Magnetism of
Extraterrestrial Materials
Pierre Rochette1, Benjamin P. Weiss2, and Jérôme Gattacceca1
1811-5209/09/0005-0223$2.50 DOI: 10.2113/gselements.5.4.223
E
xtraterrestrial materials contain a diversity of ferromagnetic phases,
ranging from common terrestrial oxides to exotic metal alloys and silicides.
Because of their great age and remote provenance, meteorites provide
a unique window on early solar system magnetic fields and the differentiation
of other bodies. Interpreting the records of meteorites is complicated by their
poorly understood rock magnetic properties and unfamiliar secondary
processing by shock and low-temperature phase transformations. Here we
review our current understanding of the mineral magnetism of meteorites
and the implications for magnetic fields on their parent bodies.
each), returned lunar samples
(>380 kg from 9 sampling sites),
submillimeter- to micron-size samples
collected on Earth and in the stratosphere (micrometeorites and interplanetary dust particles), and dust
recently sampled by the Stardust
mission as it passed through a
comet’s tail. Studying such “cosmic
dust” is a particular experimental
challenge compared to macroscopic
meteorites but provides different
information since their respective
Keywords : meteorites, paleomagnetism, shock, dynamos, magnetic fields
source regions are not the same. Several
in situ magnetometry experiments
INTRODUCTION
on planetary surfaces have also been
Solid matter in our solar system began to assemble 4.5 conducted: passive magnet experiments on Mars rovers,
the Apollo lunar surface magnetometers, and the Lunokhod
billion years ago (Ga). This material has recorded a large
2 rover magnetometer.
range of processes, including metamorphism, melting,
particle irradiation, and hypervelocity impacts. Beginning
Magnetic studies have been conducted on both chondrites
in the late 1950s (Stacey and Lovering 1959), the study of
(more or less metasomatized and/or metamorphosed aggreextraterrestrial magnetism reached a golden age during the
gates of early condensates and chondrules that presumably
era of the first lunar sample return and in situ magnetic
sample small planetesimals) and achondrites (which have
field measurements around other solar system bodies
experienced partial or full melting following accretion of
(Fuller and Cisowski 1987; Sugiura and Strangway 1987).
their parent body). About 90% of the known meteorites
The study of magnetization of extraterrestrial materials
are chondrites. The standard paradigm until recently was
(ETM) provided a rock magnetic basis for understanding
that achondrite parent bodies formed after chondrite
the origin of present fields measured by spacecraft and
parent bodies. However, recent progress in the use of radiofurnished clues for reconstructing the history of early solar
genic isotopes to time events during the first 10 million
system magnetic fields recorded by natural remanent
years of solar system history has challenged this paradigm:
magnetization (NRM) in ETM. It was soon recognized that
many achondrite parent bodies formed and differentiated
ETM contained magnetic minerals (metallic phases) and
before chondrites, and some chondrites may in fact be
were subjected to physical processes (e.g. shock and irradiamade of fragments of large achondritic bodies that have
tion) that were unfamiliar on Earth. After nearly two
been destroyed by impact. NRM in many achondrites, and
decades of relative dormancy, the field of extraterrestrial
possibly even in chondrites, is likely a record of early core
magnetism has recently been reactivated. This has been
dynamos in the parent planetesimals (Weiss et al. 2008b;
linked to the availability of new concepts and techniques,
Fig. 1). Alternatively, one may hope to retrieve the intensity
in particular high-sensitivity and high-spatial-resolution
of pre-accretionary fields, although the number of materials
rock magnetometers (e.g. Weiss et al. 2008a), and to new
that have retained such an original magnetic record is
spacecraft magnetometry observations, like the discovery
likely to be minimal due to subsequent remagnetization
of strong crustal remanence on Mars. These developments
processes. The magnetic field present in the early solar
have been accompanied by a new and deeper understanding
nebula and linked to the presumably huge early solar elecof the magnetic properties and provenance of ETM.
tromagnetic activity is a major question in astronomy, as
the magnetic field may have played a key role in controlling
ETM available for experimental studies in the laboratory
are in the form of meteorites collected at the Earth’s surface the dynamic conditions (e.g. trajectory, pressure, temperature,
and irradiation) of the accreting matter. Astronomers have
(over 50,000 in number ranging from 0.1 g to >100 kg
detected fields of the order of 100 milliteslas (mT) in the
inner part of a protoplanetary disk equivalent to our solar
system 4.5 Gy ago (Donati et al. 2005).
1 CEREGE, CNRS Aix-Marseille University, Aix-en-Provence, France
E-mail: [email protected]; [email protected]
2 Department of Earth, Atmospheric, and Planetary Sciences
54-814, Massachusetts Institute of Technology
Cambridge, MA 02139, USA
E-mail: [email protected]
E lements , V ol . 5,
pp.
223 –228
The parent bodies of meteorites and micrometeorites are
essentially unknown, except for the following achondrite
groups: the howardite–eucrite–diogenite clan (HED, inferred
to come from the second largest asteroid, Vesta) and meteorites from the Moon and Mars. Formation of meteorite
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A ugus t 2009
with implied magnetizations of 1 ampere per meter (A/m).
On the other hand, nearly half of the surface area of Mars
today generates strong magnetic anomalies equivalent in
strength to the Earth’s total surface field (tens of µT or
more) (Langlais et al. 2004) and sourced from deep crustal
magnetization with intensities of ~10 A/m. While an origin
by crustal cooling in a dynamo field (active in the first few
hundred million years of Mars history) is widely accepted
for Mars (Antretter et al. 2003; Langlais et al. 2004; Rochette
et al. 2005; Weiss et al. 2008a), the lunar case is more debated
(Runcorn et al. 1970; Fuller and Cisowski 1987; Lawrence
et al. 2008; Garrick-Bethell et al. 2009). Evidence for fields
sourced from remanent magnetization around other bodies
is elusive: it has been suggested that the roughly dipolar
fields around Mercury and the Jovian satellite Io could be
(at least partly) of remanent origin, but the dynamo hypothesis
remains more supported (Stevenson 2003). Ganymede, an
icy satellite of Jupiter, also likely has a core dynamo field.
The identification of magnetized asteroids has been more
equivocal (Acuña et al. 2002), although the color of Vesta
suggests it may be shielded from solar wind by a local
magnetosphere (Vernazza et al. 2006).
Magnetic Minerals in the Solar System
Artistic rendition of a small-body (>80 km diameter)
dynamo in a field of planetesimals 4.5 Gy ago. The
image does not reflect a real inferred density of planetesimals or the
proportion of them with active dynamos. Image courtesy Damir Gamulin
Figure 1
parent bodies at variable distance from the Sun is assumed
to explain the large range of compositions observed. The
majority of these parent bodies were likely formed in the
present asteroid belt between Mars and Jupiter. A minor
population may also have been formed elsewhere and
stored in the asteroid belt after their formation. Fireball
trajectories point toward a source in the asteroid belt for
nearly all observed meteorite falls. A cometary origin has also
been suggested for the rare (<1% of all meteorites) CI-type
of highly hydrated and porous carbonaceous meteorites
(Gounelle et al. 2006).
Contribution of Remanent Magnetism
to Present-Day Fields in the Solar System
The mean magnetic field at the Earth’s surface is composed
of the actively generated core-dynamo field (~50 µT) plus
minor contributions from remanent and induced crustal
magnetization (~15 nT at 400 km altitude) (Langlais et al.
2004; McEnroe et al. 2009 this issue). Although core dynamos
in rocky bodies were more common in the solar system in
the distant past (see below), today in the inner solar system,
they are present only in the Earth and probably Mercury.
Therefore, the surface fields of other bodies are sourced
from purely remanent magnetization in the planetary
crust. On the Moon, a small number of thin (likely <1 km
thick) crustal sources generate isolated magnetic field
anomalies (≤10 nT at 40 km altitude) (Nicholas et al. 2007),
E lements
Because iron is the second most abundant element by mass
in ETM after oxygen, it is expected that ETM may exhibit
strong magnetization due to the presence of iron-bearing
ferromagnetic minerals. The common sense definition of
a “magnetic material” is one that bears a spontaneous
magnetization at room temperature. Therefore antiferromagnets (like troilite) and minerals with a magnetic
ordering temperature below room temperature (like wüstite,
ilmenite, and Fe-bearing silicates) will not be considered
here. Except at the Martian surface (Rochette et al. 2006),
the bulk oxidation state of ETM is generally too low for
the presence of pure Fe3+ -bearing phases, so that the most
oxidized phase is the mixed-valence mineral magnetite.
Pure magnetite is present in some carbonaceous chondrites
(CI, CK, CV), and titanomagnetite is present in angrites
and Martian meteorites (Rochette et al. 2005, 2008, 2009;
Weiss et al. 2008b). Because the magnetic properties of
magnetite are so much better understood than those of
other meteoritic phases, these meteorite classes are ideal
for the study of early solar system paleomagnetism. The
oldest-known Martian meteorite, ALH 84001, contains pure
magnetite nanoparticles that were originally interpreted
as fossils of magnetotactic bacteria (see discussion in Weiss
et al. 2004 and Rochette et al. 2006). It has been suggested
that chromite may also contribute to the NRM of Martian
meteorites (Yu and Gee 2005), but it is also possible that
sulfides associated with chromite may instead be the source
of this magnetization (Weiss et al. 2008a).
The most common magnetic minerals in ETM are
Fe 0 -bearing phases that are uncommon in Earth’s surface
materials. These are mainly Fe–Ni alloys (kamacite, taenite,
tetrataenite, awaruite), but they also include phases with
the general formula (Fe,Ni) 3 X, where X = C, P, or Si, corresponding to the minerals cohenite, schreibersite, and suessite,
respectively (Rochette et al. 2008, 2009). The Fe–Ni system
is very complex due to numerous subsolidus phase transitions during cooling, exsolution, and spinodal decomposition into Ni-rich and Ni-poor lamellae, and to the formation
of many metastable phases at low temperature (Cacciamani
et al. 2006). Many of these low-temperature phases, like
tetrataenite, are unique to ETM, and the way they acquire
NRM is poorly understood (Gattacceca et al. 2003; Acton
et al. 2007). ETM are also commonly rich in sulfides.
Among sulfides present in ETM, the only magnetic phase is
pyrrhotite (Fe1-x S). Pyrrhotite plays a major role in the
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magnetic properties of Martian meteorites as well as in the
carbonaceous and R-type chondrites (Rochette et al. 2005, 2008).
Metal and sulfide phases are prone to terrestrial weathering,
which may bias the magnetic signal of meteorite finds.
A
As a result of a new exhaustive database of low-field magnetic
susceptibility (χ) for meteorites (over 4500 specimens
measured; Rochette et al. 2009 and references therein), we
now have a good picture of the abundances of magnetic
minerals, both at the meteorite scale (10–100 cm) and at
the group scale (parent-body size). Data have been averaged
in two steps: first at the meteorite scale (resulting in a mean
and standard deviation for each meteorite for which several
specimens were measured), and then at the meteorite group
scale. The magnetic susceptibility of meteorites ranges over
four orders of magnitude: logχ (with χ in 10 -9 m3/kg) varies
from 1.7 (in lunar meteorites, the least magnetic ETM) to
5.7 (pure metallic iron meteorites). Figure 2a shows that
for chondrites the standard deviation (s.d.) at the group
scale is usually low relative to inter-group differences (apart
from CM, C2, and CV carbonaceous chondrites), which
permits the use of logχ values as a rapid classification tool.
Moreover, the standard deviation at the meteorite scale is
lower than that of its group (below 0.1 on logχ, i.e. less
than 25% relative variation on χ), indicating that many
meteorites can be directly identified purely based on
susceptibility. This method has valuable curatorial applications due to its rapidity and nondestructive nature (it does
not even require subsampling). Such measurements are
now being used for preliminary classification of newly
found meteorites and for scanning established collections
to identify misclassified meteorites or mislabeled samples.
Comparing standard deviations at the meteorite and group
scales provides clues to petrogenetic processes. The chondrites, which were assembled by relatively rapid aggregation, are more homogeneous at the 1–10 cm scale than the
achondrites, which are the products of open-system melting
and long-term metamorphism (Fig. 2b, after Rochette et al.
2009). In fact, achondrites show the same type of dispersion as terrestrial magmatic rocks (basalts and granites).
Brecciation and regolithization also contribute to the variations in metal concentration. A single achondrite group,
the unbrecciated ureilites, has exceptionally low standard
deviation at the meteorite scale. Rochette et al. (2009)
proposed that this anomaly is linked to a specific postmagmatic process for the origin of metal in the ureilite: the
reduction of olivine by carbon-rich fluids.
Shock Effects
Most ETM parent bodies have been subjected to billions
of years of impacts, and all meteorites were naturally excavated from the interiors of their parent bodies by impacts.
As demonstrated by petrographic studies, most ETM show
evidence for multiple shock events, with peak pressures
typically in the range of 5 to 50 gigapascals. Such shock
events have deeply affected the structure and mineralogy
of ETM. Understanding the effects of shock on the rock
magnetic properties and the paleomagnetic record of ETM
is a critical and active area of investigation, initiated in
particular by Nagata et al. (1972).
One effect of shock processing is that ETM are often brecciated down to the millimeter scale and plastically deformed
by shock compaction. Indeed, ordinary chondrites have
been shown to be strongly anisotropic, based on magnetic
susceptibility anisotropy and chondrule shape analysis.
Moreover, the amount of magnetic anisotropy is well correlated with shock stage as derived from petrographic observations and porosity values, which is indicative of shock
compaction (Gattacceca et al. 2005).
E lements
B
Magnetic susceptibility of meteorites. (A) Mean magnetic
susceptibility (logχ, with χ in 10 -9 m3/kg) for different
chondrite groups (standard deviation is given by bar length) (after
Rochette et al. 2008). (B) Mean of logχ individual standard deviation
(s.d.) (i.e., at the meteorite scale) versus s.d. of meteorite means at
the group scale for chondrites, achondrites, and two sets of terrestrial
magmatic rocks (after Rochette et al. 2009). Only meteorite groups
with logχ < 5 are plotted to avoid dispersion due to shape anisotropy.
The highly magnetic Martian subgroup is indicated by an asterisk (*).
Figure 2
Impact can remagnetize the NRM of ETM. Remagnetization
occurs readily if an ambient field is present during passage
of the shock wave (Gattacceca et al. 2008; Funaki and
Syono 2008), but it is also possible that the impact itself
could produce a short-lived field (Hood and Artemieva
2008). This means that remanent magnetization in shocked
ETM may have no relationship to ambient parent body or
external early solar system magnetic fields. Shock remagnetization can be directly linked to a stress effect (Gattacceca
et al. 2006, 2008; Fig.3a) or, for higher shock pressures, to
shock-induced heating or mineral transformation. Depending
on shock intensity and characteristics, intrinsic magnetic
properties (in particular coercivity) may or may not be
affected by the shock. One method for identifying pre-shock
NRM is to observe random directions among clasts in a
brecciated material, as done by Gattacceca et al. (2003) and
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Weiss et al. (2008a). This must be performed by NRM determination at below-the-clast scale through millimeter-scale
subsampling (Gattacceca et al. 2003) or magnetic microscopy
(Weiss et al. 2008b; Fig. 3a, c). However, this methodology
is somewhat ambiguous because magnetic heterogeneity
in a shocked sample might be the product of processes
other than cold brecciation, like subsolidus phase transformations (Gattacceca et al. 2003) or heterogeneous shock
heating (Weiss et al. 2008a). How to distinguish between these
two outcomes is still an active area of investigation.
Magnetic microscopy is currently used in a number of
applications involving ETM. For example, the study of very
small samples (like from Stardust or future sample return
missions) and of small individual chondrules and inclusions
within a meteorite section can only be achieved by this
technique. Maps of the magnetic field component perpendicular to the section presented in Figure 3 were obtained
using a new magnetometer called the SQUID microscope,
which has unrivaled sensitivity (moment resolution of 10 -15
A·m2) and spatial resolution reaching 140 µm. The development of this instrument required overcoming the technological challenge of keeping the SQUID sensor at 4.2 K and
separated from the room-temperature sample surface by a
distance of only 140 µm. To derive magnetizations from the
field maps, an inversion is necessary, as performed for satellite
data (e.g. Langlais et al. 2004). Alternative magnetic field
sensors working at room temperature are currently being
developed to obtain higher resolution and easier operation
(avoiding the cryogenic problems), but they cannot reach
the SQUID sensitivity.
A number of ETM magnetic minerals (FeNi metal, cohenite,
pyrrhotite) undergo phase transformations under pressure,
resulting in a loss of NRM if the material has been cycled
through this phase transformation (Rochette et al. 2005).
Shock and the associated high temperatures can be responsible
for the generation of new magnetic minerals, often in the
form of nanoparticles. There is now abundant evidence
that the magnetite nanoparticles found in ALH 84001 were
generated by such a process (Golden et al. 2004). Metal
nanoparticles have recently been observed in the so-called
“black olivine” grains found in two highly shocked (>50 GPa)
Martian meteorites (Van de Moortèle et al. 2007; Fig. 4).
A
Cosmic Dust
Since the first report of abundant metal-bearing magnetic
spherules in deep-sea sediments and manganese oxide
crusts (Murray and Renard 1891), it has been shown that the
main flux of extraterrestrial matter to Earth is made up of
dust particles (<1 mm in diameter) (e.g. micrometeorites).
Much of this material has been extensively transformed
by heating and oxidation during atmospheric entry, often
to the point of complete melting. Figure 5 portrays such
B
C
Maps of magnetic fields (in nT) measured with the SQUID
microscope. (A) A thin section map of terrestrial basalt
demagnetized by two laser impacts, corresponding to the two disks
with negative field (Gattacceca et al. 2006). (B) A thin section map
of the Martian meteorite ALH 84001 (Weiss et al. 2008a). (C) Optical
photomicrograph of the thin section map shown in (B) highlighting
the strongly magnetized fusion crust (dark line at left) and chromite
grains (dark spots).
Figure 3
E lements
Shock-induced metallic FeNi nanoparticle in olivine from
the Martian meteorite NWA 2737, after Van de Moortèle
et al. (2007), as seen with high-resolution TEM. Indexed diffraction
spots are shown in inset.
Figure 4
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melted spherules, with the iron and silicate (here barredolivine) types shown. Atmospheric heating has produced
abundant magnetite, in which Cr, Ni, and other elements
substitute. Magnetite is responsible for the strong magnetization of most spherules (Suavet et al. 2009). In fact,
magnetic extraction from sediments or soil appears to be the
most efficient way to retrieve micrometeorites. According
to measurement of single spherules (in the 200–600 µm
diameter range), cosmic spherules can contribute significantly
to the characteristic NRM of sediments formed by low
accumulation rates. Lanci and Kent (2006) have also shown
that cosmic dust (size range below 1 µm) contributes significantly to the magnetization of terrestrial ice cores.
during a sabbatical year in 2001, and to V. Dekov who
provided him with the images in figure 5a, b. M. Fuller, S.
Russell, J. Feinberg, and R. Harrison are acknowledged for
their reviews, which helped to improve the initial manuscript. B.P.W. thanks the NASA Lunar Science Institute and
the NASA Mars Fundamental Research, NASA Lunar
Advanced Science and Exploration Research, and the NSF
Geophysics Programs for support.
Accretion of materials of chondritic composition results in
a significant contribution of ferromagnetic minerals to the
surfaces of the Moon and Mars. The intensity of this flux
is evident in the regolith abundances of metal and siderophile elements like Ni and Ir. On the lunar surface, metal
is most abundant in the uppermost regolith and in the smallest
grain size regolith fraction (Nagata et al. 1972; Fuller and
Cisowsky 1987). In lunar meteorites from the anorthositic
highlands (the main crustal terrane on the Moon), the
metal abundance as measured by magnetic susceptibility
correlates well with the amount of Ni and Ir, indicating that
much of the metal was derived from exogenous contamination by chondritic materials (Fig. 6). In fact, only a part of
the metal in lunar surface rocks and soils is of exotic origin.
Another large part is generated by impact-induced reduction
of Fe2+ -bearing silicates (Sasaki et al. 2001).
On the Martian surface, a large amount of metal as well
as iron sulfide has been accreted through time. However, these
reduced species are continuously oxidized by the Martian
atmosphere (Rochette et al. 2006). It has thus been suggested
that the red color of Mars is due to the oxidation of cosmic
dust rather than to the oxidation of Martian rocks, whose
iron-bearing phases (silicates and magnetite) are more resistant to oxidation by CO2 + H 2O than metal and sulfide
(Rochette et al. 2006).
Perspectives
A proper understanding of the rock magnetic properties
of minerals in ETM and the secondary processes that have
affected them (shock and low-temperature phase transformations) is essential for a grounded interpretation of the
paleomagnetic record of ETM. Although our current understanding of these issues is primitive compared to our knowledge of the history and magnetism of terrestrial rocks, many
samples already in our collections are unshocked and
contain well-understood minerals like magnetite as their
major ferromagnetic (sensu lato) phase. Furthermore, many
ETM have already been analyzed by a wide range of analytical techniques outside of rock magnetism to a level of
detail that is rarely achieved for typical Earth rocks. In
particular, great advances in analytical radioisotope geochemistry are opening new avenues for contextualizing our
magnetic exploration of the solar system. The extraterrestrial paleomagnetic record is complementary to geochemistry and petrology and provides unique paleogeophysical
clues for constraining the early differentiation and thermal
history of solar system bodies.
A
B
C
D
Examples of cosmic spherules. Spherules in (A) and (B)
were described by Murray and Renard (1891) in deep-sea
sediments (original lithography from optical microscopy observation),
iron (380 µm) and silicate (1.6 mm) types respectively. Spherules in
(C) and (D) were found by the CEREGE group in soils from the Sahara
Desert (SEM backscatter-mode image, both 300 µm diameter).
Figure 5
Acknowledgments
This paper is dedicated to Frank Stacey, who published fifty
years ago the first papers on the magnetic properties of
meteorites (together with John Lovering). These and subsequent publications from Frank Stacey have been a major
inspiration. The first author is indebted to the INGV Roma,
which hosted his conversion to extraterrestrial matter
E lements
Log-log plot of nickel concentration versus ferromagnetic
susceptibility for anorthositic lunar meteorites
(unpublished results). Ferromagnetic susceptibility is low-field
susceptibility corrected from the paramagnetic susceptibility and is
thus a direct measure of metal amount.
227
Figure 6
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