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1811-5209/09/0005-0223$2.50 DOI: 10.2113/gselements.5.4.223

Magnetism of Extraterrestrial Materials

INTRODUCTIONSolid matter in our solar system began to assemble 4.5 billion years ago (Ga). This material has recorded a large range of processes, including metamorphism, melting, particle irradiation, and hypervelocity impacts. Beginning in the late 1950s (Stacey and Lovering 1959), the study of extraterrestrial magnetism reached a golden age during the era of the first lunar sample return and in situ magnetic field measurements around other solar system bodies (Fuller and Cisowski 1987; Sugiura and Strangway 1987). The study of magnetization of extraterrestrial materials (ETM) provided a rock magnetic basis for understanding the origin of present fields measured by spacecraft and furnished clues for reconstructing the history of early solar system magnetic fields recorded by natural remanent magnetization (NRM) in ETM. It was soon recognized that ETM contained magnetic minerals (metallic phases) and were subjected to physical processes (e.g. shock and irradia-tion) that were unfamiliar on Earth. After nearly two decades of relative dormancy, the field of extraterrestrial magnetism has recently been reactivated. This has been linked to the availability of new concepts and techniques, in particular high-sensitivity and high-spatial-resolution rock magnetometers (e.g. Weiss et al. 2008a), and to new spacecraft magnetometry observations, like the discovery of strong crustal remanence on Mars. These developments have been accompanied by a new and deeper understanding of the magnetic properties and provenance of ETM.

ETM available for experimental studies in the laboratory are in the form of meteorites collected at the Earth’s surface (over 50,000 in number ranging from 0.1 g to >100 kg

each), returned lunar samples (>380 kg from 9 sampling sites), submillimeter- to micron-size samples collected on Earth and in the strato-sphere (micrometeorites and inter-planetary 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 source regions are not the same. Several in situ magnetometry experiments on planetary surfaces have also been

conducted: passive magnet experiments on Mars rovers, the Apollo lunar surface magnetometers, and the Lunokhod 2 rover magnetometer.

Magnetic studies have been conducted on both chondrites (more or less metasomatized and/or metamorphosed aggre-gates of early condensates and chondrules that presumably sample small planetesimals) and achondrites (which have experienced partial or full melting following accretion of their parent body). About 90% of the known meteorites are chondrites. The standard paradigm until recently was that achondrite parent bodies formed after chondrite parent bodies. However, recent progress in the use of radio-genic isotopes to time events during the first 10 million years of solar system history has challenged this paradigm: many achondrite parent bodies formed and differentiated before chondrites, and some chondrites may in fact be made of fragments of large achondritic bodies that have been destroyed by impact. NRM in many achondrites, and possibly even in chondrites, is likely a record of early core dynamos in the parent planetesimals (Weiss et al. 2008b; Fig. 1). Alternatively, one may hope to retrieve the intensity of pre-accretionary fields, although the number of materials that have retained such an original magnetic record is likely to be minimal due to subsequent remagnetization processes. The magnetic field present in the early solar nebula and linked to the presumably huge early solar elec-tromagnetic activity is a major question in astronomy, as the magnetic field may have played a key role in controlling the dynamic conditions (e.g. trajectory, pressure, temperature, and irradiation) of the accreting matter. Astronomers have 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).

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 mete-orites from the Moon and Mars. Formation of meteorite

Extraterrestrial 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.

Keywords: meteorites, paleomagnetism, shock, dynamos, magnetic fields

Pierre Rochette1, Benjamin P. Weiss2, and Jérôme Gattacceca1

1 CEREGE, CNRS Aix-Marseille University, Aix-en-Provence, France E-mail: rochette@cerege.fr; gattacceca@cerege.fr

2 Department of Earth, Atmospheric, and Planetary Sciences 54-814, Massachusetts Institute of Technology Cambridge, MA 02139, USA E-mail: bpweiss@mit.edu

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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 sysTEMThe 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),

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 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 antiferro-magnets (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 Fe0-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)3X, where X = C, P, or Si, corre-sponding 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 transi-tions during cooling, exsolution, and spinodal decomposi-tion 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-xS). Pyrrhotite plays a major role in the

Figure 1 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

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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.

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 applica-tions 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 chon-drites, which were assembled by relatively rapid aggrega-tion, 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 disper-sion as terrestrial magmatic rocks (basalts and granites). Brecciation and regolithization also contribute to the varia-tions 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 postmag-matic process for the origin of metal in the ureilite: the reduction of olivine by carbon-rich fluids.

sHOCk EffECTsMost ETM parent bodies have been subjected to billions of years of impacts, and all meteorites were naturally exca-vated 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 brec-ciated 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 corre-lated with shock stage as derived from petrographic obser-vations and porosity values, which is indicative of shock compaction (Gattacceca et al. 2005).

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 remag-netization 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

Figure 2 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 (*).

B

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Weiss et al. (2008a). This must be performed by NRM deter-mination 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 transfor-mations (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 perpen-dicular 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 develop-ment of this instrument required overcoming the techno-logical 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).

COsMIC DUsTSince 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

Figure 3 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 4 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.

B C

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melted spherules, with the iron and silicate (here barred-olivine) types shown. Atmospheric heating has produced abundant magnetite, in which Cr, Ni, and other elements substitute. Magnetite is responsible for the strong magne-tization 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 signif-icantly to the magnetization of terrestrial ice cores.

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 sidero-phile 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 contamina-tion 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 resis-tant to oxidation by CO2 + H2O than metal and sulfide (Rochette et al. 2006).

PERsPECTIvEsA proper understanding of the rock magnetic properties of minerals in ETM and the secondary processes that have affected them (shock and low-temperature phase transfor-mations) is essential for a grounded interpretation of the paleomagnetic record of ETM. Although our current under-standing of these issues is primitive compared to our knowl-edge 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 analyt-ical 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 geochem-istry are opening new avenues for contextualizing our magnetic exploration of the solar system. The extrater-restrial paleomagnetic record is complementary to geochem-istry and petrology and provides unique paleogeophysical clues for constraining the early differentiation and thermal history of solar system bodies.

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 subse-quent 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

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 manu-script. 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.

Figure 6 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.

Figure 5 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).

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