The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar,...
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![Page 1: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/1.jpg)
The Search for a Lunar Dynamo
Ian Garrick-Bethell
Brown University
NLSI Director’s Seminar, January 19, 2010
![Page 2: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/2.jpg)
The utility of planetary magnetism
Earth Mars
Ganymede Mercury
Moon Asteroids(in order of decreasing radius)
![Page 3: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/3.jpg)
The utility of planetary magnetism
Earth Mars
Ganymede Mercury
Moon Asteroids(in order of decreasing radius)
![Page 4: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/4.jpg)
What is the structure of the Moon?
Core evidence: seismic, moment of inertia, magnetic induction, and wobble.
Rich in heat producing elements
![Page 5: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/5.jpg)
Early views of the Moon
Pre-Apollo era: Hot Moon vs. Cold Moon
Harold Urey: primitive chondritic object
Others (e.g Shoemaker): experienced melting
Credit: Bill Hartmann
![Page 6: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/6.jpg)
In Search of a Lunar Dynamo
Luna 1, January 2, 1959Luna 2
September 12, 1959
S. DolginovMagnetometer Principal Investigator
Result: lunar dipole field at least ~10,000 weaker than the Earth’s
![Page 7: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/7.jpg)
Hot Moon
• Surveyor 5 spacecraft (1967) detected basalt.
• Apollo missions directly sampled and confirmed the volcanic origin for the lunar mare.
• The Moon had experienced at least some melting.
Surveyor 3
![Page 8: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/8.jpg)
Crustal magnetism discovered
Russell et al. 1974Apollo 15 and 16 subsatellites
![Page 9: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/9.jpg)
Crustal magnetism discovered
Apollo 16 magnetometerApollo 12 magnetometer
![Page 10: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/10.jpg)
Does crustal magnetism = dynamo?
From Mark Wieczorek’s 2009 AGU Talk
![Page 11: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/11.jpg)
The lunar rock magnetic record
Wieczorek, et al. (2006) & Cisowski and Fuller (1987)
Modern Earth field (~ 50 μT)
![Page 12: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/12.jpg)
Modern Earth field (~ 50 μT)?
Wieczorek, et al. (2006) & Cisowski and Fuller (1987)
The lunar rock magnetic record
![Page 13: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/13.jpg)
What we know and don’t know• It is clear that fields existed on Moon:
– Crustal remanence. – Paleomagnetic record.
• It is not clear whether the fields are from a dynamo or impact processes.– Doell et al. (1970): transient impact-
generated fields could magnetize rocks as a shock wave passes through them: “shock magnetized.”
![Page 14: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/14.jpg)
Rock magnetic approach
• We seek rocks with ages > 4.0 Ga.
• But we also carefully select a rock with favorable petrologic history.
![Page 15: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/15.jpg)
We started looking at a lot of old rocks
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76535 – Pristine Troctolite
• Age: 4.2-4.3 Ga
• Argon age
• Plutonic
• No shock effects
1 mm
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Why the troctolite is so important• 1) Lack of detectable shock features: remanence
is less likely due to shock effects– Restricts impact related processes.
• 2) Cooling history is well constrained: slow cooling history implies any remanence is from long-lived fields– Further restricts impact related processes.
• 3) It is very old. It is somewhat easier to accept a core dynamo at early times.
![Page 18: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/18.jpg)
Measurements
• Thermal demagnetization is the gold standard, but:
• It is destructive, rocks frequently alter (Lawrence et al. 2008).
• Our approach: first perform nondestructive AF demagnetization to understand the samples, and then if desirable, perform thermal.
![Page 19: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/19.jpg)
Alternating Field Demagnetization
z
x
y
Magnetization vectorDemag. Step 1
Sample
Ideally, trends to the origin
Demag. Step 2
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z
x
y
y
x
Magnetization
Sample
Alternating Field Demagnetization
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z
x
y
ySample
z
Alternating Field Demagnetization
![Page 22: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/22.jpg)
Display of demagnetization
=+ x,y
y,z
y
zy
x
Both Projections
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Two Samples
Magnet-like overprints: IRMs
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Easily reproduced/removed
![Page 25: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/25.jpg)
Once removed, first sample:
![Page 26: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/26.jpg)
Two samples:
![Page 27: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/27.jpg)
Second component decays to origin
![Page 28: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/28.jpg)
Second component decays to origin
![Page 29: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/29.jpg)
Second component decays to origin
MCMC
HCHC
![Page 30: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/30.jpg)
Two Magnetization Components
z
x
y
1
2Net
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Two Magnetization Components
z
x
y
2Net
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Two Magnetization Components
z
x
y
2Net
![Page 33: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/33.jpg)
Two Magnetization Components
z
x
y
Net
![Page 34: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/34.jpg)
Two Magnetization Components
z
x
y
![Page 35: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/35.jpg)
Two Magnetization Components
z
x
y
![Page 36: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/36.jpg)
Two Magnetization Components
z
x
y
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Two Magnetization Components
z
x
y
Four of our best samples show these two components: HC to MC: 142-149° apart (~10° error).
142°-149°
HC
MC
![Page 38: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/38.jpg)
Mutually Oriented Samples
145°
HC
MC
?145°
HC
MC
145°
HC
MC
![Page 39: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/39.jpg)
2/3 Mutually Oriented Samples
![Page 40: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/40.jpg)
3 Components of 3 Samples
Best fit directions
![Page 41: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/41.jpg)
3 Components of 3 Samples
MC-HCdistances:
147°123°81°
Compared with:142-149° previously
Best fit directions
![Page 42: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/42.jpg)
The rock is unshocked, so what thermal (cooling) events could
have permitted its magnetization?
Focus on the timescales for cooling events – compare with timescales for impact-generated fields.
![Page 43: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/43.jpg)
Thermal History of 76535
4.2 Ga (multiple chronometers)
![Page 44: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/44.jpg)
Thermal History of 76535
4.2 Ga (multiple chronometers)
![Page 45: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/45.jpg)
Thermal History of 76535
First Magnetization
4.2 Ga
![Page 46: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/46.jpg)
Thermal History of 76535
4.2 Ga
![Page 47: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/47.jpg)
Thermal History of 76535
4.2 Ga
![Page 48: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/48.jpg)
Thermal History of 76535
4.2 Ga
![Page 49: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/49.jpg)
Thermal History of 76535
4.2 Ga
![Page 50: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/50.jpg)
Thermal History of 76535
Post 4.2 Ga?
4.2 Ga
![Page 51: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/51.jpg)
Thermal History of 76535
Post 4.2 Ga?
4.2 Ga
![Page 52: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/52.jpg)
Constraints at 3.9 Ga
No evidence for argon disturbances
![Page 53: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/53.jpg)
Thermal History of 76535
Post 4.2 Ga?
4.2 Ga
![Page 54: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/54.jpg)
Thermal History of 76535
4.2 Ga
![Page 55: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/55.jpg)
Thermal History of 76535
4.2 Ga
Other arguments rule out importance
of very brief (~1000 s)
heating events.
![Page 56: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/56.jpg)
Rock’s size constrains heating timescale…
• If the rock was ever briefly heated, it must have been conductively heated – Vs. instantaneously due to
shock.– E.g. in an ejecta blanket.
• Time for conductive heating can be calculated: compare to impact-generated field lifetimes.
Ejecta
Rock
Soil
![Page 57: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/57.jpg)
Rock’s size constrains heating timescale…
Model as a sphere of radius 2.5 cm
The heating timescale: order 1000 seconds.Impact-generated fields: order < 100 seconds.Therefore, impact fields could not likely be a
source of magnetization post-3.9 Ga
Hot ejecta
![Page 58: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/58.jpg)
Thermal History of 76535
4.2 Ga
Rule out importance of
very brief (~1000 s)
heating events.
![Page 59: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/59.jpg)
Thermal History of 76535
4.2 Ga
First Magnetization
Second Magnetization
![Page 60: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/60.jpg)
Duration of Fields
• It is remarkable that the rock experienced two well-constrained cooling events and has two magnetization components.
• Timescale for each cooling event was much longer than the predicted lifetime of magnetic fields from impacts (max. 1 day).– These magnetization components were likely from
long-lived fields.
![Page 61: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/61.jpg)
Strength of Fields
• Inferred field strength: at least ~1 microtesla.– Determined by applying laboratory fields, and
comparing lab remanence with actual remanence.– Calibrating this technique is difficult.
• The minimum strength is greater than fields expected from the Earth, Sun, protoplanetary disk, or galactic fields.
• The most plausible source of long-lived microtesla-strength fields is a core dynamo.
![Page 62: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/62.jpg)
Why accept a dynamo?
![Page 63: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/63.jpg)
Paleofield from 76535
Modern Earth field (~ 50 μT)
![Page 64: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/64.jpg)
Structure of the Moon
Rich in heat producing elements
![Page 65: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/65.jpg)
Solid body dynamos
Earth Mars
Ganymede Mercury
Moon Asteroids
![Page 66: The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.](https://reader035.fdocuments.in/reader035/viewer/2022062713/56649f555503460f94c7933b/html5/thumbnails/66.jpg)
Thanks to
• Shelsea Peterson & Sarah Slotznick
• Gary Lofgren, Linda Watts, Andrea Mosie (Johnson Space Center)
• CAPTEM
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Lunar crustal magnetism
Impact plasmas may generate/amplify fields.Combined with simultaneous high shock pressures associated with
impacts, rock can become magnetized.
Hood and Artemieva 2008