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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

xtraterrestrial materials contain a diversity of ferromagnetic phases, each), returned lunar samples ranging from common terrestrial oxides to exotic metal alloys and silicides. (>380 kg from 9 sampling sites), submillimeter- to micron-size samples EBecause of their great age and remote provenance, provide collected on Earth and in the strato- a unique window on early solar system magnetic fields and the differentiation sphere ( and inter- of other bodies. Interpreting the records of meteorites is complicated by their planetary dust particles), and dust recently sampled by the Stardust poorly understood rock magnetic properties and unfamiliar secondary mission as it passed through a processing by shock and low-temperature phase transformations. Here we ’s tail. Studying such “cosmic review our current understanding of the mineral magnetism of meteorites dust” is a particular experimental and the implications for magnetic fields on their parent bodies. challenge compared to macroscopic meteorites but provides different Ke y w o r d s : meteorites, paleomagnetism, shock, dynamos, magnetic fields information since their respective 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 rovers, billion years ago (Ga). This material has recorded a large the Apollo lunar surface magnetometers, and the Lunokhod range of processes, including metamorphism, melting, 2 rover magnetometer. particle irradiation, and hypervelocity impacts. Beginning Magnetic studies have been conducted on both in the late 1950s (Stacey and Lovering 1959), the study of (more or less metasomatized and/or metamorphosed aggre- extraterrestrial magnetism reached a golden age during the gates of early condensates and that presumably era of the first lunar sample return and in situ magnetic sample small planetesimals) and (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 ). 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 parent bodies formed after the origin of present fields measured by spacecraft and parent bodies. However, recent progress in the use of radio- furnished clues for reconstructing the history of early solar genic 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 irradia- made 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, Fi g . 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 elec- of the magnetic properties and provenance of ETM. tromagnetic activity is a major question in astronomy, as ETM available for experimental studies in the laboratory the magnetic field may have played a key role in controlling are in the form of meteorites collected at the Earth’s surface the dynamic conditions (e.g. trajectory, pressure, temperature, (over 50,000 in number ranging from 0.1 g to >100 kg 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). 1 CEREGE, CNRS Aix-Marseille University, Aix-en-Provence, France The parent bodies of meteorites and micrometeorites are E-mail: [email protected]; [email protected] essentially unknown, except for the following achondrite 2 Department of Earth, Atmospheric, and Planetary Sciences groups: the –– clan (HED, inferred 54-814, Massachusetts Institute of Technology to come from the second largest , Vesta) and mete- Cambridge, MA 02139, USA E-mail: [email protected] orites from the and Mars. Formation of

El e m e n t s , Vo l . 5, p p . 223–228 223 Au g u s 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 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 is the second most abundant element by mass in ETM after , 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 ) 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 Fi g u r e 1 Artistic rendition of a small-body (>80 km diameter) oxidized phase is the mixed-valence mineral magnetite. dynamo in a field of planetesimals 4.5 Gy ago. The image does not reflect a real inferred density of planetesimals or the Pure magnetite is present in some carbonaceous chondrites proportion of them with active dynamos. Im a g e c o u r t e s y Da m i r Ga m u l i n (CI, CK, CV), and titanomagnetite is present in 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 parent bodies at variable distance from the Sun is assumed other meteoritic phases, these meteorite classes are ideal to explain the large range of compositions observed. The for the study of early solar system paleomagnetism. The majority of these parent bodies were likely formed in the oldest-known , ALH 84001, contains pure present asteroid belt between Mars and Jupiter. A minor magnetite nanoparticles that were originally interpreted population may also have been formed elsewhere and as fossils of magnetotactic bacteria (see discussion in Weiss stored in the asteroid belt after their formation. Fireball et al. 2004 and Rochette et al. 2006). It has been suggested trajectories point toward a source in the asteroid belt for that chromite may also contribute to the NRM of Martian nearly all observed meteorite falls. A cometary origin has also meteorites (Yu and Gee 2005), but it is also possible that been suggested for the rare (<1% of all meteorites) CI-type sulfides associated with chromite may instead be the source of highly hydrated and porous carbonaceous meteorites of this magnetization (Weiss et al. 2008a). (Gounelle et al. 2006). The most common magnetic minerals in ETM are Fe0-bearing phases that are uncommon in Earth’s surface Contribution of Remanent Magnetism materials. These are mainly Fe–Ni alloys (, , to Present-Day Fields in the Solar System tetrataenite, awaruite), but they also include phases with

The mean magnetic field at the Earth’s surface is composed the general formula (Fe,Ni)3X, where X = C, P, or Si, corre- of the actively generated core-dynamo field (~50 µT) plus sponding to the minerals , , and suessite, minor contributions from remanent and induced crustal respectively (Rochette et al. 2008, 2009). The Fe–Ni system magnetization (~15 nT at 400 km altitude) (Langlais et al. is very complex due to numerous subsolidus phase transi- 2004; McEnroe et al. 2009 this issue). Although core dynamos tions during cooling, exsolution, and spinodal decomposi- in rocky bodies were more common in the solar system in tion into Ni-rich and Ni-poor lamellae, and to the formation the distant past (see below), today in the inner solar system, of many metastable phases at low temperature (Cacciamani they are present only in the Earth and probably Mercury. et al. 2006). Many of these low-temperature phases, like Therefore, the surface fields of other bodies are sourced tetrataenite, are unique to ETM, and the way they acquire from purely remanent magnetization in the planetary NRM is poorly understood (Gattacceca et al. 2003; Acton crust. On the Moon, a small number of thin (likely <1 km et al. 2007). ETM are also commonly rich in sulfides. thick) crustal sources generate isolated magnetic field Among sulfides present in ETM, the only magnetic phase is anomalies (≤10 nT at 40 km altitude) (Nicholas et al. 2007), pyrrhotite (Fe1-xS). Pyrrhotite plays a major role in the

El e m e n t s 224 Au g u s t 2009 magnetic properties of Martian meteorites as well as in the A 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). Fi g u r e 2a shows that for chondrites the standard deviation (s.d.) at the group scale is usually low relative to inter-group differences (apart B 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 (Fi g . 2b, after Rochette et al. 2009). In fact, achondrites show the same type of disper- sion as terrestrial magmatic rocks ( and granites). Brecciation and regolithization also contribute to the varia- tions in metal concentration. A single achondrite group, the unbrecciated , 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 : the Magnetic susceptibility of meteorites. (A) Mean magnetic Fi g u r e 2 -9 3 reduction of olivine by carbon-rich fluids. susceptibility (logχ, with χ in 10 m /kg) for different chondrite groups (standard deviation is given by bar length) (after Rochette et al. 2008). (B) Mean of logχ individual standard deviation Shock Effects (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 Most ETM parent bodies have been subjected to billions magmatic rocks (after Rochette et al. 2009). Only meteorite groups of years of impacts, and all meteorites were naturally exca- with logχ < 5 are plotted to avoid dispersion due to shape anisotropy. vated from the interiors of their parent bodies by impacts. The highly magnetic Martian subgroup is indicated by an asterisk (*). 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 Impact can remagnetize the NRM of ETM. Remagnetization events have deeply affected the structure and mineralogy occurs readily if an ambient field is present during passage of ETM. Understanding the effects of shock on the rock of the shock wave (Gattacceca et al. 2008; Funaki and magnetic properties and the paleomagnetic record of ETM Syono 2008), but it is also possible that the impact itself is a critical and active area of investigation, initiated in could produce a short-lived field (Hood and Artemieva particular by Nagata et al. (1972). 2008). This means that remanent magnetization in shocked ETM may have no relationship to ambient parent body or One effect of shock processing is that ETM are often brec- external early solar system magnetic fields. Shock remag- ciated down to the millimeter scale and plastically deformed netization can be directly linked to a stress effect (Gattacceca by shock compaction. Indeed, ordinary chondrites have et al. 2006, 2008; Fi g .3a) or, for higher shock pressures, to been shown to be strongly anisotropic, based on magnetic shock-induced heating or mineral transformation. Depending susceptibility anisotropy and shape analysis. on shock intensity and characteristics, intrinsic magnetic Moreover, the amount of magnetic anisotropy is well corre- properties (in particular coercivity) may or may not be lated with shock stage as derived from petrographic obser- affected by the shock. One method for identifying pre-shock vations and porosity values, which is indicative of shock NRM is to observe random directions among clasts in a compaction (Gattacceca et al. 2005). brecciated material, as done by Gattacceca et al. (2003) and

El e m e n t s 225 Au g u s t 2009 Weiss et al. (2008a). This must be performed by NRM deter- technique. Maps of the magnetic field component perpen- mination at below-the-clast scale through millimeter-scale dicular to the section presented in Fi g u r e 3 were obtained subsampling (Gattacceca et al. 2003) or magnetic microscopy using a new magnetometer called the SQUID microscope, (Weiss et al. 2008b; Fi g . 3a, c). However, this methodology which has unrivaled sensitivity (moment resolution of 10-15 is somewhat ambiguous because magnetic heterogeneity A·m2) and spatial resolution reaching 140 µm. The develop- in a shocked sample might be the product of processes ment of this instrument required overcoming the techno- other than cold brecciation, like subsolidus phase transfor- logical challenge of keeping the SQUID sensor at 4.2 K and mations (Gattacceca et al. 2003) or heterogeneous shock separated from the room-temperature sample surface by a heating (Weiss et al. 2008a). How to distinguish between these distance of only 140 µm. To derive magnetizations from the two outcomes is still an active area of investigation. field maps, an inversion is necessary, as performed for satellite data (e.g. Langlais et al. 2004). Alternative magnetic field Magnetic microscopy is currently used in a number of sensors working at room temperature are currently being applications involving ETM. For example, the study of very developed to obtain higher resolution and easier operation small samples (like from Stardust or future sample return (avoiding the cryogenic problems), but they cannot reach missions) and of small individual chondrules and inclusions the SQUID sensitivity. within a meteorite section can only be achieved by this A number of ETM magnetic minerals (FeNi metal, cohenite, pyrrhotite) undergo phase transformations under pressure, A 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; Fi g . 4).

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. Fi g u r e 5 portrays such

B C

Maps of magnetic fields (in nT) measured with the SQUID Fi g u r e 3 microscope. (A) A thin section map of terrestrial 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 Shock-induced metallic FeNi nanoparticle in olivine from Fi g u r e 4 photomicrograph of the thin section map shown in (B) highlighting the Martian meteorite NWA 2737, after Van de Moortèle the strongly magnetized fusion crust (dark line at left) and chromite et al. (2007), as seen with high-resolution TEM. Indexed diffraction grains (dark spots). spots are shown in inset.

El e m e n t s 226 Au g u s t 2009 melted spherules, with the iron and silicate (here barred- during a sabbatical year in 2001, and to V. Dekov who olivine) types shown. Atmospheric heating has produced provided him with the images in figure 5a, b. M. Fuller, S. abundant magnetite, in which Cr, Ni, and other elements Russell, J. Feinberg, and R. Harrison are acknowledged for substitute. Magnetite is responsible for the strong magne- their reviews, which helped to improve the initial manu- tization of most spherules (Suavet et al. 2009). In fact, script. B.P.W. thanks the NASA Lunar Science Institute and magnetic extraction from sediments or soil appears to be the the NASA Mars Fundamental Research, NASA Lunar most efficient way to retrieve micrometeorites. According Advanced Science and Exploration Research, and the NSF to measurement of single spherules (in the 200–600 µm Geophysics Programs for support. 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. A B 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- C D tion by chondritic materials (Fi g . 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 Examples of cosmic spherules. Spherules in (A) and (B) Fi g u r e 5 (Rochette et al. 2006). 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 Perspectives (C) and (D) were found by the CEREGE group in soils from the Sahara A proper understanding of the rock magnetic properties Desert (SEM backscatter-mode image, both 300 µm diameter). 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 Log-log plot of nickel concentration versus ferromagnetic Fi g u r e 6 years ago the first papers on the magnetic properties of susceptibility for anorthositic lunar meteorites meteorites (together with John Lovering). These and subse- (unpublished results). Ferromagnetic susceptibility is low-field quent publications from Frank Stacey have been a major susceptibility corrected from the paramagnetic susceptibility and is inspiration. The first author is indebted to the INGV Roma, thus a direct measure of metal amount. which hosted his conversion to extraterrestrial matter

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