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Weathering the Storm
Space Weathering in the Solar System and the Laboratory
Graz in Space, 4-5th September 2008
Mark S. Bentley (mark.bentley@oeaw.ac.at)
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Contents
1. Importance of regolith
2. Mysteries of the lunar soil
3. Space weathering in the Solar System
4. Space weathering in the laboratory
5. In-situ analysis and instrumentation
6. Summary
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What is weathering?
Weathering is “the process of chemical or
physical breakdown of the minerals in rocks”
1. Physical weathering
1.
2.
3.
4.
5.
6.
Thermal expansion
Frost disintegration
Pressure release
Hydraulic action
Salt-crystal growth
Biological Weathering
x
x
x
x
x
x
2. Chemical weathering
1.
2.
3.
4.
5.
6.
Dissolution
Hydration
Hydrolysis
Oxidation
Biological
Carbonation
x
x
x
x
x
x
x
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What is regolith?
“a layer of loose, heterogeneous material covering solid rock.
It includes dust, soil, broken rock, and other related materials”
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Importance of regolith
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Importance of regolith
Airless bodies mainly considered here
Most remote sensing instruments actually measure regolith
Regolith records surface environment over geologic times
Often hard to find anything BUT regolith!
(Relatively) easy to penetrate and sample
Instrument
type
Typical
sampling depth
Data returned
X-ray
spectrometer
~ 1 – 100 μm
Elemental
composition
VISVNIR
spectrometer
~ few mm
Mineralogical
composition
Gamma-ray
spectrometer
~ 10 cm
Elemental
composition
Neutron
spectrometer
~ 100 cm
Changes in
elemental
composition
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The lunar regolith
Regolith: broken up rock material
Soil: <1 cm portion of the Regolith
Dust: < 50 µm portion of the Soil
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The lunar regolith
Impact-Glass
Bead
Agglutinate
Volcanic Glass
Bead
Rock
Chips
Impact
Glass
1 mm
Plagioclase
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The lunar regolith
© Discovery Channel 2008
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Mysteries of the lunar soil
Difference in reflectance spectra between rock and soil
- Lower albedo (darker)
- Spectral reddening (redder)
- Reduced spectral contrast (more boring ☺)
Data from RELAB, mare basalts collected at
Apollo 15 Station 4 on the rim of Dune Crater
Crushing lunar rocks does not result in the
same spectrum as the soil
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Mysteries of the lunar soil
Unusual magnetic properties of the soil:
- Magnetic susceptibility higher in regolith
- ESR “characteristic” resonance
- Curie point shows carrier is pure metallic Fe
Magnetic fraction shows greatest spectral difference
Runcorn et al, 1970
Hapke, 2001
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Mysteries of the lunar soil
The story so far…
• Regoliths are found all over the Solar System
• They are important to understand
• On airless bodies form differently from on Earth
• The lunar regolith is different from crushed rock
• Unexpected optical and magnetic properties
• These properties are due to fine metallic iron
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How does this iron form?
So we know these is “extra” iron in the regolith, not in the original soil.
Where does it come from? Possible explanations:
1. Reduction of solar wind hydrogen in impact heating
2. Solar wind sputtering
3. Re-condensation of impact vapour
Post Apollo view: reduction during impacts forms Fe in agglutinates
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Nanophase iron
Sub-microscopic iron (SMFe)
found everywhere in the regolith!
10s-100s nm in diameter
In amorphous grain coatings
Even on grains that contain no
native Fe2+
(Pieters et al., 2000)
Paradigm shift
Re-condensation of impact
and sputter products creates
SMFe.
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Baking lunar bricks
Regolith containing SMFe can
be heated and melted by a
domestic microwave!
Taylor and Meek,
“Microwave Processing of Lunar Soil”
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Weathering in the Solar System
Asteroids / Comets
Moon
Mercury
Galilean Satellites
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Weathering in the laboratory
To better understand this process, laboratory experiments are needed,
requiring:
1.
A method of simulating meteorite impacts and/or the solar wind
2.
Appropriate conditions (pressure, temperature)
3.
Appropriate samples
4.
Analyses to confirm the optical and magnetic changes
Light Gas Gun
Van der Graaf
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Pulsed laser simulation
IR Mirror
Nd:YAG 1064 nm, 10 ns, 5 Hz
Laser interface
Optical window
XY motor
controller
Pt100 ADC
Heater PSU
Heater
(RT – 500 °C)
Sample
<63 μm
Baseplate
PC
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The samples
decreasing size
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Optical analyses
Fo90
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In search of metallic iron
Mössbauer
Magnetic Susceptibility
SPM
Attempted at RT and 12 K
No obvious Fe0 signature
wt% below detection limit
SD
0.465 kHz &
4.65 kHz
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In search of metallic iron
Ferromagnetic Resonance
g ~ 2.1
ΔH ~ 65 mT
4 – 33 nm
Vibrating Magnetometry
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In-situ instrumentation
Impact & sputtering
processes
⇓
Fine-grained iron in regolith
grains
⇓
Superparamagnetism
⇓
High and freq. dependent
magnetic susceptibility
⇓
Regolith gardening
⇓
180 g
8 kHz oscillator
100 x 65 x 25 mm
Magnetic susceptibility
depth profile
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Deployment options
NOT TO SCALE ☺
Tether
~5m max
depth
Self-penetrating mole with
integrated instrument
package
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Space Weathering Summary
•
Regolith is important – it is the surface we remotely sample
•
As the lunar regolith evolves, its VNIR signature changes
•
Optical and magnetic effects caused by sub-microscopic Fe0
•
Iron is produced in micrometeorite and sputtering events
•
The longer a soil is exposed, the more mature/SMFe rich
•
Magnetic susceptibility detects SPM material and iron content
•
Would be a useful in situ instrument to determine SW degree
Request for input…
Opps for collab
Any questions…?
mark.bentley@oeaw.ac.at