Laboratory testing and data analysis of the Electrostatic Lunar Dust Analyzer (ELDA) instrument
Highlights
► Electrostatic Lunar Dust Analyzer (ELDA) instrument has been built. ► We improve the numerical data analysis of ELDA. ► ELDA is tested under vacuum using calibrated dust particles. ► ELDA can measure the mass of individual particle with a factor of two.
Introduction
The Electrostatic Lunar Dust Analyzer (ELDA) instrument is developed for future lunar landing missions to detect mobilized dust particles near the lunar surface. The ELDA instrument measures the velocity, charge, and mass of individual slow-moving dust. ELDA consists of two Dust Trajectory Sensor (DTS) units integrated with the electrostatic Deflection Field Region (DFR). The two DTS units measure the charge and velocity vector of the dust grain before and after it enters the DFR; and the grain’s mass is determined from the velocity change. Xie et al. (2011) have developed a numerical method for analyzing DTS measurements for straight-line trajectories by fitting the data to computer simulations. Duncan et al. (2011) performed the numerical analysis and optimization of the DFR. Also, a simplified ELDA prototype was tested with one DTS on air to demonstrate the measurement principles. In this article, we include the effect of gravity in the DTS data analysis and introduce a faster method for fitting the data. Further, a full version of the ELDA instrument is constructed and tested in a vacuum chamber that allows the measurement of the masses of individual dust particles.
The science and motivation for the ELDA instrument are described in detail in the preceding paper by Duncan et al. (2011); here only a brief description is given. Besides robotic and human activities, dust particles may also be mobilized due to natural processes (Grün et al., 2011). The continual bombardment of the lunar surface by micrometeoroids (Hoffmann et al., 1975, Allison and McDonnell, 1981, Zook et al., 1984) and potentially electrostatic forces (Horanyi, 1996, Colwell et al., 2007) can release dust particles from the surface. Micrometeoroids impacting airless bodies release secondary particles, a process which has been observed and characterized for the Galilean moons of Jupiter by the Galileo spacecraft (Krüger et al., 1999, Krüger et al., 2000, Krüger et al., 2003). The same process will produce dust clouds around other airless objects, such as the Moon. However, ejecta clouds around the Moon have not been observed yet due to the lack of dust detectors on past missions or insufficient sensitivity. The Lunar Dust EXperiment (LDEX) instrument is scheduled for launch onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission in 2013 (Horanyi et al., 2009). LADEE will orbit the moon and characterize the Moon's dust environment from orbit with occasional dips down to 30 km altitude. The LDEX instrument is designed for high sensitivity and capable of detecting sub-micron-sized dust particles from orbit.
The possibility of the electrostatic lofting, levitation, and transport of lunar dust has been indicated from the observations of the lunar horizon glow by the Surveyor landers (McCoy and Criswell, 1974, McCoy, 1976, Zook and McCoy, 1991) and the dust events recorded near sunrise and sunset by the Lunar Ejecta And Meteorites (LEAM) instrument left behind by Apollo 17 (Berg et al., 1973, Berg et al., 1974). Photoelectron and plasma sheaths formed above the lunar surface and the dust dynamics in the sheaths were theoretically studied and simulated (Nitter and Havnes, 1992, Nitter et al., 1998, Colwell et al., 2009, Poppe and Horanyi, 2010, Poppe et al., 2011). An analytical model of dust launching on the Moon and asteroid surfaces was recently developed by taking into account cohesive forces between dust particles (Hartzell and Scheeres, 2011). Laboratory experiments have been also conducted to investigate the physics of the electrostatic dust transport on the lunar surface. Charged dust particles were levitated in a plasma sheath due to the electrostatic force balancing the gravitational force (Sickafoose et al., 2002). Dust grains were released from surfaces exposed to plasma in the presence of an electron beam (Sheridan et al., 1992, Flanagan and Goree, 2006). Dust transport on various surfaces in plasma were demonstrated and studied, including: a) the dust spreading and hopping on a surface that repels electrons and collects ions (Wang et al., 2009, Wang et al., 2011a), b) the dust transport as a consequence of differential charging on surfaces that have different secondary electron yields (Wang et al., 2010), and c) the dust movement near the topography induced electron beam impact/shadow boundaries (Wang et al., 2011b). In spite of the progress made in both theory and experiments, the charging of dust grains and their subsequent mobilization and transport on the lunar surface still remain elusive.
Regardless of the release mechanism (ejecta particles or electrostatic lofting), only a small fraction of the particles will reach high altitudes to be detected by instruments on orbit, e.g. LDEX on LADEE. Most of the particles released by micrometeoroid impacts have a velocity below 100 m/s (Krüger et al., 2000), which corresponds to an altitude of 3 km (310 m/s corresponds to 30 km). In simplistic models for electrostatic lofting, unrealistically large charge on the dust and surface potentials are needed to elevate dust particles to high altitude (Stubbs et al., 2006). Therefore, it is necessary to detect and analyze dust particles on or near the surface. ELDA is designed to detect particles in the velocity range from approximately 1 m/s, required for the dust particle to enter the instrument, to about 100 m/s, which allows ELDA to sample the majority of the mobilized dust.
The most sensitive method for detecting slow-moving dust particles is measuring their charge. Lunar dust grains are charged by both the lunar plasma environment (Horanyi, 1996, Wang et al., 2007) and triboelectricity upon release from the surface (Sternovsky et al., 2001, Sternovsky et al., 2002). Detection methods based on momentum transfer or impact ionization are not applicable because of diminishing signal levels with decreasing velocity (Auer and Sitte, 1968). The measurement of the dust charge alone is not sufficient for the understanding of the near-surface dust environment as the charge may not be in equilibrium with the environment, and thus the size/mass of the dust remains uncertain. The flight time of the particles may not be long enough to equilibrate with the local plasma environment, and the variation of the plasma density and electron temperature with altitude may be unknown (Sternovsky et al., 2001). The particles, on the other hand, may be highly charged by triboelectricity.
The article is organized as follows. Section 2 is a description of the ELDA instrument and the calibration measurement performed in a large vacuum chamber by dropping individual dust particles into ELDA. Section 3 describes the ELDA data analysis, including: a) the numerical analysis procedure for DTS with gravity, b) the process for fitting measurements to simulations, c) the numerical model of the DFR for calculating the E-field distribution, and d) the algorithm to derive the mass based on trajectory information measured by the two DTS units. In Section 4 the numerical data analysis is applied to the experimental data and results are discussed. Section 5 presents the summary and conclusions.
Section snippets
ELDA description
The ELDA instrument concept has been described in detail by Duncan el al. (2011), and here only a brief description is given. The ELDA measures the velocity, charge, and mass of individual dust particle. ELDA consists of two Dust Trajectory Sensor (DTS) units integrated with the Deflection Field Region (DFR); see Fig. 1 for the schematics. The two DTS units measure the charge and velocity vector of individual dust particles before and after the particle's trajectory is modified by the DFR (
Case with no gravitational acceleration
The numerical method for analyzing the DTS measurement for straight-line trajectories was developed by Xie et al. (2011). The analysis is based on finding the best fit between the measurements and simulations, and there are seven independent parameters (Q, t1, t4, x1, y2, x3, y4) to be determined. Q is the total charge on the dust particle, (t1, t4) are the times of passing though planes 1 and 4, and (x1, y2, x3, y4) are the respective coordinates where the particle passes through the four
Experimental results
The particles in dust dropper are charged by triboelectrification and can be charged either positively or negatively when released from the surface. The magnitude of the dust charge varies between 0.18–1.92×10–13 C with an average value of 0.56×10–13 C. The corresponding range of charge-to-mass ratio (Q/m) is 2.6–27.7×10−5 C/kg, where m=6.93×10−10 kg is calculated from the average dust size d=108 μm and a density of 1.05 g/cm3. The corresponding surface potentials are 3–32 V. Duncan et al. (2011)
Summary and conclusions
The ELDA instrument is capable of measuring the charge, velocity, and mass of slow-moving individual dust particle relevant for the lunar near-surface environment. A full laboratory prototype version of the ELDA instrument has been built and tested in vacuum. The statistical analysis of the results for E=0 in the DFR can be used to assess the accuracy of the instrument geometry (displacements and tilts) and provide the orientation of the gravity vector with respect to the instrument. The
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