MAGNETIC HISTORY OF THE MOON AND MARS

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MAGNETIC HISTORY OF THE MOON AND MARS

• Crustal magnetization of two evolved solar system
  bodies without active dynamos.

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EARTH
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MARS
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Moon
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Generation of magnetic fields
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Observations
Moon Site magnetometer surveys Sub-satellite
magnetometers surveys Satellite surveys - Lunar
prospector Electron reflectance experiment
Apollo returned samples Lunar meteorites
Mars Satellite magnetometer surveys Electron
reflectance experiment Martian meteorites
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Apollo landing sites

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Apollo Magnetometer Site Surveys
Dyall, Parkin and Sonnett,1970
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Sub-satellite magnetometer surveys
Coleman et al., 1973
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Electron reflectance Principle
Lin, McGuire and Anderson,1974
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Results from ER experiment.
Lin, Anderson, and Hood, 1988.
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Reiner Gamma type swirls
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Apollo
Returned samples - mare basalts, breccias, and
soil
Rock magnetism
Paleomagnetism
Implications for Lunar magnetism
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Rock magnetism

• Fe in basalt,
• FeNi Melt breccia and
• Superparamagetic Fe in Soil

Nagata et al., 1972
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AF demagnetization principle
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Mare basalts
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Melt breccias
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Regolith Breccias
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Shock Remanent Magnetization
Cisowski et al., 1973
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More Shock Effects
Cisowski et al., 1973 Srnka et al., 1979
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Lunar Basalts Paleointensity Summary
Cisowski and Fuller, 1986
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Lunar paleointensity Linear f(t)
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Lunar Paleointensity log f(t)
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Lunar Prospector Profile
Mitchell et al, submitted to Icarus
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Lunar Prospector
Mitchell et al, submitted to Icarus
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Abundance and Distribution of Iron on the
MoonLucey, Taylor and Malaret (1995)
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MAG Impact Basins
Fe
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Antipodal magnetization
Hood, 1987
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On to Mars
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Mars Crustal Magnetic Anomalies
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Comparison between
Comparison between terrestrial and martian
crustal anomalies
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Models for Martian Crustal Magnetic Anomalies
Magnitude and Occurrence
Sea floor spreading (Connerney et al,1999),
Terrane accretion (Fairen et al, 2001) Igneous
model (Hammer and Brachfield 2003) Hydrothermal
alteration of igneous rocks (Harrison 2001,
Solomon et al., 2003)
Magnetic phases invoked Magnetite,
Pyrrhotite, Hematites, Hemo-ilmenties, Maghemite
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High resolution SQUID
Weiss et al., 2002
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Fine magnetite in carbonate
Magnetite in Martian meteorite ALH84001
Transmission electron microscope image showing
oriented elongated magnetites (M) and regions
containing periclase (P) in carbonate in ALH84001
(Barber and Scott, 2002).
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Paleomagnetism of ALH84001
AF Demagnetization of NRM
Soft -reversed
Soft magnetization
Hard magnetization
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AF DEMAGNETIZATIONCHARACTERISTICS
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Demagnetization of IRMs
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Magnetization of Martian Crust
Modeling suggests crust locally magnetized in
southern hemisphere to 20 km with intensity
10 A m-1 (Connerney, Nimmo, Parker).
Martian dynamo probably reversed during
magnetization of crust (Arkani-Hamad).
Intensity of magnetization acquired by rocks
depends on 1. Magnitude of Martian field. 2.
Mechanism of magnetization. 3. Magnetic
material
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Magnitude of Martian field
Studies of ALH84001 suggest it was magnetized
at 4.0 Gyr in a field 10 x smaller than
Earths field (Antretter et al., 2003
Weiss et al., 2002). Distribution of anomalies
suggests they were acquired before 4.0
Gyr. Volume of martian core is 7 Earths
core. Unlikely that martian dynamo much
stronger than geodynamo. We infer
martian surface field comparable to Earths
field gt4.0 Gyr. Note scaling of dynamo
action and effect of size of planet on
anomalies.
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Mechanism of magnetization
Generally assumed that martian magnetic
minerals formed at high temperature above Curie
point and were magnetized on cooling (TRM).
If magnetic minerals formed below Curie
point (called Chemical RM), intensity of
remanent magnetization is comparable to TRM.
In geomagnetic field CRM (or TRM) 10-2 of
saturation remanent magnetization. Not
necessary to form magnetic mineral by igneous
process.
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Formation of 30-200 nm single-domain magnetite
Quenched margins of basalts Oxidation and
exsolution of ilmenite from Ti-rich
magnetite (Nimmo, 2000). Exsolution from
plagioclase Need mechanism that hasnt
operated on Earth during past 4 Gyr.
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Magnetization model
ALH84001 contains 1 vol Fe-rich carbonate.
Carbonate contains 1 vol. SD magnetite due to
partial decomposition by impact heating
at 4.0 Gyr (Golden et al., Barber Scott).
Carbonate deposited in ALH by hydrous fluid in
crust (Bridges et al.). Lab heating can
entirely convert siderite into porous aggregate
of SD magnetite (Golden et al., Koziol and
Brearley). Twostep model a) deposit 1
siderite locally to 20 km in martian crust
from CO2-rich fluid, b) decompose siderite by
magmatic heating to give 0.5 SD magnetite.
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Decomposition of siderite to single-domain
magnetite (Koziol and Brearley, 2002)
TEM Images of thermally decomposed siderite
crystals that were transformed completely to form
clusters of single-domain magnetites by heating
briefly at 470C.
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Outline of Model (1)
In southern highlands
When dense CO2 atmosphere present and water is
stable, Fe-rich is carbonate deposited in
crust. Continued growth of crust from magmatic
intrusions causes siderite decomposition
forming single-domain magnetite. Locally
1 carbonate forms to depth of 20 km and
is converted to 0.5 SD magnetite. Earth-like
field from reversing dynamo induces intensity of
magnetization of up to 10 A m-1 prior to
4.0 Gyr. SD magnetite stable over 4 Gyr due to
lack of water, as in ALH84001.
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Suggested Origin of Single-Domain Magnetite in
Martian crust during Noachian (Hydrology after
Clifford Parker 2001)