The Electrostatic Lunar Dust Analyzer (ELDA) for the detection and trajectory measurement of slow-moving dust particles from the lunar surface
Highlights
► Dust may be levitated near the lunar surface by meteoroid bombardment or electrostatic forces. ► An instrument is designed and prototype built for the detection an analysis. ► Testing is performed in laboratory conditions. ► The data are analyzed and show that low noise and good performance is achieved.
Introduction
Astronaut operations, robotic activity, micrometeorite bombardment and electrostatic charging of the surface are mechanisms that can eject micron sized dust particles off the lunar surface. The understanding of this mobilization is of great engineering importance for future lunar missions as lunar fines pose a hazard to both equipment and astronauts. The natural processes of dust mobilization remain unresolved and/or poorly characterized. A number of remote sensing observations and in-situ measurements indicate electrostatic dust mobilization resulting from the charging of the lunar surface. These include the images taken by the television cameras on Surveyors 5, 6, and 7, which observed a distinct glow toward the western horizon shortly after sunset, dubbed as the Horizon Glow (HG; Criswell, 1973, Rennilson and Criswell, 1974). A brighter than expected lunar twilight sky was also observed by the astrophotometer on the Lunokhod-2 rover (Severny et al., 1975). Further independent optical observations were made by the Apollo 17 crew and the star-tracker cameras on the Clementine spacecraft. The Apollo crew reported the appearance of bright streamers extending in excess of 100 km above the lunar surface, when approaching the terminator region. These images were analyzed and interpreted as showing signatures of dust particles scattering sunlight at high altitudes above the Moon (McCoy and Criswell, 1974, McCoy, 1976, Zook and McCoy, 1991). The images from Clementine were very similar to those sketched by the astronauts, although the viewing geometries were quite different. Hahn et al. (2002) analyzed the Clementine images as from coronal and zodiacal light and calculated the dust content of the inner solar system. The in-situ Lunar Ejecta And Meteorite (LEAM) instrument was deployed during the Apollo 17 mission on the Moon to measure the interplanetary dust flux bombarding the lunar surface and to investigate the properties of the secondary ejecta particles (Berg et al., 1973). LEAM had three sensors, two pointed approximately towards east and west, and one detector pointed upward. All sensors have detected spurious signals close to the passage of the sunrise and sunset terminators. These findings were subsequently interpreted as being due to slow moving, highly charged lunar dust particles (Berg et al., 1974). Later theoretical and experimental studies confirmed that dust particles with <100 m/s velocity and charge Q>10−12 C could induce signals similar to those measured on the lunar surface. However, there is no understanding of processes that could lead to the ejection of particles that are large enough to carry such a charge. A more detailed review of these phenomena can be found, for example, in the articles by Horanyi (1996) and Colwell et al. (2007). Theoretical models addressing the motion of charged particles in the photoelectron and plasma sheath above the lunar surface has been developed (Nitter and Havnes, 1992, Nitter et al., 1998, Stubbs et al., 2006, Poppe and Horanyi, 2010), but a complete understanding of the charging of grains and their subsequent mobilization and transport remains elusive.
Dust particles released from the lunar surface are expected to be highly charged by triboelectricity and the ambient environment (Sternovsky et al., 2001, Sternovsky et al., 2002, Wang et al., 2007). On the sunlit side, UV induced photoelectron emission is the dominant process. The characteristic charging time can be estimated as τ≈Q/I, where Q is the equilibrium charge on the dust and I is the charging current. When exposed to direct sunlight, particles charge to about +5 V surface potential. Assuming the particles are spherical, this corresponds to charge Q≅700ϕ a, where Q has units of elementary charge e−, surface potential ϕ is in volts and the particle radius a in μm. A dust particle with a radius of 1 μm and a potential of 5 V carries a charge of 3500e−. The photoelectron emission charging current is approximately I≅5 μA/m2 (Sternovsky et al., 2008) and thus the characteristic charging time is about 35 s for a micron sized particle. The transport of particles released from and falling back to the surface takes t≅(2v0/g) seconds, where v0 is the vertical component on the initial velocity and g=1.63 ms−2, is the lunar gravitational acceleration. For v0=100 m/s, the flight time is t≅120 s, which is sufficiently long for reaching the equilibrium charge. For lower initial velocities, smaller particles, or particles elevated over the dark side of the Moon, where the charging is only due to the dilute electron density, the charging time may be longer than the time of flight. In these cases, the charge on the dust will be partly or largely determined to be the initial triboelectric charge, when released from the surface.
Recently, a series of laboratory experiments were conducted to investigate the charging and mobilization of dust under simulated conditions. The levitation of dust particles in plasma sheaths, where the electrostatic forces balance the gravitational force, was achieved by Sickafoose et al. (2002). Dust was observed to collect significant charges on surfaces exposed to plasma and was subsequently transported both horizontally and vertically (Wang et al., 2009). Transported dust was also observed on surfaces having different secondary electron yields in plasma with an electron beam, as a consequence of differential charging (Wang et al., 2010). The filling in of craters to form ‘dust ponds’, as suggested by images taken of the Asteroid Eros, had been shown to be possibly caused by topography, modulating the electron fluxes reaching the surface (Wang et al., in press).
Based on terrestrial measurements, the Moon is expected to be bombarded by about 106 kg/year of interplanetary meteoroids of cometary and asteroidal origin (Love and Brownlee, 1993, Gabrielli et al., 2004). The majority of the projectiles are in the nm to mm size range with a velocity range of 10–72 km/s. Under these conditions, and considering the regolith-type lunar surface, the excavated mass can be ∼1000 times the mass of the projectiles and most of the ejecta material is in the form of secondary dust particles. The velocity distribution of these particles is wide and a small fraction can leave the gravitational field of the Moon (escape velocity 2.3 km/s). Most of the ejecta, however, have a much lower velocity and will return to the lunar surface following ballistic orbits. The micron and submicron-sized ejected particles represent the most abundant population of impactors (Grün et al., 1985). The continual micrometeoroid bombardment thus generates a permanently present dust cloud about the Moon. Such tenuous clouds had been observed by the Galileo spacecraft flying-by the Galilean satellites (Krüger et al., 1999, Krüger et al., 2000, Krüger et al., 2003). The dust impact velocity in this case was approximately 6–8 km/s ensuring sufficient impact charge generation even from submicron-sized dust grains. The orbital velocity around the Moon is much lower (∼1.7 km/s) making the detection of dust impacts more challenging. Due to the lack of sensitive dust instruments in orbit around the Moon or on the surface, the lunar ejecta cloud has not yet been observed or characterized.
According to the model by Krüger et al. (2000), the velocity distribution of ejecta particles is of a form ψ(>u)∝(u/u0)−γ, where u0 is the characteristic lower cut-off velocity. The distribution describes the fraction of the material ejected with velocity>u and is independent of the mass of the particles. Parameter u0 is of a range from a few m/s to several hundred m/s as determined from impact experiments (Stoer et al., 1975, Hartmann, 1985). The slope of the distribution γ is approximately between 1 and 2 (Frisch, 1992). A small fraction of particles ejected with high initial velocity will reach altitudes, where they can be sampled from orbit about the Moon. For example a dust particle released vertically upward with 408 m/s velocity will reach 50 km before falling back to the surface. Most of the particles, however, are ejected with much lower velocity. The analog electronics of the instrument described in this article is optimized for the detection of dust particles from the 1 m/s to 100 m/s velocity range.
The upcoming Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, to be launched in 2013, will be equipped with both a UV–vis spectrometer and a sensitive dust detector for the characterization of the lunar dust environment from orbit about the Moon. Stubbs et al. (2010) recently analyzed the expected performance of the UV–vis instrument and predicted that light scattering by submicron sized dust particles will be readily distinguishable on the background of coronal and zodiacal light and sodium emission.
The population of dust ejected with low velocities can only be characterized by an instrument deployed on the surface. Due to the low mass and low velocity of the near-surface dust, standard measurement methods based on momentum transfer or impact energy cannot be applied. Instead, the most sensitive detection method for these particles is the measurement of the charge they carry. The charge on the dust is from triboelectrification on the surface and/or the ambient plasma and UV environment.
In this article we describe the Electrostatic Lunar Dust Analyzer (ELDA) instrument concept for the measurement of charge, mass and trajectory of micron sized dust particles. ELDA is based on the Dust Trajectory Sensor (DTS), consisting of an array of wire electrodes to measure the trajectory of a charged dust particle passing through the instrument. ELDA consists of two DTS units with a high electric field region in between, which bends the particle’s trajectory. The trajectory is measured before and after the electrostatic deflection in order to determine the particle’s mass. Sensitive electronics have been developed and integrated into the instrument, showing that it is possible to achieve the high sensitivity required to measure the charge on micron sized dust particles. The preliminary version of the instrument is tested in the laboratory by dropping individual charged particles from a dust dropper and the deflection in the high electrostatic field region is measured. Initial data analysis is performed by matching the measurements with numerical calculations.
The article is organized as follows. Section 2 contains the instrument description including the operation principle of ELDA and DTS (Fig. 1), the numerical analysis and optimization of the electrostatic deflection region, the properties of the charge sensitive electronics, and their integration. Section 3 presents the experimental setup and the measured data. The data analysis is described in Section 4, while Section 5 gives the summary and conclusions.
Section snippets
Dust detection principle
Many in-situ methods exist for the detection of dust particles in space. These include pressurized cells, detection of scattered light, induced charge measurements, momentum transfer or impact ionization and they have been reviewed, for example, by Auer (2001). The most sensitive detection method for hypervelocity (>1 km/s) particles is through impact ionization, where the charge generated upon impact on a solid surface is measured. For lower velocities, however, the generation of impact charge
Experimental results
Preliminary experimental testing of the ELDA instrument is performed by dropping small charged dust particles into the experimental setup. Fig. 9 shows the stainless steel dust sample used in the experiments. The particles are 130 μm ±20 μm in diameter and irregular in shape. The charge on the particles is almost exclusively positive and ranges from 10 to 150 fC with an average value of fC. This approximately corresponds to 6×104–106 elementary charges, 1.5–23 V surface potential or
Data analysis and numerical modeling
The terminal velocity of the falling particles is set by the balance between the gravitational force FG=mg and the drag force FD, where m is the mass of the particle and g=9.81 ms−2 the gravitational acceleration. The drag force can be calculated for an assumed spherical shape. The Reynolds number is RN=ρAvD/μ≅8, where ρA=1 kg/m2 is the density of air at room temperature, D=130 μm is the typical diameter of the particles and μ=1.84×10−5 kg m−1 s−1 is the dynamic viscosity of air. In this range, the
Summary and conclusions
An instrument capable of detecting low velocity dust particles is needed for future space missions, for example on the surface of the Moon or asteroids. Methods based on impact ionization or momentum transfer are not sensitive enough to detect micron sized particles with <100 m/s velocity. The most viable detection method is to utilize the charge on the dust. The previously developed DTS instrument is capable of both detecting the small charge and measuring the velocity vector of the dust. In
Acknowledgment
The authors acknowledge the support from NASA through the Planetary Instrument Definition and Development (PIDD) and NASA Lunar Science Institute (NLSI) programs.
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