We report on laboratory experiments to shed light on dust charging and transport that have been suggested to explain a variety of unusual phenomena on the surfaces of airless planetary bodies. We ...have recorded micron‐sized insulating dust particles jumping to several centimeters high with an initial speed of ~0.6 m/s under ultraviolet illumination or exposure to plasmas, resulting in an equivalent height of ~0.11 m on the lunar surface that is comparable to the height of the so‐called lunar horizon glow. Lofted large aggregates and surface mobilization are related to many space observations. We experimentally show that the emission and re‐absorption of photoelectron and/or secondary electron at the walls of microcavities formed between neighboring dust particles below the surface are responsible for generating unexpectedly large negative charges and intense particle‐particle repulsive forces to mobilize and lift off dust particles.
Key Points
Micron‐sized insulating dust particles are recorded to hop and mobilize under UV illumination or exposure to plasmas in laboratory
The emission and re‐absorption of photoelectron/secondary electron at dusty surfaces generate unexpectedly large charges and repulsive forces
Electrostatic processes are efficient on the surfaces of all airless bodies in space to redistribute fine dust particles
Electrostatic dust lofting has been suggested to explain a number of unresolved phenomena on airless planetary bodies such as the lunar horizon glow, the dust ponds on asteroid Eros, and the radial ...‘spokes’ in Saturn’s rings. In this paper we report laboratory measurements of the size distribution of lofted dust particles in the range of 1 to 40 μm in diameter. It is shown that the population of lofted dust increases with a decrease in dust size (i.e., smaller sized dust is easier to be lofted), which is in agreement with the theoretical expectation derived from the patched charge model (Wang et al., 2016a). Additionally, initial launch velocities are derived from transport distances of lofted particles following ballistic trajectories. It is shown that the initial velocity of lofted particles is inversely proportional to their size, in agreement with an earlier work that used a different measurement method (Carroll et al., 2020).
Despite their small masses, comets have played an extraordinary role in enhancing our understanding of cosmic physics. It was the calculation of comet Halley's orbit and the successful prediction of ...its return in 1758 that firmly established the correctness of Newton's law of universal gravitation. It was the morphology of the dusty tails of comets that provided the earliest information of the nature of the interaction of solar electromagnetic radiation with dust, and it was the orientation and structure of the plasma tails of comets that led to the discovery of the solar wind. More recently, the role of the changing dusty plasma environments of comets as natural space laboratories for the study of dust‐plasma interactions, and their physical and dynamical consequences, has been recognized. The forthcoming Rosetta‐Philae rendezvous and lander mission will provide a unique opportunity to revisit the entire range of earlier observations of dusty plasma phenomena in a single comet, as it moves around the Sun. In this topical review, motivated by the Rosetta mission, we discuss the varying modes of interaction of the comet as it approaches the Sun, and the different dusty plasma phenomena that are expected in each case, drawing on the earlier observations, including their interpretations and prevailing open questions.
Key PointsComets are a primary example where dusty plasma processes are at work.Rosetta is expected to observe dusty plasma phenomena.The interaction with the solar wind will change with distance from the Sun.
The charge state of dust particles on regolith surfaces exposed to ultraviolet radiation or plasma is investigated for understanding the role of electrostatic dust transport in the surface evolution ...of airless planetary bodies. Our charge measurement shows that the regolith dust that can be electrostatically transported or lofted carries large negative charges. This result is consistent with our “patched charge model,” which predicts that dust particles forming microcavities in the regolith surfaces can attain large negative charges by collecting photoelectrons and/or secondary electrons emitted from neighboring particles and the resulting repulsive forces between these negatively charged particles lead to their mobilization. The observed negative charge polarity is contrary to the generally expected positive charges on the regolith dust emitting photoelectrons. The measured negative charges are orders of magnitude larger than the prediction by classical charging models. Our laboratory measurements provide critical initial charging conditions for regolith dust dynamics studies.
Key Points
Regolith dust that can be electrostatically transported carries unusually large negative charges
The observed negative charge polarity is contrary to the generally expected positive charges on regolith dust emitting photoelectrons
It provides critical initial charge state for dust dynamics studies to understand the surface evolution of airless bodies
Dust particles in the approximate mass range of 10−22<m<10−20 kg produced near the Sun, due to collisions and breakup of larger interplanetary dust particles, have been shown to become entrained in ...the solar wind plasma flow. When these so‐called nano‐dust particles (NDPs) impact a spacecraft, they have been suggested to produce sufficiently large plasma clouds to cause a detectable signal in the onboard electric antennas. NDPs have been identified on the twin STEREO spacecraft, and the observed intermittent nature of their fluxes were suggested to represent the stochastic nature of their sources near the Sun. Here we show that even if the generation of NDPs remains a constant in time, their detectability near the ecliptic plane becomes intermittent due their interaction with the interplanetary magnetic fields.
Key Points
The acceleration of nano-dust to near solar wind speed is calculated
The intermittent nature of the observations can be due to the IMF configuration
Observations do not allow conclusions about the origin of these particles
The lunar surface is characterized by a collisionally evolved regolith resulting from meteoroid bombardment. This lunar soil consists of highly angular particles in a broad, approximately power law ...size distribution, with impact‐generated glasses. The regolith becomes densified and difficult to excavate when subjected to lunar quakes or, eventually, manned and unmanned activity on the surface. Solar radiation and the solar wind produce a plasma sheath near the lunar surface. Lunar grains acquire charge in this environment and can exhibit unusual behavior, including levitation and transport across the surface because of electric fields in the plasma sheath. The fine component of the lunar regolith contributes to the operational and health hazards posed to planned lunar expeditions. In this paper we discuss the mechanical response of the regolith to anticipated exploration activities and review the plasma environment near the lunar surface and the observations, models, and dynamics of charged lunar dust.
Sodium and, in a lesser way, potassium atomic components of surface-bounded exospheres are among the brightest elements that can be observed from the Earth in our Solar System. Both species have been ...intensively observed around Mercury, the Moon and the Galilean Moons. During the last decade, new observations have been obtained thanks to space missions carrying remote and in situ instrumentation that provide a completely original view of these species in the exospheres of Mercury and the Moon. They challenged our understanding and modelling of these exospheres and opened new directions of research by suggesting the need to better take into account the relationship between the surface-exosphere and the magnetosphere. In this paper, we first review the large set of observations of Mercury and the Moon Sodium and Potassium exospheres. In the second part, we list what it tells us on the sources and sinks of these exospheres focusing in particular on the role of their magnetospheres of these objects and then discuss, in a third section, how these observations help us to understand and identify the key drivers of these exospheres.
Sheaths are formed around the surfaces of airless bodies, which both collect and emit electrons. Depending on the ratio of the emitted to collected electron fluxes Γ, the sheath potential structure ...can be quite different. We present the first experimental measurements of all the three types of the sheath potentials: classical, space‐charge‐limited (SCL), and inverse. A solid surface immersed in plasmas emits secondary electrons. The potential structure changes from a monotonic classical to a nonmonotonic SCL sheath as Γ increases. At the critical electron emission with zero electric field at the surface, the sheath potential is determined by the plasma electron temperature, and Γ approaches but remains smaller than 1, in agreement with the theoretical expectation. The nonmonotonic SCL sheath persists steadily for Γ > 1. When the emitted electron density becomes larger than the plasma electron density, a monotonic inverse sheath forms with a positive surface potential relative to the ambient.
Key Points
We experimentally showed the classical, SCL, and inverse sheaths above an electron‐emitting surface
The ratio of the emitted to collected electron fluxes was examined at a SCL transition state
An inverse sheath with a positive surface potential relative to the ambient was characterized
We present the first three-dimensional fully kinetic and electromagnetic simulations of the solar wind interaction with lunar crustal magnetic anomalies (LMAs). Using the implicit particle-in-cell ...code iPic3D, we confirm that LMAs may indeed be strong enough to stand off the solar wind from directly impacting the lunar surface forming a mini-magnetosphere, as suggested by spacecraft observations and theory. In contrast to earlier magnetohydrodynamics and hybrid simulations, the fully kinetic nature of iPic3D allows us to investigate the space charge effects and in particular the electron dynamics dominating the near-surface lunar plasma environment. We describe for the first time the interaction of a dipole model centered just below the lunar surface under plasma conditions such that only the electron population is magnetized. The fully kinetic treatment identifies electromagnetic modes that alter the magnetic field at scales determined by the electron physics. Driven by strong pressure anisotropies, the mini-magnetosphere is unstable over time, leading to only temporal shielding of the surface underneath. Future human exploration as well as lunar science in general therefore hinges on a better understanding of LMAs.
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