charged dust dynamics near the lunar surface
TRANSCRIPT
LSA4 – April 9, 2014
Charged Dust Dynamics Near the Lunar Surface
Joshua Colwell and Adrienne DoveDept. of Physics, Florida Space Institute
University of Central Florida
4
Lunar Horizon Glow - 1
Rennilson and Criswell (1974): Analysis of Surveyorlander images of horizon glow after sunset.• Image is a composite;• dust cloud has been repositioned;• distance to horizon ~ 150 m suggests h~0.3 m;• angular extent suggests particle radii ~6µm.
LSA4 – April 9, 2014
Surveyor 3 Footprint
Surveyor image, April 1967.
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July 21, 2010 38th COSPAR Scientific Assembly 10
Apollo 12 astronaut imageNovember 1969
The lunar surface is not a roiling dust popper.
• Detailed models of dust levitation and dynamics in a variety of planetary enviroments
[Nitter and Havnes (1992), Nitter et al. (1994, 1998), Ip (1986)]
• Discussion of regolith removal from asteroids [Lee (1996)]
Second Round of Theoretical Studies
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Size of Levitatable Particles
Lee (1996)
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Itokawa, length~500 m, shows regions free of dust and other smooth regions.
Smooth areas at gravitational lows. Downslope movement plays a role.
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Dust “Ponds” on Eros
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July 21, 2010 38th COSPAR Scientific Assembly
Dust “Ponds” on Eros
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• 255 ponds > 30 m diameter.
• ~90% near Eros’s equator.
• Eros has obliquity near 90 degrees, suggesting solar illumination may play role in pond formation.
Properties of Eros Ponds
Shape of Eros places ponds mostly at areas of low surface gravity.
(Robinson et al. 2002, Hughes et al. 2008)
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Numerical Simulations• Dayside photoelectron sheath.• Calculate surface potential.• Calculate currents to dust particle.• Integrate equation of motion under force
of gravity and electric force.• Include effects of topography on
shadowing (surface potential) and trajectories.
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Equations of Charge and MotiondQd/dt=Ipe-Ie-Isw
Ie = πrd2enpe
8kBTpeπme
expeφdkBTpe
÷Ie = πrd
2enpe8kBTpeπme
1+eφdkBTpe
÷
I pe = π rd2eI ph0I pe = πrd
2eI ph0 exp−eφdkBTpe
÷
Isw = π rd2ensw
8kBTswπme
1 +eφdkBTsw
÷
I sw = π rd2ensw
8kBTswπme
expeφdkBTsw
÷
Φd<0 Φd >0
d 2z
dt 2=QdmdE − g E = 2 2ΦsλD 1 +
z
2λD
÷
−1
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OOPIC Simulation of Sheath
Colwell et al. 2009 (J. Aero. Eng.)
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Force BalanceExample for Moon
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Lunar Levitation Heights
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July 21, 2010 38th COSPAR Scientific Assembly
Trajectories of Lunar Dust
Launch v=200 cm/s
LSA4 – April 9, 2014
July 21, 2010 38th COSPAR Scientific Assembly
Trajectories of Lunar Dust
Launch v=200 cm/s
LSA4 – April 9, 2014
July 21, 2010 38th COSPAR Scientific Assembly
Trajectories of Lunar DustLaunch v=100 cm/sEnhanced photocurrent
LSA4 – April 9, 2014
July 21, 2010 38th COSPAR Scientific Assembly
Trajectories of Lunar Dust
Launch v=100 cm/sEnhanced photocurrent
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• Surveyor Horizon Glow particles are not floating (would be much higher, extended cloud), but are on ballistic trajectories.
• Height of HG cloud and size of LEAM (~30 cm) constrain electrostatic launch velocity at 1 m/s.
Lunar Simulations:What do They Mean?
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Effects of Topography
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• Use two numerical models to model diurnal and seasonal dust transport with topography.
• Tabulate probabilities of dust deposition in craters.
Can Charged Dust Explain Eros Ponds?
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Distribution of Ponds on Eros
Contours are areas that spend large fraction of time withSun near the horizon. Hughes et al. 2008
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“Floating” Dust Simulation of Ponds
Seasonal variations in dust deposition
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July 21, 2010 38th COSPAR Scientific Assembly
Alternate Model: “Hopping” Dust
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Does it Really Happen?Levitation Experiments
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Levitation Experiments
Sickafoose et al. (2001, 2002)
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Horizontal Transport Experiment
Dust on a conducting surface charges to a different potential and produces horizontal transport.
Colwell et al. 2009
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Horizontal Electric Fields
Heterogeneity in the surface charge produces horizontalelectric fields which transport dust across the boundary.
Insulating disk on graphite plate. Colwell et al. 2009
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New Experiments Mimicking Terminator Region
Wang et al. (2009)
0.6 cm
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Summary
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• Charged dust dynamics has observable consequences near a variety of planetary bodies:
• Spokes in Saturn’s rings; triggering mechanism uncertain;
• Dust “ponds” on Eros; may work in concert with seismic shaking;
• Removal of fines from small asteroids;• Horizon glow on the Moon; implications for future
exploration of the Moon.• Dynamics influenced by season and perhaps solar
cycle.• Electrostatic “hopping” and “floating” are two possible
modes of charged dust transport.
Open Questions
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• What are the surface potentials asteroids and moons as a function of local time and latitude, particularly near the terminator and poles?
• What are the near-surface plasma properties and electric field strengths, and how do they vary with time and latitude?
• What are the velocity and size distributions of electrostatically mobilized dust?
• What are the interparticle forces that must be overcome to launch dust?
• What are the conditions that lead to launching of particles off asteroid surfaces?