar surface during the Apollo missions (Murphy et al., 2010). After the takeoff of the lander from the lunar surface and in the first few months of observations, the signal from the laser reflector installed by Apollo 14 showed no significant degradation. However, after almost 40 years of operation of these systems on the lunar surface, the signal reflected from the laser reflector installed by the Apollo mission has become 10 times weaker. At the beginning of its operation, the signals from the laser reflector of Lunokhod-2 were an order of magnitude higher than those from the Apollo 14 reflector, but after 40 years its value became an order of magnitude lower than that of the Apollo 14 reflector. This effect may be explained by the design features of the corner reflectors and by the deposition of dust, which can reduce their reflectivity. In addition, open optics can be directly affected by particles associated with micrometeorite bombardment. In any case, analysis of the operation of these systems over several decades showed that the optical systems performed their functions quite successfully; however, the gradual degradation of optics was recorded on a time scale of the order of a decade.
Based on the results of the analysis of the impact of lunar dust on the lander systems and the activities of astronauts on the lunar surface during the six Apollo missions, all the discovered effects of lunar dust were systematized into nine categories: (1) vision obscuration, (2) false instrument readings, (3) dust coating and contamination, (4) loss of traction, (5) clogging of mechanisms, (6) abrasion, (7) problems with the thermal control system, (8) seal failures, and (9) breathing problems and other factors related to human health.
Astronauts first encountered the problem of reduced visibility during landing with the engines of the Apollo 11 lunar module running. A cloud of dust about 30 m above the surface was formed, and it became denser as the height decreased. There was a threat that one of the landing pillars would touch a large rock or fall into a small crater. Therefore, for the landing of the Apollo 14, 15, and 16 spacecraft on the lunar surface, the landing profile was corrected, and a steeper landing profile was used. However, even in these cases, it was difficult to inspect the site before contacting the surface. Also related to the visibility problem caused by dust was the fact that the landing speed sensors of the Apollo 12 and 15 spacecraft gave false readings due to the dust cloud generated by the engines that ensure the soft landing of the spacecraft (Gaier, 2005).
Lunar dust was found to be extremely abrasive. The astronauts noted that after working outside the lander, the instrument dials and sun visors of their helmets were so scratched that it was impossible to see the instrument readings. After 8 hours of operation, especially after drilling the soil, the suits and gloves had significant abrasions and, if it became necessary to perform one or two additional exits from the lander, they could lose their tightness (Gaier, 2005). For example, the spacesuit of Pete Conrad, Jr., commander of the Apollo 12 mission, which was sealed before leaving the lander for the first time, lost pressure at ~ 0.01 atm/min after the first exit and at ~ 0.017 atm/min after the second exit. Since the safe leak was 0.02 atm/min, the safety of a third exit was questionable. Dust penetrating the moving parts of the suit caused such great difficulty in movement that another exit from the lander would not have been possible (Gaier, 2005). Furthermore, all the lunar environmental sample and gas sample seals failed because of dust, and by the time they reached Earth, the samples were so contaminated as to be worthless (Gaier, 2005).
When astronauts worked on the surface, it was found that dust quickly covered all surfaces with which it came into contact, including spacesuits, astronaut’s shoes, hand tools, equipment, and the systems of the lander. The deposition of dust required the astronauts to perform additional work cleaning clothes and equipment, but this proved to be ineffective (Gaier, 2005).
Dust deposition led to such unpleasant effects as clogging of mechanisms and violation of thermal regulation processes. Reports of these problems were received from astronauts on every mission that visited the lunar surface. Serious problems were caused by a layer of dust on the surface of the radiator of the thermal control system. Attempts to remove this dust under lunar conditions were ineffective. This led to the fact that the operating temperature of some systems exceeded the expected temperature by 20°C, and some instruments of the Apollo 16 and 17 spacecraft deteriorated due to overheating. This led John Young, commander of the Apollo 16 expedition, to comment: “Dust is the number one concern in returning to the Moon” (Gaier, 2005).
However, the experience of the Apollo Program showed that the most unpleasant characteristic of lunar dust is its effects on human health, primarily due to irritation and inhalation. The Apollo crews reported that lunar dust has a pungent, gunpowder-like odor, which appears to be due to the presence of volatiles on the surface of the dust particles. Dust penetrated the astronauts’ clothes and, after removing their clothes, the astronauts found themselves covered in dust. Once in the lunar module, during the flight to Earth, in the absence of gravity, the dust spread throughout the spacecraft. The dust in this atmosphere was inhaled by the astronauts and irritated their eyes (Gaier, 2005). Dust cleaning equipment on board was ineffective. In preparations for the later Apollo missions, this property of lunar dust was taken into account, and measures were taken that somewhat reduced its effects. However, the toxicity of micron and submicron particles that were found on the spacesuit material indicates the need for constant monitoring of the concentration of particles inside the lander and in the long-term lunar habitat of the future (Christoffersen et al., 2009). Figure 5 shows Eugene Cernan, the Apollo 17 mission commander, wearing a dusty spacesuit in the lander after working on the lunar surface (NASA photo, Linnarsson et al., 2012).
The experience of the Apollo astronauts showed that the seriousness of the dust problem was underestimated in mission preparation. Apparently, this was due to the fact that the dynamics of dust associated with human activities on the surface turned out to be much more pronounced than the natural dynamics of dust on the daytime side of the Moon (Gaier, 2020). The Apollo Program focused more on studies of natural dust transport than on astronaut-induced dust transport, but dust transport associated with astronauts and robotic systems on the lunar surface can be orders of magnitude higher than dust transport due to natural processes (Kazan & Edwards, 1991). Therefore, extravehicular activity (EVA) on the lunar surface can be very dangerous for both astronauts and the equipment used.
The unusual ability of lunar dust to penetrate the seals of sealed units and to “stick” to various surfaces can be considered in terms of the dynamic properties of lunar dust particles levitating above the surface. The point is that levitating submicron and micron particles, when interacting with a surface, can manifest as more than “impactors.” By levitating, dust particles can rotate rapidly. Estimates made by Rosenfeld et al. (2016) showed that the speed of proper rotation of levitating micron and submicron particles on the illuminated side of the Moon can range from several thousand to tens of millions of revolutions per second. Furthermore, considering the impact origin of such particles, their shapes are extremely irregular and often pointed (Park, Liu, Kihm, Hill, & Taylor, 2006). All this suggests that such rapidly rotating particles resemble eastern shuriken (or ninja stars) with great destructive power (Rosenfeld et al., 2016). Apparently, this feature, in combination with the existing electrostatic charge, explains the amazing ability of lunar dust to aggressively affect the surfaces of the sensitive systems of the instruments and lander and to penetrate through hermetic seals.
The experience of implementation and analysis of the results of the Apollo Program showed that, for a new stage of active research and exploration of the Moon with human participation, additional research is needed to gain a deeper understanding of plasma–dust processes occurring in the near-surface exosphere of the Moon, of regolith characteristics, and of the properties and size distribution of dust particles. All these studies are extremely important for the development of recommendations for reducing the effect of lunar dust on engineering systems and humans on the lunar surface.
Modeling the Dynamics of Lunar Dust
Even before the discovery of the LHG, it was suggested that the lunar surface could acquire a potential of 20 to 40 V, that an electrostatic field forms at heights of several centimeters from the surface, and, as a result, that regolith microparticles can detach and levitate above the surface (Singer & Walker, 1962). Real LHG observations and evidence of levitation of regolith microparticles above the lunar surface, recorded by the television cameras of the Surveyor landing craft, led to the development of theoretical and experimental work aimed at explaining dusty plasma processes in the near-surface exosphere. The model of levitation of dust particles under the action of electrostatic forces was developed by Mishra and Bhardwaj (2019), Nitter et al. (1998), Popel and Zelenyi (2014), Popel et al. (2018), Poppe and Horanyi (2010), Sickafoose et al. (2002), and Wang et al. (2009).
The essence of the model is as follows. Solar electromagnetic radiations, as well as flows of interplanetary plasma, ions, and electrons of the solar wind, acting on the regolith, create currents through the surface. Moreover, for the illuminated side of the Moon under solar wind conditions, the photocurrent density Jph from the action of solar UV and soft X-radiation is usually an order of magnitude higher than the currents of electrons Je and ions Ji of the solar wind, or ∣Jph∣ ⪢ ∣Je∣ ⪢ ∣Ji∣, as well as secondary radiation electrons on the lunar surface (Stubbs et al., 2014). These currents depend on the surface potential, and in the equilibrium state at the formed surface potential, their sum is close to zero (Manka, 1973; Whipple, 1981). Considering that the surface of the regolith is close to a dielectric (Olhoeft et al., 1974), the acquired surface potential can be maintained for a long time (Criswell & De, 1977; De & Criswell, 1977). In this case, a double (plasma) layer with an electric field E appears between the charged surface of the Moon and the surrounding quasineutral plasma. The characteristic height of this layer is of the order of the Debye length, which corresponds to several tens of centimeters. The Coulomb force qE, the gravitational force mgL (m is the mass of the particle, gL is the acceleration of gravity on the Moon), and the van der Waals adhesion forces Fc act on dust particles lying on the surface of the Moon that have received an electric charge q. If the Coulomb repulsive force exceeds the sum of the forces holding a dust particle on the surface, qE > mgL + Fc, the particle is detached from the surface and levitates in the near-surface electric field. The condition for particle levitation is the approximate equality of the electric and gravitational forces qE ≈ mgL. The concept of dust particles levitating over the surface of the regolith was proposed in Criswell (1973) and Singer and Walker (1962). Depending on the conditions in which the lunar surface is located, the sign and magnitude of the surface potential, as well as the values of the electric field, can change significantly. This is a general picture of the formation of the potential of the regolith surface, the near-surface electric field, and the conditions for the dynamics of dust particles.
Measurements of the potential of the lunar surface were carried out using the SIDE (Suprathermal Ion Detector Experiment) experiment deployed at the landing sites of the Apollo 12, 14, and 15 spacecraft. The results of this experiment showed that the electric potential of the lunar surface in the latitude range of ± 45° from the subsolar region is about +10 V. The potential of the surface quickly decreases with an increase in the zenith angle of the Sun, and near the terminator it becomes negative, and this can be –100 V (Freeman & Ibrahim, 1975). The energy of photoelectrons knocked out of the surface of the regolith by solar UV radiation is in the range of 1 to 4 eV and the photocurrent is about 4.5 μA/m2 (Feuerbacher et al., 1973; Willis et al., 1973). Taking into account the low energy of photoelectrons and assuming Maxwellian energy distribution, the Debye length at the surface and the near-surface plasma layer is about 70 cm (Colwell et al., 2007).
Direct evidence of the existence of a photoelectron layer above the lunar surface was obtained from the results of the Charged Particle Lunar Environment Experiment (CPLEE), carried out in the Apollo 14 mission. The CPLEE instrument was able to register electron fluxes in the range of 40 to 200 eV. When the Moon crossed the geomagnetic tail, the count rate of the instrument usually changed slightly. However, during the eclipse of the Sun, which occurred when the Moon crossed the central part of the tail, the count rate of the instrument dropped to zero. The authors concluded that the CPLEE instrument, before entering the shadow and after leaving it, recorded photoelectrons. Figure 6 shows the change in the CPLEE electron counting rate when the Moon crossed the central part of the geomagnetic tail, during which the Sun eclipsed (Reasoner & Burke, 1973). On the graph, the interval of the Sun eclipse is between 5 and 9 hours (Reasoner & Burke, 1973).
Numerical modeling of the profile of the plasma layer on the illuminated side of the Moon shows that near the surface, under normal solar wind conditions, the concentration of photoelectrons is about 102 cm–3. With height, this value decreases and the electrons of the solar wind begin to have influence, and at a height of about 10 m, the plasma becomes almost electrically neutral (Colwell et al., 2007; Lisin et al., 2013, 2015; Poppe & Horanyi, 2010). In this case, the electric field vector is directed upward. Its value varies from a few volts at the surface to zero at a height of about 10 m. Figure 7 shows the calculated course of the concentration of plasma components and the change in the strength of the electrostatic field with height.
The parameters of dust particles suspended in the lunar exosphere have not been measured; however, there are numerous estimates. According to observations of near-surface luminescence, the estimated average size of suspended particles is ~ 5 μm, with a concentration of ~ 50 particles/cm2 (in column; Criswell, 1973). The speed of dust particles taking off from the surface is several tens of centimeters per second, and the concentration of submicron particles of ~ 100 nm near the surface is about 103 cm–3. The results of numerical simulation of the distribution of particle concentration with height in the polar region of the Moon are shown in Figure 8 (Popel et al., 2018). There are many other estimates of the concentration of dust particles over the illuminated surface of the Moon, but the estimates may differ by several orders of magnitude for different researchers (see Glenar et al., 2011; Stubbs et al., 2007b).
Light scattering, which was observed at high altitudes by the Lunokhod-2 astrophotometer and the astronauts of the Apollo-17 orbital module, could hardly be a consequence of the levitation of submicron particles in the near-surface double layer. To explain the glow at high altitudes, the so-called “fountain” mechanism of the dynamics of dust particles was proposed (Stubbs et al., 2006). This mechanism can be realized if the Coulomb force acting on the particle after separation from the surface is greater than the gravity force qE > mgL. In this case, dust particles of the regolith, for example, ~ 0.01 to 0.1 μm, can rise to heights up to 100 km above the lunar surface in the terminator region. However, both a reanalysis of Apollo data (Glenar et al., 2011) and subsequent remote observations from the lunar orbital spacecraft Clementine and LRO searching for LHG (Feldman et al., 2014; Glenar et al., 2014; Grava et al., 2017) could not detect glow, providing smaller upper limits than the original McCoy (1976) paper. Apparently, one of the reasons that it was not possible to see the glow of sunlight’s scattering on dusty clouds from orbiters could be the specific conditions needed for observing the glow.
The Lunar Dust Experiment (LDEX) aboard the Lunar Atmosphere and Dust Environment Explorer (LADEE; Horanyi et al., 2014) was devoted to experimental studies of the propagation of dust particles at high altitudes from the lunar surface. The measurement results showed that in the cases when the satellite was in the solar wind at altitudes of more than 3 km from the lunar surface, there was a constant cloud of dust particles, the density of which was nd < 100 m–3; when crossing the geomagnetic tail, the particle concentration was nd < 40 m–3 (Szalay & Horanyi, 2015a). The LDEX results did not confirm, as expected, an increase in the density of dust particles above the terminators, although the astronauts of the manned Apollo spacecraft observed light bursts in the vicinity of the terminator. At the same time, regular meteor showers caused persistent increased density levels of the dust cloud (Horanyi et al., 2015). Therefore, the authors of the LDEX experiment concluded that the parameters of this cloud were consistent with the ejections of microparticles formed during the constant bombardment of the lunar surface by sporadic flows of interplanetary dust (Szalay & Horanyi, 2015b).
At the terminator, in the polar regions of the Moon, where the angle of the Sun and, therefore, the photocurrent jph are close to zero, and on the night side of the Moon, the surface charge and the nature of the formation of the plasma layer are determined by the balance of the currents of ions and electrons of solar wind. Sharp changes in the illumination conditions of the regolith surface in the terminator region and natural features of the surface topography make this area extremely critical for determining the parameters of the near-surface plasma–dust environment. Nevertheless, there are model representations of dynamic processes of dust particles in these regions (Farrell et al., 2008a; Popel et al., 2015, 2018). For example, in the vicinity of the terminator, there may be a region that is an analog of the plasma layer. Such a layer creates a potential barrier in the terminator region, so that electrons are held in the plasma above the illuminated part of the Moon by electrostatic forces. The width of the plasma disturbed region associated with the terminator is on the order of the ion Debye radius (~ 10 m). Significant electric fields (E ~ 300 V/m) arise in this region, which can lead to the rise of positively charged micron dust particles to heights on the order of several tens of centimeters and the scattering of sunlight, which was observed by the Surveyor spacecraft in the region of the lunar terminator. For dust particles with sizes on the order of 100 nm, the estimate of their concentration over the dark part of the Moon is nd ~ 10–2–10–1 cm–3. Considering that the estimates of the concentration of levitating dust particles on the illuminated side of the Moon (discussed above) gave concentration values on the order of 103 cm–3, there is a significant jump in the concentration of dust in the terminator region (all numerical estimates given in this paragraph were obtained in the article by Popel et al., 2015). A model representation of the features of changes in the near-surface electric field and the course of the concentration of dust particles above the surface in the terminator region is shown in Figure 9.
The processes in the dusty plasma system of the Moon near the terminator are not fully understood. The results of measurements in the terminator region performed by the LEAM instrument deployed by the Apollo 17 astronauts on the surface of the Moon almost half a century ago in many ways have only raised questions that need to be considered.
On the shaded side of the Moon, plasma density falls (Halekas et al., 2011). At the same time, due to the fact that the speed of the electrons of solar wind is much higher than the speed of the ions (~ 1,900 km/s for electrons and ~ 45 km/s for the ions), the surface of the Moon in the shadow acquires a negative charge (Borisov & Moll, 2002; Manka, 1973) close to the value of the electron temperature. The potential gradient is directed to the surface from electrically neutral plasma outside the plasma layer. The distribution of the electric field over the surface depends significantly on the conductivity of the regolith (Borisov & Moll, 2002, 2006).
When the Moon crosses the geomagnetic tail, changes in the surface charge of the regolith can be significant due to rapid variations of fluxes in the plasma layer of the magnetosphere (Halekas et al., 2011). The study of the electrostatic potential of the surface on the night side of the Moon was carried out by the Lunar Prospector orbiter. It was found that, on the night side of the Moon, the surface potential is negative and amounts to –100 V in the high-latitude regions (the lobes) of the geomagnetic tail and from –200 V to 1 kV in the plasma layer. With an increase in solar activity, the surface potential can reach –4 kV (Halekas et al., 2008). An unexpected result was that, when the geomagnetic tail was crossed, the surface potential values turned out to be negative even on the daytime side of the Moon. This does not correspond to modern concepts of the formation of the potential of an illuminated surface and leaves questions for further experiments and theoretical analysis (Halekas et al., 2011).
The Problem of Separation of Microparticles From the Surface of the Regolith
Model concepts of dust dynamics, as a rule, did not take into account the Van der Waals adhesive forces, Fa. This is because the coupling forces between dust particles are difficult to analyze due to the wide range of sizes and shapes of dust particles. And in those cases when attempts were made to take into account the adhesion forces, their rough estimates show that the forces can be thousands or even millions of times higher than the gravity Fg of micron and submicron dust particles with radius rd (Hartzell & Scheeres, 2011; Li et al., 2006). For relatively large dust particles, which are characteristically >103 μm, the adhesion forces (Fa ∝ rd) become insignificant in comparison to the gravitational force (Fg ∝ rd3). Therefore, taking into account the van der Waals coupling forces for micron and submicron particles is crucial for explaining their separation from the surface under the action of electrostatic forces. The condition for the detachment of a dust particle lying on the surface of the regolith and having received a positive charge as a result of photo-ionization is the excess of the electrostatic force Fe = qE acting on it over the sum of the forces holding the particle on the surface: the gravitational force Fg and the adhesion force Fa (Hartzell & Scheeres, 2011; Lee, 1995). In order for the Fe force with an average electrostatic field E ~ 10 V/m on the illuminated side of the Moon to lift a speck of dust, for example, 1 micron in diameter, by overcoming only the force of gravity, it must have a sufficiently large charge q ≈ 1,500 e (Rosenfeld & Zakharov, 2020). This value takes into account the magnitude of the acceleration of gravity on the Moon gL ≈ 1.6 m/sec2 and the density of the regolith material ρ ≈ 3,000 kg/m3. To detach a micron or submicron particle from the surface of the regolith by electrostatic force, it is necessary that either its charge be very large or the electric field have a very high strength value. Attempts to resolve this issue have been made by Flanagan and Goree (2006), Sheridan (2013), Sheridan and Hayes (2011), and Sheridan et al. (1992), but the question remains open.
An article by Wang et al. (2016) presented the results of laboratory modeling of the detachment of microparticles from a surface with a loose structure that contains cavities near the upper layer of dust particles, which, apparently, is characteristic of the regolith of atmosphereless bodies. The simulation results showed that under the influence of UV illumination or plasma conditions, dust particles detach from the surface (simulating their levitation above the surface) in a manner similar to LHG in natural conditions. This result led the authors to conclude that the emission and reabsorption of photoelectrons (in the case of UV radiation) or secondary electrons (in the case of plasma) at the walls of microcavities formed between neighboring dust particles below the surface are responsible for generating unexpectedly large negative charges and intense particle repulsive forces to lift off dust particles. The results of this work were developed in several subsequent articles (Carroll et al., 2020; Hood et al., 2018; Schwan et al., 2017).
Another approach to solving this problem was discussed by Rosenfeld and Zakharov (2018, 2020). Attention was drawn to the fact that the measured near-surface field can be considered uniform only if it is assumed that the charge on the surface is uniformly distributed. In fact, the process of charging microparticles lying on the surface of the regolith is discrete and stochastic. In this case, under the action of UV radiation or plasma fluxes, fluctuating charge spots of different signs appear randomly on any part of the dielectric surface, and the resulting fluctuations of the charge density inside such a spot can exceed the average charge density on the surface by several orders of magnitude. Therefore, as estimations show, the charge of a microparticle lying inside such a charge spot and the strength of the local electric field directly above such a spot will be several orders of magnitude higher than the average value over the surface. Under these conditions, the local Coulomb force can exceed the adhesion forces and tear off the particle from the surface. Breaking away from a surface area with a high local charge density and a high local electric field, which change stochastically, the dust particle enters the region where the electric field is determined by the average surface charge, and it levitates in this region.
Thermal fluctuations of submicron particles can be another mechanism competing with the adhesion force. With such thermodynamic processes, the thermal energy of sufficiently small particles can exceed the adhesion energy between them, and then the powder will behave like a gas. This means that, as the temperature rises, the bonds between particles can weaken and conglomerates of dust particles on the surface of the regolith can decay, so that the bulk density of the upper regolith layer decreases with increasing temperature. In this case, the behavior of particles can be represented as their “boiling” on the surface. A similar mechanism was considered by Rosenfeld et al. (2016). In addition, in accordance with the principle of uniform distribution of thermal energy over the degrees of freedom, in addition to linear motions, their proper rotation should be taken into account. On the daytime side of the Moon, where the temperature of the regolith can take on a value of T ≈ 400 K (Vaniman et al., 1991), estimates show that for particles with characteristic sizes from several tens of nanometers (m ~ 10–21 kg) to several micrometers (m ~ 10–15 kg), their rotation speed can range from several thousand to tens of millions of revolutions per second. Rotating grains have been observed in laboratory experiments (Carroll et al., 2020). As dust particles tear off the surface of the Moon and levitate above it, their rotation can be one of the defining properties of their negative effects on the engineering systems of the lander and on the astronauts.
It seems that the processes of separation of dust particles from the surface and their dynamics are more complicated than previously assumed, and the existing models require correction. Apparently, to explain the detachment of particles from the surface, one should not only rely on the average description of the acquisition of an electrostatic charge by dust particles lying on the surface, but also take into account statistical processes based on the discrete nature of the interaction of plasma and UV radiation with the regolith. Preliminary consideration shows that thermodynamic processes can also play a role in the detachment of submicron particles from the surface. These models should explain how dust particles near the lunar surface manifest themselves, not only in scattering the sunlight seen above the terminator, but also in the unexpected dust activity effects encountered by American astronauts on the lunar surface during the Apollo missions.
Studies of lunar dust’s properties and dynamics carried out to date, as well as the available data on external factors to which the dust is subject, show that the interaction of the heterogeneous dust particles naturally creates a new environment with a new quality—a dusty plasma exosphere above the lunar surface. Although it seems that in its natural state this dusty exosphere has relatively modest characteristics (including density and temperature), it extremely detrimentally affects technological systems and humans. This was especially evident during the execution of the manned Apollo space program.
It is important to note that the astronauts were on the lunar surface only during the daytime of the lunar day. This time is characterized by relatively mild dusty plasma conditions, in contrast to the much greater activity in the terminator regions or during space weather disturbances. The danger can be represented by the electric fields inherent in the lunar surface, which depend on the characteristics of external influences and relief, and can cause discharges between illuminated and shaded areas of the surface (Farrell et al., 2008b; Jackson et al., 2011).
Manifestations of lunar dust, discovered during the execution of automatic and manned programs of lunar exploration, laid the foundation for the creation of models of its dynamics in the near-surface environment. Much work has been carried out on analytical, numerical, and experimental modeling of the main physical processes occurring on the surface of the Moon and during the formation of the plasma–dust exosphere above its surface. A lot has been learned, but there are open “blank spots” in our knowledge.
For example, the terminator region and regions near large-scale surface irregularities (large craters, hills), which are the regions where there are extended boundaries between the illuminated and shaded surfaces of the Moon, remain mysterious in terms of their electrical characteristics. In such areas, the conditions for the appearance of charges on the surface should radically change, and significant electric fields with a predominant horizontal component should appear.
The processes of interaction of the solar wind plasma with regolith, which compete with the action of solar UV radiation on the illuminated side of the Moon, remain unclear. With a calm solar wind, UV radiation prevails over plasma flows, and the surface potential is formed under the condition of a “two-component” flow—photoelectrons and electrons of the solar wind. However, with an active Sun, during flares, one should take into account the “three-component” current on the surface, which consists of photoelectrons and the two main components of the solar wind, protons and electrons.
An extremely important parameter for modeling the detachment of dust particles from the regolith surface is adhesion. This parameter is very difficult to evaluate, because it largely depends on the shape and properties of specific particles, which are infinite in their variety. Faced with this problem, Hartzell and Scheeres (2011) concluded that, “The method through which dust particles are launched off the surface of an airless body is currently unknown.”
The anomalous ability of lunar dust to affect surfaces and lander equipment, as well as to penetrate hermetic seals, as encountered by the Apollo astronauts, also requires physical understanding. These lunar dust influences became crucial in the Apollo program. That is why the most important conclusion about the Apollo Program was made by Eugene Cernan, the commander of Apollo 17 (Gaier, 2005). He said, “I think dust is probably one of our greatest inhibitors to a nominal operation on the Moon. I think we can overcome other physiological or physical or mechanical problems except dust.”
The experience of the Apollo Program initiated the development of a new direction of scientific and engineering research—electrostatic surface dust cleaning and mitigation of the effects of lunar dust. This area of research is aimed at creating technologies that reduce the effect of lunar dust on systems and equipment of landing vehicles, on human activities on the lunar surface, and on health and life-support systems.
Leading space agencies have been undertaking further study of the Moon. Research programs and instruments are being developed to investigate the parameters of levitating dust particles and the near-surface dust–plasma environment. It is hoped that work in the next decade will solve the mystery of the peculiarities of lunar dust and that methods and means will be proposed to minimize the negative effects of lunar dust on the engineering systems of landing vehicles and on astronauts. In addition, the planned programs of research on, and development of, the Moon require the creation of infrastructure and outposts that will serve as closed habitats for humans on the Moon. Research is already being carried out on using extraterrestrial materials for construction on the lunar surface, under vacuum, radiation, and reduced gravity. Of course, regolith and lunar dust will be the main source materials for such construction on the Moon.
- Agrell, S. O., Scoon, J. H., Muir, I. D., Long, J. V., McConnell, J. D., & Peckett, A. (1970, January 5–8). Observation of the chemistry, mineralogy and petralogy of some Apollo 11 lunar samples. In A. A. Levinson (Ed.), Proceedings of the Apollo 11 Lunar Science Conference, Houston, TX. Volume 1: Mineralogy and Petrology (pp. 93–128). Pergamon Press.
- Benaroya, H., & Bernold, L. (2008). Engineering of lunar bases: Review. Acta Astronautica, 62, 277–299.
- Berg, O. E., Wolf, H., & Rhee, J. (1976). Lunar soil movement registered by the Apollo 17 cosmic dust experiment. In H. Elsasser & H. Fechtig (Eds.), Interplanetary dust and zodiacal light (pp. 233–237). Springer.
- Bhardwaj, A., Dhanya, M. B., Alok, A., Barabash, S., Wieser, M., Futaana, Y., Wurz, P., Vorburger, A., Holmstrom, M., Lue, C., Harada, Y., & Asamura, K. (2015). A new view on the solar wind interaction with the Moon. Geoscience Letters, 2, 10.
- Borisov, N., & Mall, U. (2002). The structure of the double layer behind the Moon. Journal of Plasma Physics, 67(4), 277–299.
- Borisov, N., & Mall, U. (2006). Charging and motion of dust grains near the terminator of the Moon. Planetary and Space Science, 54, 572–580.
- Brownlee, D., Bucher, W., & Hodge, P. (1972). Primary and secondary micrometeoroid impact rate on the lunar surface: Direct measurement. In W. F. Carroll, R. Davis, M. Goldfine, S. Jacobs, L. D. Jaffe, L. Leger, B. Milwitzky, & L. Nickle (Eds.), Analysis of Surveyor 3 material and photographs returned by Apollo 12 (pp. 143–150). NASA-STIO.
- Carrier, W. D., III. (1973). Lunar regolith grain size distribution. The Moon, 6, 250–263.
- Carrier, W. D., III, Olhoeft, G. R., & Mendell, W. (1991). Physical properties of lunar surface. In G. H. Heiken, D. T. Vaniman, & B. M. French (Eds.), The lunar sourcebook (pp. 475–594). Cambridge University Press.
- Carroll, A., Hood, N., Mike, R., Wang, X., Hsu, H. W., & Horányi, M. (2020). Laboratory measurements of initial launch velocities of electrostatically lofted dust on airless planetary bodies. Icarus, 352, 113972.
- Chin, G., Brylow, S., Foote, M., Garvin, J., Kasper, J., Keller, J., Litvak, M., Mitrofanov, I., Paige, D., Raney, K., Robinson, M., Sanin, A., Smith, D., Spence, H., Spudis, P., Stern, S. A., & Zuber, M. (2007). Lunar Reconnaissance Orbiter overview: The instrument suite and mission. Space Science Review, 129, 391–419.
- Christoffersen, R., Lindsay, J. F., Noble, S. K., Meador, M. A., Kosmo, J. J., Lawrence, J. A., Brostoff, L., Young, A., & McCue, T. (2009). Lunar dust effects on spacesuit systems: Insights from the Apollo spacesuits (NASA/TR-2009–214786). NASA.
- Clarke, A. C. (1961). A fall of moondust (p. 224). Gollancz.
- Clay, D. R., Goldstein, B. E., Neugebauer, M., & Snyder, C. W. (1975). Lunar surface solar wind observations at the Apollo 12 and Apollo 15 sites. Journal of Geophysical Research, 80(13), 1751–1760.
- Colwell, J. E., Batiste, S., Horanyi, M., Robertson, S., & Sture, S. (2007). Lunar surface: Dust dynamics and regolith mechanics. Review of Geophysics, 45, RG2006/2007.
- Criswell, D. R. (1973). Horizon-glow and the motion of lunar dust. In R. J. L. Grard (Ed.), Photon and particle interactions with surfaces in space (pp. 545–556). D. Reidel Publishing.
- Criswell, D. R., & De, B. R. (1977). Intense localized charging in the lunar sunset terminator region. 2. Supercharging at the progression of sunset. Journal of Geophysical Research, 82, 1005–1007.
- De, B. R., & Criswell, D. R. (1977). Intense localized photoelectric charging in the lunar sunset terminator region. 1. Development of potentials and fields. Journal of Geophysical Research, 82, 999–1004.
- Dyal, P., Parkin, C. W., & Daily, W. D. (1974). Magnetism and the interior of the Moon. Review of Geophysics: Space Physics, 12, 568–591.
- Farrell, W. M., Stubbs, T. J., Delory, G. T., Vondrak, R. R., Collier, M. R., Halekas, J. S., & Lin, R. P. (2008). Concerning the dissipation of electrically charged objects in the shadowed lunar polar regions. Geophysical Research Letters, 35, L19104.
- Farrell, W. M., Stubbs, T. J., Halekas, J. S., Delory, G. T., Collier, M. R., Vondrak, R. R., & Lin, R. P. (2008). Loss of solar wind plasma neutrality and effect on surface potentials near the lunar terminator and shadowed polar regions. Geophysical Research Letters, 35, L05105.
- Feldman, P. D., Glenar, D. A., Stubbs, T. J., Retherford, K. D., Gladstone, G. R., Miles, P. F., Greathouse, T. K., Kaufmann, D. E., Parker, J. W., & Stern, S. A. (2014). Upper limits for a lunar dust exosphere from far-ultraviolet spectroscopy by LRO/LAMP. Icarus, 233, 1–34.
- Feuerbacher, B., Willis, R. F., & Fitton, B. (1973). Electrostatic charging and formation of composite interstellar grains. In R. J. L. Grard (Ed.), Photon and particle interactions with surfaces in space (pp. 415–426). D. Reidel Publishing.
- Flanagan, T. M., & Goree, J. (2006). Dust release from surfaces exposed to plasma. Physics of Plasmas, 13, 123504.
- Frank, L. A. (1985). Plasmas in the Earth’s magnetosphere. Space Science Review, 42, 211–240.
- Freeman, J. W., & Ibrahim, M. (1975). Lunar electric fields, surface potential and associated plasma sheaths. The Moon, 8, 103–114.
- Gaier, J. R. (2005). The effects of lunar dust on EVA systems during the Apollo missions. NASA TM-2005–213610.
- Gaier, J. R. (2020, February 11–13). The impact of dust on lunar surface equipment during Apollo. In Lunar Planetary Conference: The Impact of Lunar Dust on Human Exploration, Houston, Texas. LPI Contribution No. 2141, id. 5002.
- Ganapathy, R., Keays, R. R., Laul, J. C., & Edward, A. (1970, January 5–8). Trace elements in Apollo 11 lunar rocks: Implications for meteorite influx and origin of moon. In A. A. Levinson (Ed.), Proceedings of the Apollo 11 Lunar Science Conference, Houston, TX. Volume 2: Chemical and Isotope Analyses (pp. 1117–1142). Pergamon Press.
- Glenar, D. A., Stubbs, T. J., Hahn, J. M., & Wang, Y. (2014). Search for a high-altitude lunar dust exosphere using Clementine navigational star tracker measurements. Journal of Geophysical Research: Planets, 119(1), 2548–2567.
- Glenar, D. A., Stubbs, T. J., McCoy, J. E., & Vondrak, R. R. (2011). A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust. Planetary and Space Science, 53, 1695–1707.
- Graff, J. C. (1993). Lunar soil grain size catalog. NASA-RP-1265.
- Grava, C., Stubbs, T. J., Glenar, D. A., Retherford, K. D., & Kaufmann, D. E. (2017). Absence of a detectable lunar nanodust exosphere during a search with LRO’s LAMP UV imaging spectrograph. Geophysical Research Letters, 44(10), 4591–4598.
- Grun, E., & Horanyi, M. (2013). A new look at Apollo17 LEAM data: Nighttime dust activity in 1976. Planetary and Space Science, 89, 2–14.
- Grun, E., Horanyi, M., & Sternovsky, Z. (2011). The lunar dust environment. Planetary and Space Science, 59, 1672–1680.
- Grun, E., Zook, H. A., Fechtig, H., & Giese, R. H. (1985). Collisional balance of the meteoritic complex. Icarus, 62, 244–272.
- Halekas, J. S., Delory, G. T., Lin, R. P., Stubbs, T. J., & Farrell, W. M. (2008). Lunar Prospector observations of the electrostatic potential of the lunar surface and its response to incident currents. Journal of Geophysical Research, 113, A09102.
- Halekas, J. S., Saito, Y., Delory, G. T., & Farrell, W. M. (2011). New views of the lunar plasma environment. Planetary and Space Science, 59, 1681–1694.
- Hapke, B., & Sato, H. (2016). The porosity of the upper lunar regolith. Icarus, 273, 75–83.
- Hartzell, C. M., & Scheeres, D. J. (2011). The role of cohesive forces in particle launching on the Moon and asteroids. Planetary and Space Science, 59, 1758–1768.
- Hayne, P. O., Bandfield, J. L., Siegler, M. A., Vasavada, A. R., Ghent, R. R., Williams, J.-P., Greenhagen, B. T., Aharonson, O., Elder, K. M., Lucey, P. G., & Paige, D. A. (2017). Global regolith thermophysical properties of the Moon from the Diviner Lunar Radiometer Experiment. Journal of Geophysical Research: Planets, 122, 2371–2400.
- Hood, N., Carroll, A., Mike, R., Wang, X., Schwan, J., Hsu, H. W., & Horanyi, M. (2018). Laboratory investigation of rate of electrostatic dust lofting over time on airless planetary bodies. Geophysical Research Letters, 45(24), 13206–13212.
- Horányi, M., Sternovsky, Z., Lankton, M., Dumont, C., Gagnard, S., Gathright, D., Grün, E., Hansen, D., James, D., Kempf, S., Lamprecht, B., Srama, R., Szalay, J. R., & Wright, G. (2014). The Lunar Dust Experiment (LDEX) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission. Space Science Review, 185, 93–113.
- Horanyi, M., Szalay, J. R., Kempf, S., Schmidt, J., Grün, E., Srama, R., & Sternovsky, Z. (2015). A permanent, asymmetric dust cloud around the Moon. Nature, 522(7), 324–326.
- Jackson, T. L., Farrell, W. M., Killen, R. M., Delory, G. T., Halekas, J. S., & Stubbs, T. J. (2011). Discharging of roving objects in the lunar polar regions. Journal of Spacecraft and Rockets, 48(4), 700–703.
- Jones, J., & Brown, P. (1993). Sporadic meteor radiant distributions—Orbital survey results. Monthly Notices of the Royal Astronomical Society, 265(3), 524–532.
- Kallio, E., Dyadechkin, S., Wurz, P., & Khodachenko, M. (2019). Space weathering on the Moon: Farside–nearside solar wind precipitation asymmetry. Planetary and Space Science, 166, 9–22.
- Katzan, C. M., & Edwards, J. L. (1991). Lunar dust transport and potential interactions with power system components (NASA Contractor Report 4404). NASA Office of Management. Scientific and Technical Information Program.
- Keller, L. P., & McKay, D. S. (1993). Discovery of vapor deposits in the lunar regolith. Science, 261, 1305–1307.
- Lee, L.-H. (1995). Adhesion and cohesion mechanisms of lunar dust on the moon`s surface. Journal of Adhesion Science and Technology, 8, 1103–1124.
- Leontovich, A. K., Gromov, V. V., Dmitriev, A. D., Lozhkin, V. A., Pavlov, P. S., & Rybakov, A. V. (1974). Physical and mechanical properties of lunar soil sample in a research chamber in a nitrogen medium. In A. P. Vinogradov (Ed.), Lunar grunt iz Morya Izobilia [Lunar soil from the Mare Fecunditatis] (pp. 563–570). Nauka.
- Li, Q., Rudolph, V., & Peukert, W. (2006). London-van der Waals adhesiveness of rough particles. Powder Technology, 161, 248–255.
- Linnarsson, D., Carpenter, J., Fubini, B., Gerde, P., Karlsson, L. L., Loftus, D. J., Prisk, G. K., Staufer, U., Tranfield, E. M., & van Westrenen, W. (2012). Toxicity of lunar dust. Planetary and Space Science, 74, 57–71.
- Lisin, E. A., Tarakanov, V. P., Petrov, O. F., Popel, S. I., Dol’nikov, G. G., Zakharov, A. V., Zelenyi, L. M., & Fortov, V. E. (2013). Effect of the solar wind on the formation of a photoinduced dusty plasma layer near the surface of the Moon. JETP Letters, 98(11), 664–669.
- Lisin, E. A., Tarakanov, V. P., Popel, S. I., & Petrov, O. F. (2015). Lunar dusty plasma: A result of interaction of the solar wind flux and ultraviolet radiation with the lunar surface. Journal of Physics: Conference Series, 653, 012139.
- Liu, Y., Park, J., Schnare, D., Hill, E., & Taylor, L. A. (2008). Characterization of lunar dust for toxicological studies. II: Texture and shape characteristics. Journal of Aerospace Engineering, 21(4), 272–279.
- Liu, Y., & Taylor, L. A. (2011). Characterization of lunar dust and a synopsis of available lunar simulants. Planetary and Space Science, 59, 1769–1783.
- Loftus, D. J., Rask, J. C., McCrossin, C. G., & Tranfield, E. M. (2010). The chemical reactivity of lunar dust: From toxicity to astrobiology. Earth, Moon and Planets, 107, 95–105.
- Mahmood, A., Mitchell, J. K., & Carrier, W. D., III. (1974, March 18–22). Grain orientation in lunar soil. In Proceedings of the Fifth Lunar Science Conference, Houston, Texas, Volume 3 (A75-39540 19-91, pp. 2347–2354). Pergamon Press.
- Manka, R. H. (1973). Plasma and potential at the lunar surface. In R. J. L. Grard (Ed.), Photon and particle interactions with surfaces in space (pp. 347–361). D. Reidel Publishing.
- McCoy, J. E. (1976). Photometric studies of light scattering above the lunar terminator from Apollo solar corona photography. In Proceedings of the Seventh Lunar Science Conference (pp. 1087–1112). Pergamon Press.
- McKay, D. S., Cooper, B. L., Taylor, L. A., James, J. T., Thomas-Keprta, K., Pieters, C. M., Wentworth, S. J., Wallace, W. T., & Lee, T. S. (2015). Physicochemical properties of respirable-size lunar dust. Acta Astronautica, 107, 163–176.
- McKay, D. S., Heiken, G., Basu, A., Blanford, G., Simon, S., Reedy, R., French, B. M., & Papike, J. (1991). The lunar regolith. In G. H. Heiken, D. T. Vaniman, & B. M. French (Eds.), The lunar sourcebook (pp. 285–356). Cambridge University Press.
- Mishra, S. K., & Bhardwaj, A. (2019). Photoelectron sheath on lunar sunlit regolith and dust levitation. The Astrophysical Journal, 884, 5.
- Mishra, S. L., & Misra, S. (2018). An analytical investigation: Effect of solar wind on lunar photoelectron sheath. Physics of Plasmas, 25(2), 023702.
- Mitrofanov, I. G., Sanin, A. B., Boynton, W. V., Chin, G., Garvin, J. B., Golovin, D., Evans, L. G., Harshman, K., Kozyrev, A. S., Litvak, M. L., Malakhov, A., Mazarico, E., McClanahan, T., Milikh, G., Mokrousov, M., Nandikotkur, G., Neumann, G. A., Nuzhdin, I., Sagdeev, R., … Zuber, M. T. (2010). Hydrogen mapping of the lunar South Pole using the LRO neutron detector experiment LEND. Science, 330, 483–486.
- Murphy, T. W., Jr., Adelberger, E. G., Battat, J. B. R., Hoyle, C. D., McMillan, R. J., Michelsen, E. L., Samad, R. L., Stubbs, C. W., & Swanson, H. E. (2010). Long-term degradation of optical devices on the Moon. Icarus, 208, 31–35.
- Nitter, T., Havnes, O., & Melandsø, F. (1998). Levitation and dynamics of charged dust in the photoelectron sheath above surfaces in space. Journal of Geophysical Research, 103(A4), 6605–6620.
- Olhoeft, G. R., Frisillo, A. L., Strangway, D. W., & Sharpe, H. (1974). Temperature dependence of electrical conductivity and lunar temperatures. The Moon, 9, 79–87.
- Park, J. S., Liu, Y., Kihm, K. D., Hill, E., & Taylor, L. A. (2006). Submicron particle size distribution of Apollo 11 lunar dust. In R. B. Malla (Ed.), Earth and Space: 10th Biennial ASCE International Conference on Engineering, Construction and Operations in Challenging Environments (pp. 200–205). American Society of Civil Engineers. Curran Associates, Inc.
- Park, J. S., Liu, Y., Kihm, K. D., & Taylor, L. A. (2006). Micro-morphology and toxicological effect of lunar dust. Lunar and Planetary Sciences, XXXVII, 2193.
- Parker, E. N. (1965). Dynamical theory of the solar wind. Space Science Reviews, 4(5–6), 666–708.
- Peterson, W. K., & Shelley, E. G. (1984). Origin of the plasma in a crosspolar cap auroral feature (theta aurora). Journal of Geophysical Research, 89, 6729.
- Pieters, C. M., & Noble, S. K. (2016). Space weathering on airless bodies. Journal of Geophysical Research: Planets, 121, 1865–1884.
- Popel, S. I., & Zelenyi, L. M. (2014). Dusty plasmas over the moon. Journal of Plasma Physics, 80(6), 885–893.
- Popel, S. I., Zelenyi, L. M., & Atamaniuk, B. (2015). Dusty plasma sheath-like structure in the region of lunar terminator. Physics of Plasmas, 22(12), 123701.
- Popel, S. I., Zelenyi, L. M., Golub, A. P., & Dubinskii, A. Y. (2018). Lunar dust and dusty plasmas: Resent development, advances, and unsolved problems. Planetary and Space Science, 156, 71–84.
- Poppe, A., & Horányi, M. (2010). Simulations of the photoelectron sheath and dust levitation on the lunar surface. Journal of Geophysical Research, 115(A08106), 1–9.
- Reasoner, D. L., & Burke, W. J. (1973). Measurement of the lunar photoelectron layer in the geomagnetic tail. In R. J. L. Grard (Ed.), Photon and particle interactions with surfaces in space (pp. 369–387). D. Reidel Publishing.
- Rennilson, J. J., & Criswell, D. R. (1974). Surveyor observations of lunar horizon-glow. The Moon, 10, 121–142.
- Rich, F. J., Reasoner, D. L., & Burke, W. J. (1973). Plasma sheet at lunar distance: Characteristics and interactions with the lunar surface. Journal of Geophysical Research, 78, 34.
- Rosenfeld, E. V., Korolev, A. V., & Zakharov, A. V. (2016). Lunar nanodust: Is it a borderland between powder and gas? Advances in Space Research, 58, 560–563.
- Rosenfeld, E. V., & Zakharov, A. V. (2018). Dust shedding from a dielectric surface in plasma as a result of charge fluctuations. Physics of Plasmas, 25, 103703.
- Rosenfeld, E. V., & Zakharov, A. V. (2020). Charge fluctuations on the sunlit surface of airless bodies and their role in dust levitation. Icarus, 338, 113538.
- Saito, Y., Yokota, S., Tanaka, T., Asamura, K., Nishino, M. N., Fujimoto, M., Tsunakawa, H., Shibuya, H., Matsushima, M., Shimizu, H., Takahashi, F., Mukai, T., & Terasawa, T. (2008). Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP‐PACE onboard SELENE (KAGUYA). Geophysical Research Letters, 35, L24205.
- Schwan, J., Wang, X., Hsu, H. W., Grün, E., & Horanyi, M. (2017). The charge state of electrostatically transported dust on regolith surfaces. Geophysical Research Letters, 44(7), 3059–3065.
- Severny, A. B., Terez, E. I., & Zvereva, A. M. (1975). The measurements of sky brightness on Lunokhod-2. The Moon, 14, 123–128.
- Sheridan, T. E. (2013). Charging time for dust grain on surface exposed to plasma. Journal of Applied Physics, 113, 143304.
- Sheridan, T. E., Goree, J., Chiu, Y. T., Rairden, R. L., & Kiessling, J. A. (1992). Observation of dust shedding from material bodies in a plasma. Journal of Geophysical Research, 97(A3), 2935–2942.
- Sheridan, T. E., & Hayes, A. (2011). Charge fluctuations for particles on a surface exposed to plasma. Applied Physics Letters, 98, 091501.
- Sickafoose, A. A., Colwell, J. E., Horányi, M., & Robertson, S. (2002). Experimental levitation of dust grains in a plasma sheath. Journal of Geophysical Research, 107(A11), 1–11.
- Singer, S. F., & Walker, E. H. (1962). Electrostatic dust transport on the lunar surface. Icarus, 1(2), 112–120.
- Snelling, A. A., & Rush, D. E. (1993). Moon dust and the age of the solar system. Creation Ex-Nihilo Technical Journal, 7(1), 2–42.
- Spahn, F., Sachse, M., Seiß, M., Hsu, H.-W., Kempf, S., & Horányi, N. (2019). Circumplanetary dust populations. Space Science Reviews, 215, 11.
- Stubbs, T. J., Farrell, W. M., Halekas, J. S., Burchill, J. K., Collier, M. R., Zimmerman, M. I., Vondrak, R. R., Delory, G. T., & Pfaff, R. F. (2014). Dependence of lunar surface charging on solar wind plasma conditions and solar irradiation. Planetary and Space Science, 90, 10–27.
- Stubbs, T. J., Vondrak, R. R., & Farrell, W. M. (2006). A dynamic fountain model for lunar dust. Advances in Space Research, 37, 59–66.
- Stubbs, T. J., Vondrak, R. R., & Farrell, W. M. (2007). Impact of dust on lunar exploration. In H. Krüger & A. L. Graps (Eds.), Dust in planetary systems, SP-643 (pp. 239–244). ESA Publications.
- Stubbs, T. J., Vondrak, R. R., Farrell, V. M., & Collier, M. R. (2007). Predictions of dust concentrations in the lunar exosphere. Journal of Astronautics, 28(4), 166–167.
- Szalay, J. R., & Horanyi, M. (2015a). The search for electrostatically lofted grains above the Moon with the Lunar Dust Experiment. Geophysical Research Letters, 42(1), 5141–5146.
- Szalay, J. R., & Horanyi, M. (2015b). Annual variation and synodic modulation of the sporadic meteoroid flux to the Moon. Geophysical Research Letters, 42(2), 10580–10584.
- Tsurutani, B. T., Jones, D. E., & Sibeck, D. G. (1984a). The two-lobed structure of the distant (X > 200 RE) magnetotail. Geophysical Research Letters, 11, 1066.
- Tsurutani, B. T., Jones, D. E., Slavin, J. A., Sibeck, D. G., & Smith, E. J. (1984b). Plasma sheet magnetic fields in the distant tail. Geophysical Research Letters, 11, 1062.
- Vaniman, D., Reedy, R., Heiken, G., Olhoeft, G., & Mendell, W. (1991). The lunar environment. In G. H. Heiken, D. T. Vaniman, & B. M. French (Eds.), The lunar sourcebook (p. 27). Cambridge University Press.
- Vaverka, J., Richterová, I., Pavlu, J., Šafránková, J., & Němeček, Z. (2016). Lunar surface and dust grain potentials during the Earth`s magnetosphere crossing. The Astrophysical Journal, 825, 133.
- Wang, X., Horanyi, M., & Robertson, S. (2009). Experiments on dust transport in plasma to investigate the origin of the lunar horizon glow. Journal of Geophysical Research, 114, A05103.
- Wang, X., Schwan, J., Hsu, H. W., Grün, E., & Horanyi, M. (2016). Dust charging and transport on airless planetary bodies. Geophysical Research Letters, 43(1), 6103–6110.
- Whipple, E. C. (1981). Potentials of surfaces in space. Reports on Progress in Physics, 44, 1197–1250.
- Wieser, M., Barabash, S., Futaana, Y., Holmström, M., Bhardwaj, A., Sridharan, R., Dhanya, M. B., Wurz, P., Schaufelberger, A., & Asamura, K. (2009). Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space. Planetary and Space Science, 57, 14–15.
- Willis, R. F., Anderegg, M., Feuerbacher, B., & Fitton, B. (1973). Photoemission and secondary electron emission from lunar surface material. In R. J. L. Grard (Ed.), Photon and particle interactions with surfaces in space (pp. 389–401). D. Reidel Publishing Co.
- Wurz, P., Rohner, U., Whitby, J. A., Kolb, C., Lammer, H., Dobnikar, P., & Martín-Fernández, J. A. (2007). The lunar exosphere: The sputtering contribution. Icarus, 191, 486–496.
- Zook, H. A. (1975, March 17–21). The state of meteoritic matter on the moon. In Proceedings of the Sixth Lunar Science Conference, Houston, Texas, Volume 2 (A78-46668 21-91, pp. 1653–1672). Pergamon Press.
- Zook, H. A., Potter, A. E., & Cooper, B. L. (1995). The lunar dust exosphere and Clementine lunar horizon glow. Lunar and Planetary Science, 26, 1577–1578.