the search for a lunar dynamo ian garrick-bethell brown university nlsi director’s seminar,...
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The Search for a Lunar Dynamo
Ian Garrick-Bethell
Brown University
NLSI Director’s Seminar, January 19, 2010
The utility of planetary magnetism
Earth Mars
Ganymede Mercury
Moon Asteroids(in order of decreasing radius)
The utility of planetary magnetism
Earth Mars
Ganymede Mercury
Moon Asteroids(in order of decreasing radius)
What is the structure of the Moon?
Core evidence: seismic, moment of inertia, magnetic induction, and wobble.
Rich in heat producing elements
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
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
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
Crustal magnetism discovered
Russell et al. 1974Apollo 15 and 16 subsatellites
Crustal magnetism discovered
Apollo 16 magnetometerApollo 12 magnetometer
Does crustal magnetism = dynamo?
From Mark Wieczorek’s 2009 AGU Talk
The lunar rock magnetic record
Wieczorek, et al. (2006) & Cisowski and Fuller (1987)
Modern Earth field (~ 50 μT)
Modern Earth field (~ 50 μT)?
Wieczorek, et al. (2006) & Cisowski and Fuller (1987)
The lunar rock magnetic record
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.”
Rock magnetic approach
• We seek rocks with ages > 4.0 Ga.
• But we also carefully select a rock with favorable petrologic history.
We started looking at a lot of old rocks
76535 – Pristine Troctolite
• Age: 4.2-4.3 Ga
• Argon age
• Plutonic
• No shock effects
1 mm
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.
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.
Alternating Field Demagnetization
z
x
y
Magnetization vectorDemag. Step 1
Sample
Ideally, trends to the origin
Demag. Step 2
z
x
y
y
x
Magnetization
Sample
Alternating Field Demagnetization
z
x
y
ySample
z
Alternating Field Demagnetization
Display of demagnetization
=+ x,y
y,z
y
zy
x
Both Projections
Two Samples
Magnet-like overprints: IRMs
Easily reproduced/removed
Once removed, first sample:
Two samples:
Second component decays to origin
Second component decays to origin
Second component decays to origin
MCMC
HCHC
Two Magnetization Components
z
x
y
1
2Net
Two Magnetization Components
z
x
y
2Net
Two Magnetization Components
z
x
y
2Net
Two Magnetization Components
z
x
y
Net
Two Magnetization Components
z
x
y
Two Magnetization Components
z
x
y
Two Magnetization Components
z
x
y
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
Mutually Oriented Samples
145°
HC
MC
?145°
HC
MC
145°
HC
MC
2/3 Mutually Oriented Samples
3 Components of 3 Samples
Best fit directions
3 Components of 3 Samples
MC-HCdistances:
147°123°81°
Compared with:142-149° previously
Best fit directions
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.
Thermal History of 76535
4.2 Ga (multiple chronometers)
Thermal History of 76535
4.2 Ga (multiple chronometers)
Thermal History of 76535
First Magnetization
4.2 Ga
Thermal History of 76535
4.2 Ga
Thermal History of 76535
4.2 Ga
Thermal History of 76535
4.2 Ga
Thermal History of 76535
4.2 Ga
Thermal History of 76535
Post 4.2 Ga?
4.2 Ga
Thermal History of 76535
Post 4.2 Ga?
4.2 Ga
Constraints at 3.9 Ga
No evidence for argon disturbances
Thermal History of 76535
Post 4.2 Ga?
4.2 Ga
Thermal History of 76535
4.2 Ga
Thermal History of 76535
4.2 Ga
Other arguments rule out importance
of very brief (~1000 s)
heating events.
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
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
Thermal History of 76535
4.2 Ga
Rule out importance of
very brief (~1000 s)
heating events.
Thermal History of 76535
4.2 Ga
First Magnetization
Second Magnetization
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.
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.
Why accept a dynamo?
Paleofield from 76535
Modern Earth field (~ 50 μT)
Structure of the Moon
Rich in heat producing elements
Solid body dynamos
Earth Mars
Ganymede Mercury
Moon Asteroids
Thanks to
• Shelsea Peterson & Sarah Slotznick
• Gary Lofgren, Linda Watts, Andrea Mosie (Johnson Space Center)
• CAPTEM
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