The Search for a Lunar Dynamo Ian Garrick-Bethell Brown University NLSI Director’s Seminar, January 19, 2010.

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