The lunar dust environment

Each year the Moon is bombarded by about 10⁶ kg of interplanetary micrometeoroids of cometary and asteroidal origin. Most of these projectiles range from 10 nm to about 1 mm in size and impact the Moon at 10–72 km/s speed. They excavate lunar soil about 1000 times their own mass. These impacts leave a crater record on the surface from which the micrometeoroid size distribution has been deciphered. Much of the excavated mass returns to the lunar surface and blankets the lunar crust with a highly pulverized and “impact gardened” regolith of about 10 m thickness. Micron and sub-micron sized secondary particles that are ejected at speeds up to the escape speed of 2300 m/s form a perpetual dust cloud around the Moon and, upon re-impact, leave a record in the microcrater distribution. Such tenuous clouds have been observed by the Galileo spacecraft around all lunar-sized Galilean satellites at Jupiter. The highly sensitive Lunar Dust Experiment (LDEX) onboard the LADEE mission will shed new light on the lunar dust environment. LADEE is expected to be launched in early 2013.Another dust related phenomenon is the possible electrostatic mobilization of lunar dust. Images taken by the television cameras on Surveyors 5, 6, and 7 showed a distinct glow just above the lunar horizon referred to as horizon glow (HG). This light was interpreted to be forward-scattered sunlight from a cloud of dust particles above the surface near the terminator. A photometer onboard the Lunokhod-2 rover also reported excess brightness, most likely due to HG. From the lunar orbit during sunrise the Apollo astronauts reported bright streamers high above the lunar surface, which were interpreted as dust phenomena. The Lunar Ejecta and Meteorites (LEAM) Experiment was deployed on the lunar surface by the Apollo 17 astronauts in order to characterize the lunar dust environment. Instead of the expected low impact rate from interplanetary and interstellar dust, LEAM registered hundreds of signals associated with the passage of the terminator, which swamped any signature of primary impactors of interplanetary origin. It was suggested that the LEAM events are consistent with the sunrise/sunset-triggered levitation and transport of charged lunar dust particles. Currently no theoretical model explains the formation of a dust cloud above the lunar surface but recent laboratory experiments indicate that the interaction of dust on the lunar surface with solar UV and plasma is more complex than previously thought.     

Characterization of lunar dust and a synopsis of available lunar simulants

Understanding the formation and evolution of the soil and dust of the Moon addresses the fundamental question of the interactions of space with the surface of an airless body. The physical and chemical properties of the lunar dust, the <20 μm portion of lunar soil, are key properties necessary for studies of the toxicity and the electrostatic charging of the dust. These properties have been largely overlooked until recent years. Although chemical and physical studies of the <20 μm portion of lunar soil have been the topic of several studies, there is still need for further studies, primarily of the <1 μm particles. This paper presents a review of the studies of lunar dust that have been conducted to date. As many preparations for future exploration or science activities on the Moon require testing using lunar soil/dust simulants, we also include a brief review of past and current simulants.     

Mitigation of lunar dust adhesion by surface modification

Dust has been recognized as one of the greatest hazards in continued lunar exploration due to its tenacious adhesion to everything with which it comes into contact. Unfortunately, there is little known about the mechanisms of adhesion on widely varying surface types: van der Waals and electrostatic forces are the dominant forces under consideration here. Surface energy, roughness, mechanical properties and electronic properties are all known to contribute to the adhesion characteristics. An optimal solution to mitigate dust adhesion would be to identify the dominant components of the adhesive force and to reduce that force by surface modification. In this study, an ion beam process was used to modify (treat) the surfaces of three dramatically different materials spanning the range of conductor (black Kapton), semiconductor (silicon), and insulator (quartz). Adhesive forces between less than JSC-1 lunar simulants and these virgin/treated surfaces were measured in vacuum using a centrifugal force detachment method. We found that JSC-1 particles adhered less to treated silicon and quartz surfaces, correlated with a reduction in van der Waals force due to a reduced surface energy. The large reduction in adhesion for treated black Kapton is mainly due to the large decrease in the electrostatic (image) force that results from reduced contact charging. Materials in space and on the lunar surface will be directly exposed to high-energy ultraviolet radiation prior to being covered by dust, so the UV irradiation effects on surface adhesion were also examined. Both virgin and treated quartz surfaces are most affected by the UV-irradiation, showing dramatically increased adhesion.     

An integrated laser anemometer and dust accumulator for studying wind-induced dust transport on Mars

Windborne dust is one of the most important and dynamic factors affecting the Martian surface and its atmosphere, yet there lacks a detailed physical understanding how it is transported. We present a miniature laser-based optoelectronic instrument for use on a Mars lander. It integrates sensors capable of quantifying important parameters needed for the understanding and modeling of dust transport on Mars, these include wind speed, wind direction, suspended dust concentration, dust deposition and removal rates as well as the electrification of the Martian dust. Dust electrification has been seen from experimental simulations to be of considerable importance to the processes of adhesion and cohesion, specifically prompting the formation of low mass density dust aggregates. Testing of this prototype instrument has been performed under simulated Martian conditions in a wind tunnel facility. The results and analysis of its functionality will be presented.     

The Electrostatic Lunar Dust Analyzer (ELDA) for the detection and trajectory measurement of slow-moving dust particles from the lunar surface

Micron and submicron-sized dust particles can be lifted from the lunar surface due to continual micrometeoroid bombardment and electrostatic charging. The characteristics of these dust populations are of scientific interest and engineering importance for the design of future equipment to operate on the lunar surface. The mobilized grains are expected to have a low velocity, which makes their detection difficult by traditional methods that are based on momentum transfer or impact energy. We describe a newly developed instrument concept, the Electrostatic Lunar Dust Analyzer (ELDA), which utilizes the charge on the dust for detection and analysis. ELDA consists of an array of wire electrodes combined with an electrostatic deflection field region, and measures the mass, charge, and velocity vector of individual dust grains. The first basic prototype of the ELDA instrument has been constructed, tested and characterized in the laboratory. The instrument is set up to measure over a velocity range 1–100 m/s and is sensitive to particles from an approximate mass range from 2×10⁻¹⁶ to 10⁻¹¹ kg, depending on the charge state and velocity.     

A review on East Asian dust storm climate, modelling and monitoring

In arid and semi-arid area of Asia, dust storms occur frequently. Asian dust storms have a major impact on the air quality of the densely populated areas of China, Korea and Japan, and are important to the global dust cycle. In extreme cases, they result in the loss of human lives and disruptions of social and economic activities. In recent years, systematic research on Asian dust storms has been carried out. Much progress has been made in the development of integrated dust storm monitoring and modeling systems by making use of advanced numerical models, satellite remote sensing and GIS data. In this paper, we summarize the recent achievements in Asian dust storm research with an emphasis on dust climatology, modeling and satellite monitoring. The concept of integrated dust storm monitoring and modeling system is described and a summary of the developments in key research areas is given, including new dust models and techniques in satellite remote sensing and system integration. We then discuss the current research frontiers and the challenges for future studies.     

Disintegration of dust aggregates in interstellar shocks and the lifetime of dust grains in the ISM

We have studied to influence of grain porosity on the dust destruction mechanism in interstellar shocks. Our results show, that fluffy aggregates of dust grains can be broken up easily in shocks.     

The electrical properties of Mars analogue dust

Dust is a major environmental factor on the surface and in the atmosphere of Mars. Knowing the electrical charge state of this dust would be of both scientific interest and important for the safety of instruments on the Martian surface. In this study the first measurements have been performed of dust electrification using suspended Mars analogue material. This has been achieved by attracting suspended dust onto electrodes placed inside a Mars simulation wind tunnel. The Mars analogue used was from Salten Skov in Denmark, this contained a high concentration of ferric oxide precipitate. Once suspended, this dust was found to consist of almost equal quantities of negatively (46±6%) and positively (44±15%) charged grains.These grains were estimated to typically carry a net charge of around 10⁵e, this is sufficient to dominate the processes of adhesion and cohesion of this suspended dust. Evidence is presented for electrostatic aggregation of the dust while in suspension. Development of a simple instrument for measuring electrical charging of the suspended dust on Mars will be discussed.     

Optical properties of dust from Jupiter family comets

To try to define specific physical properties of the dust of Jupiter-family comets (JFCs), we compare the light scattered by them. Amongst the more than 1000 JFCs, less than 200 are numbered, 40 of them being rather bright. In the present work we use data from the latter. In situ observations of three nuclei show low albedo surfaces. The albedo of the dust particles in the coma is low, with generally a red colour. The A(α)fρ product is a measure of cometary activity and secular changes. Images of different regions (jets and fans) give indications on the nucleus rotation and position of the emitting areas, as compared to the position of the rotation axis. Differences in physical properties between the particles in different regions are pointed out by differences in the linear polarization of the scattered light and by spectral variations in brightness and polarization. Jupiter family comets are considered as dust-poor comets. Tails and trails’ studies give an estimation of the size distribution of the particles. However the dust production rates depend on the largest particles (up to centimetre size), which are mainly observed in the trails where large dark compact particles are found. These dark particles are also responsible for the high polarization in the inner most coma of some comets. The meaning, in terms of physical properties, of the linear polarization is discussed through different examples such as 2P/Encke, 9P/Tempel 1 or the fragments of 73P/Schwassmann-Wachmann 3. Cometary outbursts and splitting events show that the properties of the dust ejected from the interior of the nucleus are similar to the ones of more active comets (new or with larger semi-major axis).     

Evidence from IRAS for a very young, partially formed dust band

The relative proportions of asteroidal and cometary materials in the zodiacal cloud is an ongoing debate. The determination of the asteroidal component is constrained through the study of the Solar System dust bands (the fine-structure component superimposed on the broad background cloud), since they have been confidently linked to specific, young, asteroid families in the main belt. The disruptions that produce these families also result in the injection of dust into the cloud and thus hold the key to determining at least a minimum value for the asteroidal contribution to the zodiacal cloud. There are currently known to be at least three dust band pairs, one at approximately 9.35° associated with the Veritas family and two central band pairs near the ecliptic, one of which is associated with the Karin subcluster of the Koronis family. Through careful co-adding of almost all the pole-to-pole intensity scans in the mid-infrared wavebands of the Infrared Astronomical Satellite (IRAS) data set, we find strong evidence for a partial Solar System dust band, that is, a very young dust band in the process of formation, at approximately 17° latitude. We think this is a confirmation of the M/N partial band pair first suggested by Sykes [1988. IRAS observations of extended zodiacal structures. Astrophys. J. 334, L55-L58]. The new dust band appears at some but not all ecliptic longitudes, as expected for a young, partially formed dust band. We present preliminary modeling of the new, partial dust band which allows us to put constraints on the age of the disruption event, the inclination and node of the parent body at the time of disruption, and the quantity of dust injected into the zodiacal cloud.     

The effect of surface topography on the lunar photoelectron sheath and electrostatic dust transport

The dayside near-surface lunar plasma environment is electrostatically complex, due to the interaction between solar UV-induced photoemission, the collection of ambient ions and electrons, and the presence of micron and sub-micron sized dust grains. Further complicating this environment, although less well understood in effect, is the presence of surface relief, typically in the form of craters and/or boulders. It has been suggested that such non-trivial surface topography can lead to complex electrostatic potentials and fields, including “mini-wakes” behind small obstacles to the solar wind flow and “supercharging” near sunlit-shadowed boundaries (Criswell, D.R., De, B.R. [1977]. J. Geophys. Res. 82 (7); De, B.R., Criswell, D.R. [1977]. J. Geophys. Res. 82 (7); Farrell, W.M., Stubbs, T.J., Vondrak, R.R., Delory, G.T., Halekas, J.S. [2007]. Geophys. Res. Lett. 34; Wang, X., Horányi, M., Sternovsky, Z., Robertson, S., Morfill, G.E. [2007]. Geophys. Res. Lett. 34, L16104). In this paper, we present results from a three-dimensional, self-consistent, electrostatic particle-in-cell code used to model the dayside near-surface lunar plasma environment over a variety of local times with the presence of a crater. Additionally, we use the particle-in-cell model output to study the effect of surface topography on the dynamics of electrostatic dust transport, with the goal of understanding previous observations of dust dynamics on the Moon and dust ponding on various asteroids.     

Determination of dust properties and magnetic fields from polarimetric and photometric observations of stars (Review)

Verschiedene Modelle zur Darstellung der interstellaren Extinktion und Polarisation werden betrachtet. Dabei wird den Ausrichtungsmechanismen, wie dem Davies-Greenstein-Mechanismus und der Ausrichtung durch suprathermische Rotation, und ihren Näherungen besondere Aufmerksamkeit zu teil. Es wird geschlußfolgert, daß die Größe der Staubteilchen sowie die Starke und Richtung des interstellaren Magnetfeldes aus den Extinktions- und Polarisationsdaten abgeschätzt werden können. Die Anwendung der theoretischen Ergebnisse auf Sterne in der galaktischen Ebene wird erörtert.     

Effects of two-temperature electrons, external oblique magnetic field, concentration of charged dust grains and higher-order nonlinearity on dust ion-acoustic solitary waves in Saturn's E-ring

The purpose of the present work is to investigate some nonlinear properties of the dust ion-acoustic (DIA) solitary waves in a four-component hot-magnetized dusty plasma consisting of charged dust grains, positively charged ions and two-temperature isothermal electrons. Applying a reductive perturbation theory, a nonlinear Korteweg-de Vries (KdV) equation for the first-order perturbed potential and a linear inhomogeneous KdV-type equation for the second-order perturbed potentials are derived. Stationary solutions of these coupled equations are obtained using a renormalization method. A method based on energy consideration is used to obtain a condition for stable solitons. The effects of two different types of isothermal electrons, external oblique magnetic field, concentration of negatively (positively) charged dust grains and higher-order nonlinearity on the nature of the DIA solitary waves are discussed. The numerical results are applied to Saturn's E-ring.     

Fractal properties of solar magnetic fields

We study the spatial properties of solar magnetic fields using data from the Solar Vector Magnetograph of the Marshall Space Flight Center (MSFC) (FeI 5250.2 Å) and SOHO/MDI longitudinal magnetic field measurements (Ni 6767.8 Å) (96-min full-disk maps). Our study is focused on two objects: the fractal properties of sunspots and the fractal properties of the spatial magnetic field distribution of active and quiet regions considered as global structures. To study the spatial structure of sunspots, we use a well-known method of determining the fractal dimension based on an analysis of the perimeter—area relation. To analyze the fractal properties of the spatial magnetic field distribution over the solar surface, we use a technique developed by Higuchi. We have revealed the existence of three families of self-similar contours corresponding to the sunspot umbra, penumbra, and adjacent photosphere. The fractal coefficient has maxima near the umbra—penumbra and penumbra—photosphere boundaries. The fractal dependences of the longitudinal and transverse magnetic field distributions are similar, but the fractal numbers themselves for the transverse fields are larger than those for the longitudinal fields approximately by a factor of 1.5. The fractal numbers decrease with increasing mean magnetic field strength, implying that the magnetic field distribution is more regular in active regions.     

Electrical stress and strain in lunar regolith simulants

Experiments to entrain dust with electrostatic and fluid-dynamic forces result in particulate clouds of aggregates rather than individual dust grains. This is explained within the framework of Griffith-flaw theory regarding the comminution/breakage of weak solids. Physical and electrical inhomogeneities in powders are equivalent to microcracks in solids insofar as they facilitate failure at stress risers. Electrical charging of powders induces bulk sample stresses similar to mechanical stresses experienced by strong solids, depending on the nature of the charging. A powder mass therefore “breaks” into clumps rather than separating into individual dust particles. This contrasts with the expectation that electrical forces on the Moon will eject a submicron population of dust from the regolith into the exosphere. A lunar regolith will contain physical and electrostatic inhomogeneities similar to those in most charged powders.     

Sunspots and large-scale magnetic fields at various heights

Height variation of the magnetic field structure over groups of sunspots for heights ranging from the photosphere to the source surface (R = 2.5 Ro, where Ro is the radius of the Sun) is examined. For all heights, starting from the photospheric level, groups of sunspot are shown as being independent of long-lived boundaries of large-scale structures rotating with a period shorter than the Carrington period. At heights of 1–1.5 Ro, there is a clear relation between sunspot groups and boundaries separating the head and tail sunspots in the groups (the Hale boundaries). The rotation periods of these structures are close to the Carrington period, their lifespan being less than three to five rotations. The maximal intensity of the solar magnetic field drops by two orders when height increases from H = 1 to H = 1.1 Ro. Further decrease in intensity proceeds gradually (dropping by one order from H = 1.1 to 2.5 Ro). The results obtained can be considered as evidence that large-scale magnetic field structures and long-lived boundries between them (the lines dividing polarities of the magnetic field or zero lines) all exist irrespective of sunspot fields being generated by other sources than sunspots. At the photospheric level, active regions fields are superimposed on these structures.     

Effects of vapor-phase deposition processes on the optical,chemical, and magnetic properties OE the lunar regolith

The processes of solar wind sputtering and meteoritic impact vaporization have created materials in the lunar regolith which were deposited from a vapor phase. Although the quantity of such exotic condensed substances should theoretically be comparable with that of materials which have been melted by impacts, their existence in the fines has not been generally recognized. We have investigated the physical and chemical properties of materials deposited from vapors generated by hydrogen-ion sputtering and thermal evaporation of lunar and artificial ferrosilicates. Both processes are highly reducing. The deposits are enriched in Fe, have large, nonselective, optical absorptivities, and contain abundant sub-microscopic, superparamagnetic grains of metallic Fe which exhibit the characteristicg=2.1 ESR resonance. The sputter-deposited films are enriched in heavy elements. Thus the hypothesis that the lunar fines contain several percent of materials deposited from the vapor phase accounts in a natural manner for many of the unusual optical, physical and chemical properties of lunar soils. The vapor-deposits are probably concentrated in the agglutinate particles of the regolith.     

Gas kinetics and dust dynamics in low-density comet comae

Extensive regions of low-density cometary comae are characterized by important deviations from the Maxwell-Boltzmann velocity distribution, i.e. breakdown of thermodynamic equilibrium. The consequences of this on the shapes of emission and absorption lines, and for the acceleration of solid bodies due to gas drag, have rarely been investigated.These problems are studied here to aid in the development of future coma models, and in preparation for observations of Comet 67P/Churyumov-Gerasimenko from the ESA Rosetta spacecraft. Two topics in particular, related to Rosetta, are preparation for in situ observations of water, carbon monoxide, ammonia, and methanol emission lines by the mm/sub-mm spectrometer MIRO, as well as gas drag forces on dust grains and on the Rosetta spacecraft itself.Direct Simulation Monte Carlo (DSMC) modeling of H₂O/CO mixtures in spherically symmetric geometries at various heliocentric distances are used to study the evolution of the (generally non-Maxwellian) velocity distribution function throughout the coma. Such distribution functions are then used to calculate Doppler broadening profiles and drag forces.It is found that deviation from thermodynamic equilibrium indeed is commonplace, and already at 2.5 AU from the Sun the entire comet coma displays manifestations of such breakdown, e.g., non-equal partitioning of energy between kinetic and rotational modes, causing substantial differences between translational and rotational temperatures. We exemplify how deviations from thermodynamic equilibrium affect the properties of Doppler broadened line profiles. Upper limits on the size of liftable dust grains as well as terminal grain velocities are presented. Furthermore, it is demonstrated that the drag-to-gravity force ratio is likely to decrease with decreasing cometocentric distance, which may be of relevance both for Rosetta and for the lander probe Philae.     

The dust trail of Comet 67P/Churyumov-Gerasimenko between 2004 and 2006

We report on observations of the dust trail of Comet 67P/Churyumov-Gerasimenko (CG) in visible light with the Wide Field Imager at the ESO/MPG 2.2 m telescope at 4.7 AU before aphelion, and at with the MIPS instrument on board the Spitzer Space Telescope at 5.7 AU both before and after aphelion. The comet did not appear to be active during our observations. Our images probe large dust grains emitted from the comet that have a radiation pressure parameter β<0.01. We compare our observations with simulated images generated with a dynamical model of the cometary dust environment and constrain the emission speeds, size distribution, production rate and geometric albedo of the dust. We achieve the best fit to our data with a differential size distribution exponent of −4.1, and emission speeds for a β=0.01 particle of 25 m/s at perihelion and 2 m/s at 3 AU. The dust production rate in our model is on the order of 1000 kg/s at perihelion and 1 kg/s at 3 AU, and we require a dust geometric albedo between 0.022 and 0.044. The production rates of large (>) particles required to reproduce the brightness of the trail are sufficient to also account for the coma brightness observed while the comet was inside 3 AU, and we infer that the cross-section in the coma of CG may be dominated by grains of the order of .     

Spectral properties of hydrogen,helium, methane,and ammonia at thermal infrared wavelengths

We present some results of a theoretical and laboratory program to determine the thermal infrared spectral properties of the principal gaseous constituents of the atmosphere of Jupiter. G. Birnbaum has measured laboratory spectra in the 16 to 1000 um wavelength range for hydrogen and hydrogen-helium mixtures at Jovian temperatures. These are compared to theoretically computed spectra in order to determine the temperature dependence of the line strengths in the pressure-induced rotational band and the overlap parameters from the translational band. Existing spectral data for methane do not agree well with measurements of the ν₄ band at room temperature. A revised allocation of line intensities is proposed. Existing data for the ν₂ (10 um) band of ammonia do agree reasonably well with measurements at room temperature and at ?77δC, but there are some important discrepancies which remain to be explained.