Electrostatic Mechanics of Dust Lofting and Transport on Airless Planetary Bodies

Transcription

1 University of Colorado, Boulder CU Scholar Undergraduate Honors Theses Honors Program Spring 2017 Electrostatic Mechanics of Dust Lofting and Transport on Airless Planetary Bodies Joseph Schwan Follow this and additional works at: Part of the Other Physics Commons, Physical Processes Commons, and the Plasma and Beam Physics Commons Recommed Citation Schwan, Joseph, "Electrostatic Mechanics of Dust Lofting and Transport on Airless Planetary Bodies" (2017). Undergraduate Honors Theses This Thesis is brought to you for free and open access by Honors Program at CU Scholar. It has been accepted for inclusion in Undergraduate Honors Theses by an authorized administrator of CU Scholar. For more information, please contact

2 ELECTROSTATIC MECHANICS OF DUST LOFTING AND TRANSPORT ON AIRLESS PLANETARY BODIES by Joseph Schwan In Partial Fulfillment of the Requirements for the Honors Degree in Engineering Physics at the University of Colorado, Boulder Defense Date: [April 3, 2017] Prof. Mihàly Horànyi, Advisor School of Physics University of Colorado, Boulder Dr. Xu Wang IMPACT Lab at LASP Laboratory of Atmospheric and Space Physics Research Scientist Prof. Tobin Munsat, Honors Committee Member School of Physics University of Colorado, Boulder Prof. Todd Murray, External Department School of Mechanical Engineering University of Colorado, Boulder

3 ELECTROSTATIC MECHANICS OF DUST LOFTING AND TRANSPORT ON AIRLESS PLANETARY BODIES Schwan, Joseph (B.S., Engineering Physics) Thesis directed by Prof. Mihàly Horànyi with assistance of Dr. Xu Wang. ABSTRACT As airless planetary bodies are directly exposed to solar ultraviolet radiation and plasma, an environment that fosters dust charging within planetary regolith will result, producing dust mobilization and transport. A multitude of unsolved in-situ planetary observations linked to electrostatic processes exist, ranging from dust ponds on the asteroid Eros [1], to intermittently appearing radial spokes in Saturn s rings [2,3], and even to the lunar horizon glow [4,5]. Available charging models could not adequately explain any of these phenomena. Through careful experimentation a new Patched Charge Model has been developed at the NASA/SSERVI s Institute for Modeling Plasma, Atmospheres and Cosmic Dust (IMPACT) at the University of Colorado at Boulder [6]. Experiments have proven that emission and re-absorption of photoelectrons and/or secondary electrons at the walls of dust microcavities generate large negative charge patches and particle-particle repulsive forces which mobilize dust particles. Experimentation focused on micron sized particles under conditions of ultraviolet illumination, electron-beam, and plasma with electron-beam. Dust was lofted to heights exceeding 2.5 cm, which translates to lofting heights of the same order of magnitude required to produce the lunar horizon glow [4,7]. Contrary to previous charging models and consistent with the patched charge model, dust under all three conditions were measured as having large negative net charges. Surface ii

4 mobilization characterizing experiments are underway, investigating magnetic field interaction and surface morphology changes. Greater understanding of electrostatic dust mobilization and transport will enhance current understanding of surface evolution of airless bodies and is expected to assist in explaining unconsidered surface processes. Keywords: dust, charging, electrostatic, transport, lofting, mobilization, planetary, electron-beam, plasma, ultraviolet. iii

5 [Dedicated to the dust that was wiped away and thrown in the trash.] iv

6 ACKNOWLEDGEMENTS To begin, I would like to thank my mentor Dr. Xu Wang for his guidance and patience throughout this research, without which I would have been completely at a loss. I would also like to thank my advisor Professor Mihàly Horànyi for affording me the opportunity to work in his lab and supplying assistance and council that pushed for exploration into new facets within this research. My thanks also exts to the rest of my thesis committee in Professor Tobin Munsat and Professor Todd Murray. I would also like to thank everyone in the IMPACT lab that has helped me over the years, from advice, to motivation, to ling a helping hand. Thank you all. This work was supported by the National Aeronautics and Space Administration (NASA)/Solar System Exploration Research Virtual Institute s (SSERVI) Institute for Modeling Plasma, Atmospheres, and Cosmic Dust (IMPACT). Partial support is also acknowledged by the contract JPL at the University of Colorado from Rosetta, which is a European Space Agency (ESA) mission with contributions from its member states and NASA. This work was partially inspired by discussions within International Team 336: Plasma Surface Interactions with Airless Bodies in Space and Laboratory at the International Space Science Institute, Bern, Switzerland. v

7 TABLE OF CONTENTS ABSTRACT ACKNOWLEDGEMENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS AND ABBREVIATIONS ii v viii ix xi CHAPTER 1. Introduction Research Motivation Previous Charging Models The Shared Charge Model Adjustments Affecting Forces 4 CHAPTER 2. A New Charging Mechanism Experimental Equipment Standard Experimental Apparatus Standard Experimental Conditions Equipment and Components Used Comparative Experiments Plasma with and without Electron Beam Conditions Surface Potential Difference Solely Electron Beam Condition Dust Transport Observations The Patched Charge Model Proposed Charging Mechanism Cavity Effect Replication Test Patched Charge Model Predictions 19 CHAPTER 3. Charge Properties Dust Charge Polarity Polarity Experiment Setup Polarity Experiment Results Net Dust Charge Magnitude Net Charge Magnitude Experiment Setup Net Charge Magnitude Experiment Results Predictions vs. Measurements 25 CHAPTER 4. Dust Lofting Lofted Particle Size Analysis Particle Lofting Tests Analysis Program 27 vi

8 4.1.3 Particle Sizes Lofted Particle Launch Velocities and Peak Heights Particle Velocity Analysis Program Lofted Particle Velocities In-Situ Lofting Implications 35 CHAPTER 5. Surface Mobilization Prolonged Surface Feature Change Due to Dust Mobilization Size Depence of Dust Mobilization Geometry Depence of Dust Mobilization Composition Depence of Dust Mobilization Magnetic Field Experiments Roughness Measurement 42 CHAPTER 6. Conclusions General Conclusions Ongoing Research 45 APPENDIX A. Code Implimented for analysis 46 A.1 Particle Size Distribution Analysis Code 46 A.2 Particle Velocity Analysis Code 48 A.3 Surface Roughness Analysis Code 51 REFERENCES 54 vii

9 LIST OF TABLES Table 1 Charge and force calculations for previous models in laboratory and lunar conditions. Table 2 Charge and force calculations with the patched charge model for UV, electron beam, and plasma and electron beam conditions viii

10 LIST OF FIGURES Figure 1 Surveyor 7 images of the lunar horizon glow. 2 Figure 2 Standard experimental setup in vacuum chamber. 8 Figure 3 Electric field profiles above dust surface in bottom and top filament experimental setups. Figure 4 Horizontal potential scans 1 mm above dust surface in bottom and top filament experimental setups. Figure 5 Images of dust transport and lofting trajectories in a) UV, b) electron beam, and c) plasma and electron beam experiments. Figure 6 Schematic of dual disk Langmuir probe experimental setup for patched surface potential measurements. Figure 7 Schematic of dual disk Langmuir probe experimental setup for patched surface potential measurements. Figure 8 Charge polarity test of silica microspheres after exposure to electron beam conditions Figure 9 - Diagram of experimental setup for net charge experiments. 23 Figure 10 Histograms of particles' net charge magnitude resulting from twelve tests in each listed condition. Figure 11 Measured and predicted mean net particle charge magnitudes with standard deviations. Figure 12 Example of a) focally cropped image, and b) subtracted image, similar to those used for particle size calculation. Figure 13 Size distributions of lofted dust particles after exposure to UV illumination or plasma and electron beam conditions. Figure 14 Example of trajectory analysis program with: green - new particle detected, pink - particle in transit detected, and red - particle no longer present Figure 15 Example of frame by frame particle tracking by hand. 32 ix

11 Figure 16 Measured lofted particle height as a function of initial vertical speed of dust particle. Figure 17 Mars simulant m in diameter smoothing over time when exposed to UV illumination conditions Figure 18 The dust ponding phenomenon observed on the asteroid Eros. 38 Figure 19 Mars simulant m (top), m (middle), and < 38 m (bottom) in diameter after 50 minutes of exposure to UV illumination. Figure 20 Similarly sized irregular and spherical silica dust particles exposed to electron beam conditions over time. Figure 21 Mars simulant m in diameter without (top row) and with (bottom row) a magnetic field before (left column) and after (right column) exposure to UV illumination conditions for 2 hours. Figure 22 Dust surface roughness as quantified by area in shadow produced from a horizontal light source x

12 LIST OF SYMBOLS AND ABBREVIATIONS UV SE Ultraviolet Light Secondary Electron MATLAB Matrix Laboratory RGB fps Red/Green/Blue Image Matrix Frames per Second xi

13 CHAPTER 1. INTRODUCTION The material presented throughout this thesis will cover a range of topics, from the unexplained observed phenomena that prompted the investigation, to the value and implications of the mechanism responsible. Data will be presented and interpreted in following chapters, however within this chapter the motivation for the following research will be given in addition to previous charging models. 1.1 Research Motivation In 1960 s during the Apollo missions, a distinct unpredicted glow was observed above the dusk horizon by the Surveyor 5, 6, and 7 cameras on the lunar surface (Figure 1). A planetary body will typically experience a horizon glow at sunrise or sunset due to its possession of an atmosphere to scatter the incoming solar light, however as the moon lacks such an atmosphere this glowing was unexplained. In lieu of an atmosphere, dust would be capable of scattering light in a way that could account for the glow, though this would present new problems. Forward scattered light from a cloud of dust averaging ~10 micron in diameter at around 1 m above the surface is the current supposition for what causes the glow. The resulting question from this model is how are the dust particles lofted in a way that can sustain such conditions. 1

14 Figure 1 Surveyor 7 images of the lunar horizon glow. Secondary ejecta from micrometeoroid bombardment was ruled out due to the long duration and high intensity of the glow, which left the dominant theory to be electrostatic dust lofting resulting from the photoelectron sheath above the lunar surface. The lunar horizon glow as a phenomena likely due to dominant electrostatic forces is not alone. For instance, the intermittently appearing radial spokes within the rings of Saturn first observed in the Voyager missions [2,3], the high-altitude ray-pattern streamers above the lunar surface reported by Apollo astronauts [8,9], and the smooth deposits of fine grain dust known as dust ponds accumulating in the craters like on the asteroid Eros [1] and comet 67P [10] as observed by the Rezvous-Shoemaker and Rosetta missions respectively are all theorized to involve electrostatic dust charging and interactions. 2

15 Likewise are the unexpectedly smooth surface of Saturn s icy moon Atlas [11], highly porous surfaces of asteroids [12], and dust release from comet 67P at its low activity level [13]. Even phenomena such as the feldspathic material indicated on high-albedo markings on the lunar surface [14] and the lunar dusty exosphere [15] have been tied to electrostatic charging. It has even been suggested that electrostatic dust activity may be responsible for the degradation of lunar surface reflectors [16]. Modeling of these phenomena have posed problems as the initial conditions which dictate how much charge a dust particle will receive and how the dust particle will react with its surroundings requires an understanding of how the particle and its surroundings are charged. Several dedicated laboratory experiments had previously demonstrated dust particle mobilization and lofting can be produced from surfaces exposed to varying plasma environments [17-21]. However, none were able to provide an adequate explanation of the charging mechanism. The patched charge model provides viable and experimentally confirmed predictions of these initial conditions, and will assist in the accurate modeling of dusty phenomena. 1.2 Previous Charging Models The Shared Charge Model As stated in Section 1.1 there have been previous models for dust charging on airless planetary bodies, however none of them have been able to fully account for lofting effects or give adequate reason for alternate phenomena like ponding. The dominant theory until recently was the so-called shared charge model [17], which assumes each dust 3

16 particle on a surface will acquire the same charge. Gauss Law provides an expression for the charge on a dust particle as shown below in Equation 1, Q = 4πε 0 a 2 E (1) where ε 0 is the vacuum permittivity constant, a is the radius of a dust particle, and E is the sheath electric field at the dusty surface which will be inversely proportional to the Debye length λ De Adjustments In order to compensate for discrete electron and ion fluxes applied to dusty surfaces, the shared charge model is adjusted slightly to accommodate charge fluctuation theory [21]. The dictating equation for charge fluctuation theory is shown below in Equation 2, δq rms e = CT e e (2) where δq rms is the root-mean-squared magnitude of the charge fluctuation, e is the elementary charge (the charge magnitude of an electron or proton), C is the capacitance of an isolated particle and is defined as C = 4πε 0 a, and T e is the electron temperature. This theory effectively predicts slightly larger charges will be collected on particle surfaces which will in turn allow for greater inter-particle repulsion and greater sheath effects. Calculated values for these models under laboratory conditions and lunar conditions can be seen in Table Affecting Forces 4

17 With particle charges accounted for the forces acting on each particle are as follows. The electric field force resulting from the plasma sheath, F e = QE, (3) where Q is the net charge of an individual particle and E is the electric field observed by the particle. Next, the repulsive force between two identically charged particles, F c = 1 4 πε 0 ( Q 2a ) 2, (4) where ε 0 is the vacuum permittivity constant and a is the average radius of a dust particle. Following that is the gravitational force acting on a particle, F g = mg, (5) where m is the mass of a dust particle and g is the gravitational acceleration. The gravitational acceleration is dictated by the equation, g = GM r 2, (6) where G is the gravitational constant, M is the mass of the planetary body the particle is resting on, and r is the distance from the centre of the massive body planet. Cohesive forces between particles is more complicated due to inconsistent surface contact between particles and will be estimated using the following equation, 5

18 F co = CS 2 (2a), (7) where C = D m 2, D is the Hamaker constant ( J for lunar dust), and S is an index of the particle cleanliness (~0.13 in Earth s atmosphere and 0.88 on the lunar surface during the day [23]). Cohesive forces estimated for spherical particles are predicted to be similar in magnitude as the net electrostatic force, as shown in previous experiments [24]. A visualization of the acting forces can be seen in Figure 6a and the force calculation results are shown below in Table 1. Table 1 Charge and force calculations for previous models in laboratory and lunar conditions. Parameters Laboratory case (shared charge model) Laboratory case (charge fluctuation theory) Lunar case (dayside) E (V/m) a (μm) Q (C) Fe (N) Fc (N) Fg (N) Fes / Fg As shown in the last row of Table 1, the forces acting on a dust particle with either applied model fall several orders of magnitude short of overcoming the gravitational force alone. When considering that lofted or shifting particles will also have to overcome cohesion forces it becomes evident that neither model can accurately account for past observations or experiments. 6

19 CHAPTER 2. A NEW CHARGING MECHANISM In this chapter the new patched charge model, the experiments that led to its proposal, and the consequences it has will be described in detail. The standard experimental setup and equipment will also be described in order to provide a greater understanding of all experiments outlined in this thesis. 2.1 Experimental Equipment Standard Experimental Apparatus The majority of experiments described in this thesis were carried out in a stout cylindrical vacuum chamber, 50 cm in diameter and 28 cm tall made out of stainless steel. The chamber is lined with a graphite coated removable copper sheet to reduce chamber effects. In addition to having four multi-use ports on the sides of the chamber, the top of the chamber contains five multi-use ports, and the bottom contains one centered multi-use port. Both the top center port and the bottom port are dedicated to the experiment emission sources. Varying types, sizes, and mixtures of dust will be used during experiments, however all samples will be contained in a rubber crater, a sheet of rubber 2 mm tall with a 1.9 cm diameter hole used as a container. Both dust and container will be placed atop a plate, typically made of graphite that can have its electric potential controlled or left floating. Experiments involving a different plate substrate will note the material used as well as the reason for exchanging the material. 7

20 Figure 2 Standard experimental setup in vacuum chamber. Emission sources used throughout experimentation switch between a hot tungsten filament and a xenon excimer UV lamp with a peak spectral intensity at the 172 nm wavelength with a spectral width of 14 nm FWHM. The tungsten filament is used to emit 120 ev beam electrons which upon contact with any neutral gaseous atom will produce a plasma. Argon gas will be used to pressurize the chamber during experiments and will be the dominant form of plasma. Finally, most experiments will contain video data recorded through one of the multi-use ports with either the commercially available Sony Panasonic HC-VX870 running at 30 frames per second (fps) or the high speed Phantom V2512 running at 5000 fps. Experiments using the Phantom V2512 will be noted as doing so. A representation of this setup can be seen in Figure Standard Experimental Conditions 8

21 With the confinement now defined, four conditions will be used repeatedly in experiments described throughout this thesis, these conditions consist of: 1) Plasma and Electron Beam: argon gas maintaining a controlled pressure ~0.5 mtorr with emission of ~120 ev electrons from a hot tungsten filament at varying emission currents while in the top filament configuration, 2) Electron Beam: pressures lower than 10-5 Torr with emission of ~120 ev electrons from a hot tungsten filament at various emission currents while in the top filament configuration, 3) UV Illumination: pressures lower than 10-5 Torr with photon emission peaking at a wavelength of 172 nm, 4) solely thermal plasma: argon gas maintaining a controlled pressure of ~0.5 mtorr with emission of ~120 ev electrons through the use of a hot tungsten filament emitting from the bottom configuration so that the plate that the dust was resting on blocked any electron beam effects Equipment and Components Used Data used throughout this thesis was collected primarily through video recordings and analysis using the two cameras listed above (Phantom V2512 and Sony Panasonic HC- VX870), however probes and alternate experimental apparatus were used for investigations into specific properties. All probes used (dual-disk Langmuir probe, emissive probe, polarization grid, and electrometer) were constructed, tested, and calibrated within the lab, some being designed and built specifically for an experiment. Similarly probe circuits such as the emissive probe and Langmuir probe circuits used to collect data were constructed within the lab. Other utilized equipment such as power supplies, data acquisition cards, pumps, chambers, and components like the xenon excimer UV lamp were commercially produced. 9

22 2.2 Comparative Experiments In order to isolate a cause for dust transport, several tests were carried out and compared using the standard setup of dust (in this case JSC-1 mars simulant m in diameter) held in a 2 cm diameter, 2 mm deep rubber hole placed atop a graphite plate Plasma with and without Electron Beam Conditions For these tests hot tungsten filament was placed above or below the dust as shown in Figure 2 producing an electron beam and an argon thermal plasma resulting from neutral argon atoms colliding with beam electrons. Under the top filament configuration dust particles would be simultaneously exposed to a 120 ev electron beam and an argon thermal plasma at a pressure of 1.0 mtorr, while the bottom filament configuration would only be exposed to the argon thermal plasma as the plate supporting the dust and its container would block the electron beam. When vertical scans were taken at the dusty surface under each condition they appeared to have roughly the same electric fields at around 16 V/cm as shown in Figure 3, much larger than the previous photoelectron sheath estimate of 0.5 V/cm [25]. 10

23 Figure 3 Electric field profiles above dust surface in bottom and top filament experimental setups. Dust was not observed to move while exposed to solely plasma through the use of a bottom filament which is consistent with previous experiments [17]. Despite otherwise identical conditions, dust motion was only observed during the top filament configuration which suggested the necessity for additional investigation into conditions with and without direct electron beam exposure. It should also be noted that exposure to solely plasma not producing motion Surface Potential Difference To determine the apparent effect of exposure to an electron beam on a dusty surface the conditions described above in Section were repeated for horizontal emissive probe scans. Horizontal scans using an emissive probe were taken at a height 1 mm above the dust surface and scanning on an arced path, tracing over the graphite plate, the dust, and its 11

24 containment. As shown below in Figure 4, the solitary thermal plasma effects from the bottom filament did not cause any major difference in potential within the dust region as opposed to the solid graphite plate. However, the potential observed by the electron beam and thermal plasma showed a large potential difference with the potential above the solid surface being 17 V more positive than over the dusty surface. This change in potential indicates a lower SE emission yield from the dusty surface. Figure 4 Horizontal potential scans 1 mm above dust surface in bottom and top filament experimental setups. Additional tests were done with silica (SiO2) dust in covered and smooth solid surface configurations (i.e. the same material with different apparent roughness) at identical plasma conditions. The potential above the solid silica (glass) was nearly 14 V more positive than that of the dusty surface where emitted SEs could be captured by neighboring dust particles [18, 22, 26, 27]. 12

25 2.2.3 Solely Electron Beam Condition As the top filament configuration produced motion and the differentiating factor was the presence of direct surface exposure to an electron beam, the different methods of initializing the electron beam conditions were tested. The standard filament conditions produced a 120 ev, 1 ma electron beam from a hot filament at the top of the vacuum chamber. For these tests the plasma density was kept under a pressure of 10-6 Torr which is effectively negligible and removes most thermal plasma effects. For one of test the filament voltage was set first at -120 V and the current was then applied, and for the other a heating current was applied and the filament voltage followed. Both methods resulted in the same final filament conditions, however only the condition with voltage set first produced dust motion. It was also observed that in the case of setting the current then increasing the voltage magnitude the surface potential followed the voltage, maintaining flux equilibrium and effectively supressing SE creation. In contrast, when the voltage was set and the current followed the surface potential became less negative ing at -55 V, due to the flux balance between SEs and beam electrons. This suggested the importance of SE in dust mobilization Dust Transport Observations These comparative experiments suggested that the absorption of SEs within the dusty surface plays a critical role in dust charging and mobilization. Similarly, this charging and mobilization process of dust should also be true for any photoelectrons emitted by the dusty surface while undergoing UV illumination. In order to examine the effect of the 13

26 emitting electrons in dust mobilization, recordings were made of lofting dust in the conditions 1), 2) and 3) listed above. Figure 5 Images of dust transport and lofting trajectories in a) UV, b) electron beam, and c) plasma and electron beam experiments. Observations showed that under conditions of UV illumination, solely electron beam, and electron beam with plasma, all resulted in electrostatic lofting while pure thermal plasma did not. This implies that photo- and secondary electrons hold a key role in the charging mechanism. It should also be noted that these experiments are the first to show conclusive evidence of dust charged and lofted by exclusively UV radiation. 2.3 The Patched Charge Model 14

27 2.3.1 Proposed Charging Mechanism Resulting from the observed charging differences and dust motion conditions, a new patched charge model was formulated. Particles, whether irregular or not, will have gaps between them due to random positioning in the same way a pile of Legos will t to have gaps between blocks. These microcavities, the empty areas between particles, will generally be around the diameter of a particle (a) in length and exist throughout any layer of a dusty surface. With that said, the gaps between particles on the surface layer allow for photons and/or electrons and ions to penetrate into deeper dust layers, similar to the way light may pass through leaves to the forest floor. These energetic particles will be incident to subsurface dust particles and will cause them to generate photo- and/or secondary electrons. Some of these emitted electrons will be absorbed by surface patches of the neighboring dust particles that make up the microcavity, regions shown in red on Figure 6b. These surfaces are shielded from direct exposure to incoming photons and/or electrons and ions and as a result will always charge to Q r, obtaining a negative potential φ r with respect to the ambient plasma potential φ p. The directly exposed surfaces are depicted in Figure 6b as blue patches and will be charged in accordance with Equation 1 to Q b, attaining a potential φ b with a depence on the Debye length λ De. 15

28 Figure 6 a) Illustration of forces acting on a dust particle, and b) Simplified diagram of microcavity charging mechanism. The magnitude of φ r deps primarily on the photo- and/or secondary electron energies, implying that e(φ r φ b ) will reflect the photo- and/or secondary electrons from reaching the red surface patches at charging equilibrium. To determine the charge attained by either type of region Equation 1 can be used. With this said, the electric field within the microcavities will be different to that seen by the exposed surface layer, being on the order of φ r φ p a and φ b φ p λ De respectively. Generally the Debye length will be much larger than the size of a microcavity (λ De a) and the surface to plasma potential differences will be of the same order of magnitude (φ b φ p is close to φ r φ p ), so the electric field within the microcavity will be larger (E r E b ), leading to the charge amassed in the shielded red region to be much greater than that of the directly exposed surface (Q r Q b ). Due to the relatively large surface charges being at such close regions, 16

29 the repulsive force between particles will dominate (F c F e ). As a result of this charging the net charge is thus Q = Q b + Q r, however as Q r is so much larger than Q b, Q Q r 0.5 C ( ηt ee ). (8) e In this equation C = 4πε 0 a and is the capacitance of an isolated particle, T ee is the emitted electron temperature in electron Volts (ev), and η is an experimental factor larger than 1 to account for the enhanced surface potential due to collection of high-energy electrons in the emitted electron distribution. The term ηt ee e is itself an approximation of the potential attained by the collecting surface patch with respect to the emitting surface patch within the microcavity. Empirically the value for η will lie between 4 and 10 based on experiments described in Section [28]. Finally the factor of 0.5 is attributed to the assumption that the surface area of a microcavity patch will be at most half of the total particle surface area Cavity Effect Replication Test To examine whether the predicted difference between shadowed and exposed regions existed a dual disk Langmuir probe was created consisting of two 6.35 mm disks separated by a layer of insulating ceramic. The probe would be oriented parallel to the plate so as to have a top disk directly exposed to the emission source and a bottom disk that was shadowed from any direct effects. Emission sources for these experiments were either a UV lamp or tungsten filament. The probe was held 3 mm above the surface of a stainless steel plate biased to a potential of φ s. 17

30 Figure 7 Schematic of dual disk Langmuir probe experimental setup for patched surface potential measurements. The probes when exposed to UV illumination, electron beam, and plasma with electron beam produced positive potentials on the top disk (φ top of +3 V, +2 V, and +4 V respectively) and negative potentials on the bottom disk (φ bottom of -2.5 V, -22 V, and - 12 V respectively) with respect to the applied φ s. The top probe was observed to be at similar potentials with the plate, whereas the bottom probe charged to negative potentials resulting from photo- or SE collection. The difference in potential between the plate and the bottom disk (φ bottom φ s ) is more negative than T e e for both photoelectrons and secondary electrons (around -0.3 V [29] and around -3 V [6] respectively). This is likely due to the exted high-energy tail within emission distributions. For electron beam conditions the maximum attainable energy of a SE could reach 65 ev. In the case of these UV experiments, the UV lamp emits a wavelength of 172 nm, providing 7.2 ev photons. Given the work function for stainless steel with an oxidized layer is 4.4 to 5.5 ev [30] the 18

31 maximum energy of photoelectrons emitted from the plate is 1.7 to 2.8 ev effectively limiting the bottom disk potential to a maximum magnitude of 1.7 to 2.8 V. These limitations agree with the stated results and the values attained for (φ bottom φ s ) were used to approximate (φ r φ b ) in the calculation of Q r shown in Table 2. Table 2 Charge and force calculations with the patched charge model for UV, electron beam, and plasma and electron beam conditions. Parameters UV Electron beam Plasma and electron beam E (V/m) a (μm) Qb (C) r - b (V) Qr (C) Fe (N) Fc (N) Fg (N) Fco (N) Fes / Fg Patched Charge Model Predictions The patched charge model makes two key predictions about the dust charging process. First, Dust particles on a dusty surface that emits photo- and/or secondary electrons are able to attain a negative net charge. Second, the dust particle s net charge can attain higher magnitudes than the charges predicted using previous models. These unusually large charges were also demonstrated through recent computer simulation work [31]. Further examination of experimental results that verify these charge predictions will be in Section

32 CHAPTER 3. CHARGE PROPERTIES Within this chapter experiments that were done to determine the polarity and net charge of a dust particle will be explained in detail. The experimental results will then be compared to values predicted by the patched charge model. 3.1 Dust Charge Polarity Polarity Experiment Setup For the following experiments two distinct types of particles were used: irregularly shaped m diameter JSC-1 Mars simulant and 42.3 ± 1.1 m diameter silica microspheres. These dust particles were charged through exposure to one of four conditions described in Section Both polarity and charge measurement tests were performed after charging the dust patch until a mobilization effect was observed had and then turning off the charging source and quickly taking data. Due to the high electrical resistivity of these dust particles, it is expected that the dust particle charges remain approximately unchanged while under vacuum conditions. Charge polarity of dust particles was then measured through moving in a gridded electrode 5 mm above the dust surface. This electrode then had either a positive or negative voltage applied to it with a maximum of ±3 kv, this would attract particles with the opposite net charge causing them to pass through the electrode and be clearly observable. Before exposure to any of the afore mentioned charging conditions each trial was tested for any uninted background charges, such as tribocharges, by running both electrode polarities to their maximum values. No particles were found to have any background charges large enough to produce any observations. 20

33 0.5 cm 0.5 cm Polarity Experiment Results After exposure to solely thermal plasma no motion was observed and there were no attracted particles of either kind observed for either polarity. As thermal plasma does not produce photo- or secondary electrons and at least one type is required for the new patched charge model, this result is consistent with predictions and previous experimental results [28]. For the other three conditions, as either photo- and/or secondary electrons would be present it was expected that the dust would charge negatively and be attracted to the electrode only when a positive voltage was applied. This is exactly what happened for all but the case of silica microspheres charged by UV radiation as they were not caused to move from application of either grid polarity. Figure 8 Charge polarity test of silica microspheres after exposure to electron beam conditions.no particles were attracted by the negatively charged electrode up to the maximum charge of -3 kv, whereas a large flux of dust particles were observed being attracted to the electrode at voltages as low as +0.5 kv. An example of this can be seen in the electron beam charged silica microsphere experiment shown in Figure 8. These results confirm the patched charge model s prediction that the electrostatically lofted dust particles 21

34 would all have a negative net charge, even for the case of UV radiation where a positive charge is usually expected due to photoemission from dust particles. 3.2 Net Dust Charge Magnitude Net Charge Magnitude Experiment Setup Net charge magnitude was measured using a similar technique as done in the polarity experiments, differing through the use of the grid as a grounded gate and the use of a Faraday cup probe to collect charge data. The Faraday cup apparatus consisted of a copper inner cup, co-axially embedded in a copper outer cup which was charged to +3 kv in order to accelerate particles from the dust surface into the Faraday cup. The outer cup had a copper cap sealing the inner cup inside but for a small 1 mm diameter hole in the center of the cap to allow particles in. The gridded electrode was grounded and used to shield the dust from the electric field produced by the outer cup. When everything was prepared and in place with the outer cup and Faraday cup voltages giving steady readings, the grid was moved from its position between the dust and the Faraday cup apparatus, effectively opening the gate and allowing the particles to be attracted through the hole and into the Faraday cup (see Figure 9). When a particle is attracted through the hole and enters the Faraday cup it creates an image charge, which is an inversed response that compensates for the sudden change in the electric field. This image charge is amplified through an electrometer built by Li-Hsia Yeo (a graduate student working in the IMPACT lab) and saved as a voltage fluctuation signal. Signals distinct from background noise are then interpreted to particle charge using a calibration curve produced through the use of 22

35 capacitor discharges. Attracted dust particle charges and polarities will be measured, however dust particle sizes will remain unknown. This measurement procedure was performed as it avoids perturbing the electrometer results as well as the charged dust while setting the outer cup voltage. Figure 9 - Diagram of experimental setup for net charge experiments Net Charge Magnitude Experiment Results As dust that had been charged solely through thermal plasma showed no capacity for attraction within the polarity test, experiments centered on the other three conditions. Both types of dust was tested under all three conditions for twelve identical trials to amass a significant number of particle signals. Figure 3, shown below, contains these six conditions with vertical lines denoting the average and median charges for a given condition. 23

36 Figure 10 Histograms of particles' net charge magnitude resulting from twelve tests in each listed condition. It should be noted that the charge distributions of both types of particles are broad, even for the uniformly sized and shaped microspheres. Based on our dust transport experiments (described in Section 4.1), dust particles accelerated into the Faraday cup are expected to have a broad size distribution which will result in broad charge distribution due to charge being proportional to dust particle diameter. In the case of the irregularly-shaped simulant, the measured size distributions of lofted particles are 20.3 ± 13.1 m under UV illumination and 44.9 ± 30.1 m under plasma and electron beam conditions (see Section 4.1.3). Despite being sifted to fall within the range of 38 to 45 m diameter, smaller residual particles aggregated and were not fully eliminated. This process and its effects are described more completely in Section 4.1, however it should be evident that with residual particles present, aggregates larger than 45 m in diameter can form due to inter-particle cohesion and be lofted as observed in dust 24

37 transportation experiments (described in Section 4.1), thus providing a large particle size distribution. Similar aggregation is also expected to occur for silica microspheres which too will lead to the formation of large particulates and broad size distributions. Additionally, the charges on approximately equal-sized single particles may have large variations due to the variation in shape and size of microcavities where highly varied electric fields on surfaces and surroundings are created. No silica microspheres were registered by the electrometer after exposure to UV illumination. This could be due to a difference in cohesive forces between the silica microspheres and irregularly-shaped Mars simulant due to their different compositions and shape [32], and possibly the different geometry of the respective microcavities formed by the samples. 3.3 Predictions vs. Measurements Using the average lofted particle diameter for each of the conditions as found in Section 4.1, it was possible to create charge magnitude predictions using the patched charge model. The spherical particle charge magnitudes predicted by using Equation 8 shows a general agreement with the measured charges of the irregularly-shaped simulants with the exception of the UV radiation condition as shown in Figure 11. This is due to the predicted value being beneath the measurement threshold leaving all data gathered to consist of the more rare large charge magnitudes. 25

38 Figure 11 Measured and predicted mean net particle charge magnitudes with standard deviations. It should be noted that all measured charges are greater than 10 5 e in magnitude, which is already two orders of magnitude larger than the charges predicted with previous models (less than e) [28]. In summary, our charge measurements confirmed the predictions of the patched charge model, showing that dust particles can be electrostatically transported or lofted on the surfaces of airless planetary bodies due to their large negative net charges, which is in direct contradiction of previous charging model predictions. 26

39 CHAPTER 4. DUST LOFTING Within this chapter the analysis of dust lofting characteristics will be done recognizing lofted dust size distributions, launch velocity, and height distributions in three relevant conditions (plasma and electron beam, solely electron beam, and UV illumination) as well as any implications of the determined values. 4.1 Lofted Particle Size Analysis Particle Lofting Tests When investigating trs in motion it is prudent to separate experiments into distinct different conditions. This chapter will utilize the charging conditions 1), 2), and 3) described in Section in conjunction with 38 to 45 m diameter JSC-1 Mars simulant centred underneath the emission source, resting on a graphite plate that was electrically floated, and contained in the afore mentioned 2 cm diameter 2 mm high rubber enclosure Analysis Program After each of the tests described above had been taken and recorded, they were put through a program running in Matrix Laboratory (MATLAB). To see this program in its entirety refer to Appix A.1. The analysis program works by taking two selected frames of the recording and subtracting the later frame s pixel values from the earlier frame s pixel values. This leaves only what was different between the frames, the moved dust. Discrepancies are then turned into binary values using a threshold pixel brightness to 27

40 separate any shadows or reflections from actual dust. During this analysis, the value given to the red color in the RGB matrix was weighted more highly due to the hue of the simulant used cm Figure 12 Example of a) focally cropped image, and b) subtracted image, similar to those used for particle size calculation. After cropping the image to regions in focus and disregarding the dust within the container the program then goes through the remaining matrix row by row until it contacts a highlighted dust particle. Upon finding a particle the program finds the edges of said particle and indexes the size and location so as to prevent counting a particle twice. The program continues this process until the entire image has been analyzed. Using the known dimensions of the hole containing the dust each indexed particle s size is evaluated with respect to depth in the image and the number of pixels that the particle takes up. The results are then output in a file that contains the particle coordinates within the image, the number of pixels the particle takes up, and the corresponding size of the particle. From there plots and post analysis interpretations can be made cm Particle Sizes Upon analyzing the images and determining the sizes for each of the particles it became apparent that the sifted particle range allowed smaller residual particles to cling together and make it into the dust sample. The sifting process thus acts as more of an upper 28

41 bound to single particle sizes and merely reduces the number of residual particles. Similarly, when lofting particles it has become evident that aggregates held together by large cohesive forces can be lofted, reaching sizes up to 140 m in diameter. Figure 13 Size distributions of lofted dust particles after exposure to UV illumination or plasma and electron beam conditions. As shown above in Figure 13 the size distribution of lofted particles deps greatly on the emissions that the dust is exposed to. In the case of plasma and electron beam or just electron beam there is a long densely populated distribution where the defined single particle range from sifting only accounts for 12% of the lofted dust. The majority of lofted 29

42 dust appears to be residual particles (~51%), however in this case the tail exts all the way to ~140 mm diameter particles and makes up a significant fraction of the lofted population. Particle lofting as a result of solely UV illumination had not been observed until the experiments stated within this thesis were conducted. As with the case of plasma and electron beam conditions, residual particles make up the majority of lofted particles (~90%) with rates decreasing as particle size increases. Unlike the previous case few large particles are lofted as a result of UV illumination and the resulting photoelectron charging leaving a mere 6% of lofted particles within the sifted single particle range and very few lofted aggregates (~4%). The distinct difference in the lofted particle size distributions can be traced to the amount of charge amassed on the microcavity charge patches as larger particles will require larger net charges in order to be lofted. As the energy input by UV illumination is significantly lower than by electron beam, any resulting photoelectrons from the UV illumination will have much less energy than the SE produced via electron beam. This energy difference will permit the electron beam to attain a greater patch charge than UV illumination as shown on Table Lofted Particle Launch Velocities and Peak Heights Particle Velocity Analysis Program Using the recordings described in Section as well as additional tests of alternately sized particles an additional analysis was done to determine the velocity of lofted particles. As high quality videos with the camera used could only be attained at 29 30

43 0.25 cm frames per second we were limited to indistinct blurs at differing heights to represent particles in transit. Unfortunately these blurs made it impossible to discern the size of the moving particle, however these blurs would generally be the most visible at the peak of a trajectory due to the particle being at its slowest when turning around. Knowing the particle s maximum height could then provide a rough approximation of the particle velocity using the equation below, v z = 2gh, (9) where g is the gravitational acceleration applied to the dust particle and h is the maximum height attained by the particle. Using image subtraction from three frames it could be discerned whether a particle was indeed in transit by seeing if it wasn t there one frame before and if it will not be there one frame after. Using image subtraction in this way particles that were appeared in this kind of motion were highlighted in pink. Each frame was then analyzed to find the zenith of these pink trajectories in a way similar to what was described in Section 3.1.2, then indexed with respect to location and frame. Figure 14 Example of trajectory analysis program with: green - new particle detected, pink - particle in transit detected, and red - particle no longer present. 31

44 0.25 cm Using the known range for where the lofted particle could have been emitted from and the location of its zenith an approximate range of velocities could be made for each detected particle. For each detected particle the program would then output an indexed frame, the particle s peak location, and the calculated velocity ranges. To ensure consistency several hundred particles were tracked by hand and compared to the program output achieving almost identical results. The program in its entirety can be seen in Appix A.2. As noted, the quality of recordings would not allow for particle velocities to be mapped with particle size. Using the Phantom V2512 high speed camera at 5000 fps, a crisper image was captured and particle boundaries could be clearly seen over the course of their trajectories. A high contrast filtered image can be seen below in Figure 15. Figure 15 Example of frame by frame particle tracking by hand. Due to the short well defined time between frames, an accurate velocity could be calculated by the difference in particle location. Also due to the crispness in image quality the particle area could be calculated from the number pixels taken to represent it. Trajectories were 32

45 tracked manually and had their two dimensional velocities mapped to specific particle sizes allowing for the creation of Figure Lofted Particle Velocities The heights attained by dust particles were in general, greater than initially predicted from simple ballistic trajectories. This was due to initial calculations ignoring the added upward acceleration provided by the amassed negative charges within the dust surface and the plasma sheath producing an electric field and the accompanying electric force F e. This alters the lofting height equation from being, H = v z0 2, to being, 2g 2 v z0 H = 2 (g Q, m E) (9) where v z0 is the initial vertical speed and E is the electric field observed by the lofted particle. To reduce the observed velocity discrepancy the electric force was then accounted for with E = 40 V/cm (the average measured plasma sheath field value) and using three charge to mass ratios, effectively determining the likely charge to mass ratio s range. 33

46 Figure 16 Measured lofted particle height as a function of initial vertical speed of dust particle. In the case of irregularly shaped particles sifted to be between 38 and 45 m long, when exposed to electron beam conditions the maximum measured height achieved was 1.9 ± 0.3 cm with an initial speed of 0.6 ± 0.1 m/s. A particle with roughly equivalent launching conditions would achieve a height of 0.11 ± 0.05 m on the lunar surface, which is on the order of magnitude needed to attain the lunar horizon glow (within 1 m of the surface) [4,33]. According to the patched charge model, for a 10 m diameter lunar dust particle to reach this height it would need to be charged to 1.7 ± C, via absorption of photoelectrons 31.4 ± 5.1 ev in energy (this is shown to be quite possible in Section 3.3). Using the average size of lofted Mars simulant and its mass density of 1.9 g/cm 3, a 34

47 corresponding charge to mass ratio ( Q m ) of C/kg can be estimated, which is close to the lower limit found in other experiments [28]. 4.3 In-Situ Lofting Implications In contrast to the results of the shared charge model and charge fluctuation theory (see Table 1), the patched charge model predicts force magnitudes capable of overcoming gravitational forces and potentially cohesive forces under reasonable conditions. To make these predictions the charge estimates are made using the relation and data shown in Section 2.3.2, as well as other measured values. For example, the acting electric field and the average lofted particle size for a given condition. Calculated results for each of the three tested conditions is listed in Table 2. In space, photoelectron production peaks at energies of 12.4 to 17.7 ev [28] and in soft X-ray radiation photon energies can reach hundreds of electron volts. As a result photoelectrons can attain energies larger than 10 ev reaching up to hundreds of electron volts. Provided the estimated flux of 0.1 ma/m 2 for 30 to 40 ev photoelectrons [34], the charging time to attain the required lofting charge magnitude is around 10 minutes, which is significantly less than the surface illumination time of near 15 days. SEs are the dominant charging force on night-side surfaces as it is not uncommon for electrons with energies in the tens of electron volts to exist in space. Emissions of SE from the night-side lunar surface have been observed [35] and were roughly 3 times lower in energy than those measured from a single dust particle in laboratory [36] which indicates 35

48 microcavity absorption of particles. As a result, dust is expected to mobilize on all surfaces of an airless planetary body. 36

49 CHAPTER 5. SURFACE MOBILIZATION Observations relating to surface smoothing, creation of surface patterns, and general surface mobilization will be addressed within this chapter. Work on surface mobilization is ongoing and understanding of certain phenomena is expected to grow as more experiments are conducted. 5.1 Prolonged Surface Feature Change Due to Dust Mobilization In addition to dust lofting, dust has also been observed to roll on dusty surfaces. The combination of these two motions will result in a surface mobilization effects. As shown in Figure 17, exted exposure to a charging condition produces dust particle mobilization that causes smoothing within dusty surfaces cm Figure 17 Mars simulant m in diameter smoothing over time when exposed to UV illumination conditions. 37

50 This effect could directly explain the dust pond phenomena as observed on the asteroid Eros [1] (Figure 18) and comet 67P [10]. Figure 18 The dust ponding phenomenon observed on the asteroid Eros. Experiments involving exposure to UV illumination or electron beam conditions have demonstrated that dust composition, size, and geometry all play a role in the dust mobilization process. The details of these experimental results are described in the following subsections Size Depence of Dust Mobilization Experiments utilizing irregularly shaped particles demonstrated that exposure to identical conditions will cause differing sized dust particles to respond with differing dominant motions, as shown in Figure 19. Smaller particles t to attain high charge to mass ratios and as a result are lofted in a more dramatic fashion than larger particles which t to shift and roll on the surface before attaining a charge large enough to overcome both gravity and cohesion. The mobilization of dust particles m and < 38 m in diameter tapered in activity to a complete stop with surfaces that had not been completely 38

51 smoothed after exted exposure to UV illumination. However, the mobilization of intermediately sized particles (38-45 m in diameter) resulted in complete surface smoothness with no apparent reduction in surface activity over time. Size depencies of dust mobilization effects can be due to the filling of microcavities and cohesion between small particles. 0.5 cm 0.5 cm 0.5 cm Figure 19 Mars simulant m (top), m (middle), and < 38 m (bottom) in diameter after 50 minutes of exposure to UV illumination Geometry Depence of Dust Mobilization Geometries of particles has also proven to be a large factor in determining electrostatically charged particle motion. When comparing irregularly shaped silica (SiO2) dust to microspheres of the same composition it became evident that irregularly shaped particles were lofted more frequently than their similarly sized spherical counterparts (Figure 20). This is likely attributed to both the irregularly shaped particles having a larger 39

52 ratio of microcavity to dust than spherical geometries, but also the cohesive forces between irregularly shaped particles having a wide range in magnitude when compared to the relatively uniform cohesive forces between microspheres. As a result, irregularly shaped particles can accumulate more charge to overcome potentially lesser cohesive forces and become lofted more easily than microspheres as a result. 0.5 cm 0.5 cm Figure 20 Similarly sized irregular and spherical silica dust particles exposed to electron beam conditions over time Composition Depence of Dust Mobilization Differences in composition may also have an effect on dust lofting and transport. This was generally observed through trials with mars simulants, lunar simulants, and silica dust of the same sizes. Variations in dust particle mass density and inter-particle cohesion due to differing composition may be responsible for differing dust mobilization. 5.2 Magnetic Field Experiments When placed in the presence of a magnetic field, motion within a sample alters significantly. This experiment will be helpful for the understanding of formation of high albedo swirl-shaped markers on the lunar surface within magnetic anomaly regions. One 40

53 hypothesis for these swirl formations is that electrostatic dust transport will redistribute fine-sized dust particles in response to magnetic field patterns. Tests were done with UV illumination as photon motion will not be affected by magnetic fields and one can observe how photoelectron charging might change under magnetic conditions. As shown above, surfaces without induced magnetic fields mobilize uniformly, however when two stacked 3 mm diameter rare earth magnets were embedded within the plate even with the surface and placed beneath the dust sample, the majority of dust motion halted. Dust was only observed to be moving directly over the magnetic field, where the magnetic field-lines would be perpicular to the surface (i.e., the magnetic cusp region). 0.5 cm 0.5 cm 0.5 cm 0.5 cm Figure 21 Mars simulant m in diameter without (top row) and with (bottom row) a magnetic field before (left column) and after (right column) exposure to UV illumination conditions for 2 hours. 41