The Lunar Surface: A Dusty Plasma Laboratory

The Lunar Surface: A Dusty Plasma Laboratory
M. Horanyi, D. Brain, A. Colette, K. Drake, E. Grün, S. Kempf, T. Munsat,
S. Robertson, Z. Sternovsky, X. Wang & CCLDAS Team
NASA Lunar Institute: Colorado Center for Lunar Dust and Atmospheric Studies
Outline:
1) Dusty Plasmas
2) Outstanding Lunar Issues
3) Laboratory Experiments
4) Space-born Experiment (LDEX)
5) Surface-Experiments
Fall AGU 2011
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Dusty Plasmas
e, i1 , i1 , i1 , … and dust+/duste and dust +
dust
Fall AGU 2011
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and dust
2
Dusty Plasmas
New physics:
Dust is many orders of magnitude heavier than ions and can
carry many orders of magnitude larger + or – time dependent
charge.
new temporal & spatial scales
unusual dynamics
new waves & instabilities
Dust charge:
electron and ion fluxes
secondary and photoelectrons
dust – dust collisions
Fall AGU 2011
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Unresolved lunar issues
Fall AGU 2011
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CeO2 (Cerium-oxide) experiments
UV
lamps
Single-sided
planar
Langmuir
probe
Tantalum Ceramic
coating
foil
Collector
Biased
grid
Langmuir Probe
CeO2
Zr surface
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CeO2 experiments
Over CeO2
Over Zr
1.4
1.4
float
float
1.2
0V
1.2
-20 V
-20 V
-40 V
Current (mA)
1.0
0.5
0.8
0.4
0.6
Current (µA)
Current (mA)
1.0
0.4
0.2
0.0
-40
0V
-20
0.3
0.2
0.1
0.0
0
Bias
Voltage (V)
-0.1
+20 V bias
+10 V bias
+5 V bias
0 V bias
-5 V bias
-10 V bias
-20 V bias
-40 V
0.8
0.6
0.4
0.2
0.0
20
40
-40
(a)
-20
0
Bias Voltage (V)
20
40
Derivative (arb. units)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
30
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10
(b)
A. Dove
P41C-1635 (Thursday)
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Motivation
Dust Levitation
Dusty Sheath
Potential
Motivation
Dust Levitation
Dusty Sheath
Dusty Plasma Sheath
Grain Levitation
XXXXXX
Colwell et al.: LUNAR REGOLITH AND DUST DYNAMICS
Motivation
Dust Levitation
Dusty Sheath
ng the Sheath
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Jamey Szalay
Dusty Sheath Simulation
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Dusty Sheath Simulation
Arnas et al., 2001
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Dusty Sheath Simulation
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Plasma interaction with a magnetic dipole field
Dipolar Magnetic field
(~600 G at the surface)
IEA
Emissive
probe
Vacuum
pump
N
S
Insulating
surface
Filament
!
Effects of Surface Magnetic Fields
Electrons are magnetized (re < d)
Ions are un-magnetized (ri > d)
Fall AGU 2011
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Potential contours above the surface along the dipole axis
Non-monotonic sheath
!
•  More positive surface potential than the bulk to reflect the ions back into the
plasma.
•  Potential minimum due to collisions and magnetic-mirror-trapping.
Horizontal fluctuations
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Surface potential and electric field profiles
•  Fluctuation due to
shielding, focusing !
•  Highest electric fields at
the cusps may enhance
the dust transport.
!
Xu Wang
P43F-06 (Thursday)
I
II
III
Fall AGU 2011
IV
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LADEE: LDEX
MCP detector for ions (high sensitivity) and integrated signal
E-field
Ions
e-
Incoming
dust
Hemispherical
target
Target collects electrons (low sensitivity)
Fall AGU 2011
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Testing and Initial Calibration
•  Performed at the CCLDAS facility
•  Velocity range 1–8 km/s
LADEE velocity
•  Size range 0.2 – 1.5 µm
•  Total number of particles shown
•  Black: particles detected by LDEX
•  Red: not detected by LDEX
•  65% is detected
Velocity [km/s]
•  Dust flux is reduced by the 3 grids
over the aperture (90% open area)
and the 90% duty cycle of LDEX
•  (0.9)4 = 0.656
13
Fall AGU 2011
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The measurement establishes a direct link of the grains composition to its origin on the surface (compositional
mapping). In the course of the mission a dust spectrometer is collecting many thousands of submicron samples from
a greater part of the entire surface and determines their origin and composition. It thus combines in situ analysis,
which otherwise could only be achieved by a lander, with the much better surface coverage of a remote sensing
method.
LDEX Expectations
Figure 1. Schematics of dust spectrometry of a planetary surface. Ejecta particles lifted by micro-meteoroid
impacts from the satellites' surface are analyzed in situ by a sensor in combination with a high-resolution dust
mass spectrometer. By tracing back the trajectory to the surface compositional maps of the surface are
generated.
The analysis of emitted solids is complementary to studies by remote sensing methods (e.g. by infrared
spectroscopy) and analysis of the gas phase (by an ion and neutral mass spectrometers). It is important that both,
solid and gas phases be measured. For example, Cassini’s dust detector CDA (Srama et al., 2004) found sodium salts
in the dust particles from Saturn’s satellite Enceladus (e.g. Postberg et al., 2009), while high-resolution spectroscopy
and the Cassini INMS did not detect any sodium in the emerging plume gas or at the moon’s surface (Schneider et
al., 2009; Waite et al., 2009). Only the combination of all methods provides a conclusive picture. There is another
remarkable advantage of ‘surface dust spectrometry’: Whereas remote sensing methods determine the compositional
average of a certain area, a dust spectrometer is able to identify individual constituents of that area on a submicron
level. Ideally, the contribution of certain minerals and compounds to a specific geological formation can be
quantified.
The process of creating ejecta from impacting micrometeroids is very efficient: the total mass is of the order of a few
thousand times of the impactor’s mass (Koschny & Grün., 2001). The predicted density of ejecta grains in the lunar
dust exosphere at an altitude of 50 km is shown in Fig. 2. In addition, during meteor showers, the ejecta population is
expected to dramatically increase with a spatial distribution showing strong deviations from spherical symmetry for a
duration of a few days.
Fall AGU 2011
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Z. Sternovsky
P43A-1666
(Thursday)
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LDEX +
New Experiments: LDEX-2 for LADEE-2!
Time-of-flight mass spectrometer.
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Dusty Plasma Surface Package: EPB
SIDE: Suprathermal Ion Detector
Experiment
S. Robertson & A. Colette
P41C-1634 (Thursday)
Fall AGU 2011
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Summary
1)  Dust, dust charging, and dusty plasma effects are important
to understand the near surface environment of the Moon and
all airless objects in the solar system.
2)  These issues are of interest for both basic plasma science
and engineering.
3)  Small-scale laboratory experiments enable the understanding
of the physics, and lead to new measurement and instrument
concepts.
Fall AGU 2011
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