Magnetic meteorites Magnetic meteorites and the early solar system

James F J Bryson, Francis 1 Ancient and present-day magnetic activity on Nimmo and Richard J Harrison describe what meteorite rocky bodies in the solar system Downloaded from magnetism can tell us about planetary body radius time approx. field comment dynamo activity early in the (km) (Myr ago) intensity (µT) Earth 6371 0, 500, 3500 50, 50, 30 evidence of continuous field from history of the solar system. ~3500 Myr ago to present day Venus 6050 0 0 unknown ancient field http://astrogeo.oxfordjournals.org/ Mars 3390 0, 4000 0, 50 field likely to be present prior to 4000 Myr ago Ganymede 2630 0 >0.7 unknown ancient field Mercury 2440 0 ~0.3 field generated at present day Moon 1740 0, 3500, 4000 0, 20, 20–100 very early field uncertain, evidence of recent (1000 Myr ago) weak fields Vesta 263 0, 3690 0, >2 field recorded by meteorites thought to be crustal remanence imparted by earlier dynamo at University of California, Santa Cruz on September 2, 2015 pallasite parent ~200 4500 100 field intensity could have been body altered by mantle-hosted metal

oday, the Earth generates a mag- properties, Mercury’s magnetic field and netic field through convection of the crustal remanence (Anderson et al. 2011). Telectrically conducting molten iron in These magnetic observations are being its outer core. Core convection is governed used to reconstruct the present day and past by the thermal and chemical processes behaviour of Mercury’s interior and, along that operate deep within our planet; thus with other measurements made by MES- measurements of the intensity and direc- SENGER, we are starting to understand the tion of the magnetic field can provide processes driving this curious body. insights into the thermochemical state of Another planet of interest is Venus, the Earth’s interior. Crustal rocks can also which, despite similarities in size and record and preserve a memory of the field composition to the Earth, does not gener- they experienced as they were forming. ate a magnetic field at the present day. Paleomagnetic measurements can there- However, unlike the Earth, Venus’s surface fore provide records of ancient magnetic is not tectonically active, which may inhibit activity and, by extension, the internal con- the heat flux through its mantle. In this ditions of our planet in the past (Tarduno et scenario, Venus’s mantle temperature is al. 2014). A combination of paleomagnetic expected to be increasing at present, which and present-day magnetic measurements would inhibit core convection and hence therefore allow us to study the long-term the generation of a planetary magnetic and large-scale evolution of our planet over field (Nimmo 2002). This deduction is a billions of years; this method could also good example of how magnetic activity potentially allow us to predict how it may (or the lack of it in this case) can be tied to behave in the future. the large-scale thermal and compositional Beyond the Earth, planetary magnetic conditions of a planet; it is this aspect of fields on other terrestrial bodies (inner solar planetary magnetic fields that make them system planets, moons and asteroids) can attractive in planetary sciences. also be measured relatively easily from A different class of planetary bodies orbital and fly-by satellite observations of interest from a magnetic perspec- (table 1). For example, the MESSENGER tive are asteroids (rocky bodies with mission has recently reached the end of its radii <~500 km). Asteroids display a extended mission to measure, among other range of sizes, surface compositions and

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1 Schematic of dynamo generation within a partially differentiated asteroid. (a) Thermally driven convection (red and blue curved arrows) can generate magnetic fields (curved black lines) for a range of heat flux values out of the core (straight red lines). (b) Compositionally driven convection (curved yellow arrows) can generate magnetic fields via the rejection of light elements from the advancing solid (straight yellow arrow) that migrate upwards under gravity. This process depends on the growth of the solid (straight grey arrows), which in turn is dictated by the heat flux out of the core. Downloaded from http://astrogeo.oxfordjournals.org/ morphologies and, because of their rela- of our own planet. Here we describe the by the core, it is controlled by the mantle. tively small size, they are cold and inactive main mechanisms of planetary magnetic Core convection can result directly from at the present day. For example, no aster- field generation, followed by a summary the transfer of heat within a planet as it oid has been observed to be generating a of paleomagnetic results from each type of cools (figure 1). If heat is extracted from the magnetic field. However, this may not have meteorite, and finally the implications of core at a greater rate than can be conducted been the case during the early solar system, these measurements for theories of plan- along the adiabat, the result is fluid motion when these bodies were hot. Most meteor­ etary evolution and formation. that can create magnetic fields. Calcula- ites are samples of asteroids and display tions of the spatial and temporal release of at University of California, Santa Cruz on September 2, 2015 widely varying compositions and textures, Planetary magnetic fields heat in an asteroid-sized body suggest that containing some of the oldest structures in Many planetary magnetic fields are this process could have powered convec- the solar system. Paleomagnetic measure- dynamic, displaying both spatial and tem- tion in them only during the first ~10– ments of meteorites can therefore provide poral variations in their intensity, direction 50 Myr of the solar system (Elkins-Tanton et insight into the internal processes that and polarity (Gubbins 2008). This behaviour al. 2011). Thermally driven magnetic fields acted within asteroids, and allow us to peer implies that such planetary fields are the on asteroids are therefore predicted to have back in time to the conditions present as the result of a self-exciting dynamo driven by been early and relatively short-lived. solar system was forming (Weiss et al. 2010). motion of an electrically conductive fluid in Compositional convection is driven Crucially, because asteroids cooled much a planetary core, rather than a permanent by the solidification of the core. Once a faster than the Earth, meteorites could con- magnet (Stevenson 2003). The liquid part body has cooled to the point where the tain magnetic information from the entire of a core can move for various reasons. The core temperature is below its freezing active lifespan of a planetary body, so could sources of motion often relate temperature, the core will be used to study processes that are yet to to the large-scale and long- “Core convection can start to solidify. As it does occur on our own planet (e.g. complete term evolution of a planetary result directly from so, light elements (such as core solidification). Indeed, paleomagnetic body; one aim of planetary the transfer of heat in S, O, C, Si) are rejected from measurements of meteorites have been magnetic studies is to deduce a planet as it cools” the advancing solid, and the used to study a wide range of phenomena the mechanism driving resulting changes in buoy- such as planetary collisions, the collapse motion in the core and, by extension, the ancy of the liquid part of the core can lead of the protoplanetary dust cloud, and how thermal and chemical conditions within a to fluid motion (Nimmo 2009). This is the asteroids evolved to the inactive bodies we planetary body. Beyond mechanical driving case at the present day in the Earth, where know today. of the core fluid (which has been proposed the solidification of the inner core produces In this review we focus on paleomagnetic to have occurred on the ancient Moon (Le a light-element enriched liquid layer at the measurements of meteorites, in particular Bars et al. 2011), the two primary mecha- boundary between the inner and outer what these experiments can tell us about nisms of convection relate to thermal and core that moves upwards under gravity, the thermochemical evolution of their par- compositional buoyancy of the core liquid. creating convection (Fearn & Loper 1981). ent asteroids. The results of these measure- Although the details of this process depend Whether this mechanism was capable of ments have important implications for the on many physical properties of a planetary generating magnetic activity on asteroid- structure and formation of these bodies body (e.g. size, core composition, mantle sized bodies has been a matter of debate, and have been used to study the thermo- composition, etc), ultimately core convec- because of uncertainties about the direction chemical processes that govern planetary tion is dictated by the cooling of the body. of core solidification in these small bodies. evolution. Interestingly, these results all This is governed by the ability of a body The relatively large internal pressure gradi- point to similarities between asteroids to transfer heat out of the core, and hence ent within an Earth-sized body results in and the Earth, allowing us to think of depends on the thermal properties of the bottom-up core solidification (i.e. from the asteroids as small, accelerated analogues mantle. So, although the field is generated centre, towards the core–mantle boundary).

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2 Reproduced from Fu et al. (2014a). (A) Light microscope image of a section of the Allende meteorite. (B) Magnitude of total magnetic field 100 μm above the section. (C) Vertical component of the magnetic field 100 μm above the section. The arrow marks an individual chondrule. The magnetic fields were measured using a scanning SQUID microscope. Downloaded from

Due to their smaller size, many asteroids represent chondritic material that did melt on Earth such as remagnetization on entry, have been predicted to have solidified from while it was part of its parent asteroid. further shock, weathering, and quality of the top down, i.e. from the core–mantle Chondrites and achondrites are sub­ care during collection and curation. The

boundary, towards the centre. Although divided based on minor element and iso- remanence carried by achondrites could http://astrogeo.oxfordjournals.org/ this direction of solidification does not pre- topic compositions, and it has been argued also contain primary and secondary com- clude compositional convection, it would that each subcategory represents a distinct ponents, although they are not expected to have to operate through a different mecha- parent asteroid (Weiss & Elkins-Tanton have retained any pre-accretionary infor- nism to that acting within the Earth. 2013). Current theories of planetary forma- mation. The major technological achieve- Importantly, as the density contrast pro- tion suggest that these meteorite classes ment in paleomagnetism over the past duced by the rejection of light elements is represent fundamentally different bodies 10–15 years has been the development of greater than that caused by liquid cooling, that differ primarily in their timing of accre- sophisticated techniques capable of meas- compositional convection is a much more tion relative to the timescale of radioactive uring and decoding the magnetization his- efficient mechanism of field generation decay of 26Al (~3 Myr), which is the primary tory within bulk rocky meteorite samples. than thermal convection (Nimmo 2009). It source of heat in these bodies. From a paleo- Indeed, many of the early paleomagnetic at University of California, Santa Cruz on September 2, 2015 is therefore expected that thermal convec- magnetic perspective, plan­et­ary melting measurements made on rocky meteorites tion may have been relatively sparse in the is crucial because this process provides a are now thought to be inaccurate as a result early solar system. Nevertheless, because viable method for planetary differentia- of the quality of past techniques (specifi- of the young age of most of the magnetic tion (the separation of rocky and metallic cally those that involve potential sample remanences that have been found in the components under gravity to form a mantle alteration through heating). studies, the majority of paleomagnetic and core), which is a stringent requirement Recently, chondrites have been measured results from meteorites have been linked to for a planetary magnetic field. Hence, the paleomagnetically by means of the physical thermally driven convection as a result of remanent magnetization within chondrites separation and isolation of pre- and post- the uncertainties regarding core solidi- and achondrites is expected to differ and accretionary phases. A chondritic meteorite fication (Weiss et al. 2010). The ability of could be used to elucidate the details of that has been particularly well studied is an asteroid to generate thermally driven asteroid structure and differentiation. the Allende CV carbonaceous chondrite magnetic activity has key implications for Rocky meteorites are notoriously dif- (which is rich in organic molecules). The its thermochemical structure at the time the ficult to measure and interpret paleomag- material within this large meteorite experi- field was generated. netically. Many contain nanometre-scale, enced both aqueous and thermal alteration stable magnetic inclusions that are typically while it was part of its parent asteroid. Rocky meteorites reliable paleomagnetic recorders. But the Despite this alteration, this meteorite had Most rocky meteorites are thought to overall remanence of a rocky meteorite can previously been thought to contain accu- represent the mantles or crusts of asteroids, be a complex amalgamation of numerous rate records of pre-accretionary magneti- and are divided into two major categories: components, each of which could reflect a zation. However, a recent paleomagnetic chondrites and achondrites (Scott 2011). range of primary and/or secondary pro- study of individual chondrules extracted Chondrites consist of primitive material cesses. For example, the overall remanence from this meteorite suggests this is not believed to have formed in the solar nebula carried by a chondrite is expected to consist likely (figure 2, Fu et al. 2014a). Instead, the and then accreted to form asteroids. The of a pre-accretionary component, carried conditions within the parent body removed building blocks of chondrites include by the chondrules themselves and other this early magnetization, and the rema- chondrules (mm-sized spherical inclusions primitive phases, as well as a component nence in the Allende meteorite is likely to that were once molten) and Ca–Al-rich reflecting any potential magnetic activity represent purely post-accretionary pro- inclusions (CAIs, the oldest solids in the on the parent body, induced through ther- cesses. Paleomagnetic studies of subsam- solar system). Evidence suggests that these mal and/or chemical alteration. A range ples of the Allende meteorite that acquired phases were heated when they were part of secondary processes can also adversely a magnetization ~9–10 Myr after solar of their parent asteroid although, crucially, alter the primary remanence of a meteorite system formation suggest that it recorded they did not melt during this time. In such that it can be difficult to interpret their a unidirectional post-accretionary field contrast, rocky achondrites lack primitive magnetization: processes on the parent with an intensity of ~20 µT (the present-day material and show large degrees of thermal body such as late-stage thermal or aqueous Earth field is ~30–60 µT), consistent with alteration. It is believed that achondrites alteration, shock and cold brecciation, and a planetary magnetic field (Carporzen et

4.38 A&G • August 2015 • Vol. 56 • www.astrongeo.com Magnetic meteorites al. 2011). Interestingly, subsamples of the also been performed on rocky achon- about dynamo activity. This is particularly Allende meteorite that correspond to a drites originating from a range of parent true for the Moon, where the remanence magnetization acquisition age of ~40 Myr bodies. One class of achondrites that is acquisition age of lunar samples (both show a much weaker field (Fu et al. 2014a). of particular interest is the HED (howar- meteorites and Apollo mission samples) Considering the early ages, this trend could dite–eucrite–diogenite) meteorites. This spans almost 4 billion years. Paleomagnetic be interpreted as the decay of the thermally diverse and numerous group contains the measurements therefore provide a record driven magnetic fields that are predicted only achondrites that originate from an of lunar magnetic activity, and hence the to operate shortly after the birth of the asteroid to have their parent body – the internal conditions of the Moon, across its solar system. Chondritic parent bodies are asteroid Vesta – confidently identified (Fu et entire cooling history (Weiss & Tikoo 2014). not expected to have generated magnetic al. 2012, Binzel & Xu 1993). This allows the These measurements show that the lunar fields, because they have not differentiated measured properties of these meteorites dynamo field intensity was initially strong or formed a core. These observations are to be linked to the measured properties of (40–100 µT), then decreased relatively sud- inconsistent with this hypothesis, and the Vesta, with the prospect of linking these denly (to <10 µT), staying at this weak value implications for the structure of chondritic data to direct observations of differentia- for billions of years. Very young samples parent bodies are discussed below. tion, core activity and planetary melting. (<7 Myr old) include tantalizing evidence Using advanced scanning magnetic Fu et al. (2012) performed paleomagnetic that the lunar core was still active then. microscopies, the pre-accretionary measurements on the ALHA81001 HED The Moon has also been studied exten- Downloaded from remanence within individual chondrules meteorite and suggest that it cooled in sively from a dynamo modelling perspec- extracted from the Semarkona LL ordinary the presence of planetary magnetic field tive. By comparing these model results to chondrite (ordinary chondrites make up with an intensity of >2 µT. This meteorite the measured record, the lunar dynamo ~85% of meteorites on Earth and contain also shows a relatively young remanence has been proposed to have been driven by

low levels of organic molecules) have acquisition age (3.69 Gyr), inconsistent with various mechanisms over its history. The http://astrogeo.oxfordjournals.org/ been used to estimate the strength of the thermally driven magnetic activity early earliest lunar dynamo activity is thought pre-accretionary magnetic field (Fu et al. in the evolution of the solar system. These to originate from thermal convection. The 2014b). This field was generated at the same authors argue that this meteorite experi- longevity of this mechanism is uncertain time as chondrules and other early solids enced a secondary magnetic field gener- as a result of the unknown water content formed and the solar nebula collapsed. The ated by the crustal remanence induced of the lunar mantle; this property affects Semarkona meteorite displays no thermal by an extinct thermally driven dynamo. mantle thermal conductivity (Evans et al. or chemical alteration, making it the ideal The intensity of this original dynamo field 2014). Once the Moon had cooled and could sample to investigate this ancient process. would have to have been between ~10 and no longer generate this field, it has been pro- The inferred field intensity of 54 ± 21 µT 100 µT. The presence of a crustal remanence posed that some convection was mechani- has been linked to mecha- has also been proposed as cally forced as a result of surface impacts at University of California, Santa Cruz on September 2, 2015 nisms of mass and angular “Neither the Moon nor the origin of the apparent (Le Bars et al. 2011). The following period of momentum transfer within Mars generate a field, absence of space weathering dynamo activity is thought to result from the protoplanetary disc as but their crusts hold a from solar winds on Vesta precessional motion of the Moon that led well as supporting chondrule magnetic remanence” (Fu et al. 2012). to flow of the core liquid (Dwyer et al. 2011). formation through nebular Direct evidence of early Finally, recent and long-lived magnetic shock or planetesimal collisions. magnetic activity recorded by an achon- activity may be the result of compositional Bulk ordinary chondrite samples are dritic asteroid has been demonstrated convection arising from core solidification particularly difficult to study paleomag- within the Angrite meteorites (Weiss et (Laneuville et al. 2014). These last models netically because the remanence carriers al. 2008a). This group originates from an predict that the Moon should still be gen- have low coercivity. Despite these difficul- unknown parent asteroid and displays erating a magnetic field; simultaneous top- ties, there is evidence that the LL chondrites excellent paleomagnetic recording capabili- down core solidification has been proposed contain a directionally heterogeneous ties. Indeed, paleomagnetic measurements as a way of addressing this discrepancy. remanence down to the mm-scale (Gattac- of three of the meteorites in this group Martian meteorites often contain abun- ceca et al. 2003). Within thermally altered suggest they experienced an early magnetic dant magnetic phases (containing up to LL chondrites, this remanence could be field of intensity ~10–20 µT. This early and 15 wt% magnetic oxides, Gattacceca et al. interpreted as either remanence acquisition relatively intense magnetic field is consist- 2014), and paleointensity measurements of in a weak or null field, cold brecciation or ent with the Angrite meteorites directly these samples suggest Mars experienced late-stage remagnetization. If this rema- recording the dynamo field itself. This a field of ~50 µT (Weiss et al. 2008b). This is nence is shown to correspond to magneti- study was one of the first to demonstrate compatible with the strong crustal rema- zation in a null field, this observation could that asteroids were capable of generating nence in Mars’s southern hemisphere, be used to argue that some chondrites do in magnetic activity, and paved the way for which is probably the result of an earlier fact originate from undifferentiated bodies. future studies as well as providing a funda- dynamo field that shut down ~4 Gyr ago. It is intriguing to note that subsamples of mental insight into the internal conditions H ordinary chondrites display a direction- of asteroids. Stony-iron meteorites ally homogeneous remanence, suggestive Two other classes of achondrites that Stony-iron meteorites consist of roughly of magnetization by a dynamo field (Weiss warrant particular discussion are lunar equal amounts of rocky and metallic mat- & Elkins-Tanton 2013). Because of the dif- and martian meteorites. Neither the Moon erial, and are divided into two main groups: ficulties in measuring these samples with nor Mars generate global fields at the the pallasites and meso­siderites. The conventional paleomagnetic techniques, present day, but the crusts of both bodies metal in these meteorites has a relatively this observation is yet to be investigated in display a magnetic remanence. Although large grain size and so consists of multiple detail. But this remanence could be the first these meteorites (and Apollo samples in magnetic domains, making it a poor and tantalizing glimpse of dynamo activity on the case of the Moon) originate from larger unreliable paleomagnetic recorder. As a an ordinary chondritic parent body. bodies than the others considered here, result, accurate paleomagnetic measure- Paleomagnetic measurements have they are nonetheless extremely informative ments on bulk samples of these meteorites

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3 (a) Image of the Imilac pallasite. The scale bar corresponds to cm increments. (b) Optical microscopy image of the metal next to an olivine crystal. The sample has been etched to highlight the microstructures that are generated in meteoritic metal. The cloudy zone (CZ) is visible as a brown rim around the plessite.

remained elusive for decades. However, Cloudy zones magnetization from that of the entire recent measurements using novel tech- Despite the majority of the metal in the sample with traditional paleomagnetic niques from a pair of pallasites have been pallasites being a poor paleomagnetic techniques. Instead, Bryson et al. (2015) used to investigate both the origin of these recorder, there are specific regions that used magnetic microscopy to image the Downloaded from meteorites and the processes that governed could provide reliable paleomagnetic infor- magnetization of just the CZ in the Imilac the evolution of their parent body. mation. The pallasites typically cooled at and Esquel pallasites (figure 4). Images The pallasites consist of centimetre- rates of <10 K/Myr (Yang et al. 2010), which of the nm-scale magnetization across the sized olivine crystals embedded within permits low-temperature phase transitions width of the CZ (corresponding to different

a continuous Fe–Ni matrix. The presence that are kinetically hindered on laboratory remanence acquisition ages), showed that http://astrogeo.oxfordjournals.org/ and proximity of these phases led to the timescales. For example, slow cooling of the the magnetic field experienced by the Imilac original proposal that these meteorites metal in these meteorites allowed lamellae meteorite remained at ~120 µT for 4–8 Myr, originate from the core–mantle boundary of the bcc Fe-rich phase kamacite to form, while the field recorded by the Esquel mete- of an asteroid. Tarduno et al. (2012) noted resulting in the characteristic Widmanstat- orite decreased from ~80 µT to a plateau at that some of the olivine crystals in the ten microstructure associated with mete- 30 µT, before finally falling to ~0 µT over a Imilac and Esquel pallasites contain strings oritic metal. As these lamellae form and period of 5–10 Myr (figure 5). These values of nanometre-scale Fe inclusions. These advance, Ni is rejected from the bcc crystal are consistent with those found by Tarduno particles are small enough to be reliable structure and accumulates in et al. (2012), further suggesting paleomagnetic recorders, and by isolating the surrounding fcc taenite “A core can only that the pallasite parent body olivine crystals from their surrounding phase. This generates a vari- generate a planetary produced an intense mag- at University of California, Santa Cruz on September 2, 2015 metal matrix these authors were able to ation in the Ni content in the magnetic field if it is at netic field. By relating values obtain the first reliable measurements of taenite adjacent to the lamel- least partly liquid” of the cooling rates measured the field generated by the pallasite parent lae interface. The Ni content from the size of the structures asteroid. Single-crystal paleomagnetic decreases from ~50% down to the bulk within the CZ to those predicted from measurements show that these meteorites composition (typically ~10%) over ~1–20 µm, planetary cooling models, these authors experienced field intensities of >70 µT, depending on the meteorite (Goldstein et argue that the Imilac and Esquel meteorites which are consistent with a relatively al. 2009). On slow cooling, metal with this recorded their magnetizations during the intense planetary magnetic field. The Ni concentration range can order to form early and late stages of core solidification, presence of this remanence raises ques- the phase tetrataenite, which consists of respectively. Within this time period, the tions about the origin of the pallasites. The alternating atomic layers of Fe and Ni and field could have resulted from composi- nm-scale Fe particles record the magnetic is intrinsically an extremely magnetically tional convection. To test this hypothesis, field as they cooled below the relatively hard material (intrinsic coercivity >2 T). For the field trends were predicted on the basis low temperature of 633 K. A core can only Ni concentrations between ~45% and 25%, of calculations of the core–mantle boundary generate a planetary magnetic field if it is tetrataenite forms via spinodal decomposi- heat flux and inner-core growth rate. These at least partly liquid, i.e. when it was at tem- tion as nm-scale islands in an intergrown trends were found to agree with the experi- peratures >1200 K. Planetary cooling mod- region known as the cloudy zone (CZ, mental results, with the Imilac meteorite els show that the core–mantle boundary is figure 3). This consists of small islands of a recording a dipolar field, and the Esquel expected to have been only slightly cooler magnetically hard phase, characteristic of a meteorite recording a dipolar–multipolar than the core; in other words, this region reliable paleomagnetic recorder (Bryson et field transition, the multipolar regime and would not have been capable of recording al. 2014). Hence, by studying the magnetic then the eventual shut down of magnetic a magnetization while the core was liquid. remanence of the CZ in isolation, meteoritic activity associated with complete core solid- Therefore, the pallasites could not originate metal could provide reliable paleomag- ification. The experimental observations from this depth in their parent asteroid. netic information. Furthermore, the CZ are, therefore, all consistent with a field Tarduno et al. (2012) propose instead that forms locally at later times for lower local generated by compositional convection. the pallasites originate from the upper- to Ni contents, i.e. at a further distance from Despite stony-iron meteorites having mid-mantle of a 200 km radius asteroid, the kamacite lamellae (Bryson et al. 2014a). been considered as unreliable samples and the presence of metal is likely to be Thus, the spatially resolved magnetization for paleomagnetic studies, it is now clear the result of a planetary impact between a across the CZ corresponds to a time- that they contain a wealth of informa- larger body and smaller body (~1/10 of the resolved record of the magnetic activity tion. Indeed, this family of meteorites has mass of the larger body). During this event, experienced by this intergrowth. illuminated some key aspects of planetary the core of the smaller body could have However, the CZ forms only a small collisions and thermochemical evolution. been injected into the mantle of the larger total volume within a meteorite; it is body; the pallasites are samples of the unfeasible to extract and study this region Iron meteorites resulting metal–silicate mixture. from bulk samples or isolate its remanent Iron meteorites are comprised entirely of

4.40 A&G • August 2015 • Vol. 56 • www.astrongeo.com Magnetic meteorites meteoritic metal and are thought to repre- 4 5 μm field-of-view sent asteroid cores. These meteorites are XPEEM images of the therefore the result of destructive planetary CZ in (a) the Imilac collisions during the early solar system. and (b) the Esquel Due both to the unreliable recording nature pallasite based on a of meteoritic metal and their proposed ori- figure from Bryson gin, these meteorites have not been widely et al. (2015). Blue and studied paleomagnetically. However, red correspond to they display the same microstructures as positive and negative those seen in the metal of the pallasites, projections of the so they could potentially contain reliable magnetization of the paleomagnetic information. To date, this sample surface onto approach has only been applied to the the X-ray beam direc- Tazewell IIICD iron meteorite (Bryson et tion. The CZ forms al. 2014b), which has been studied using over time, with the the same magnetic microscopy technique oldest region adjacent as that used to study the metal within the to the tetrataenite Downloaded from pallasites. Somewhat unsurprisingly, this (TT) rim. The strength meteorite does not appear to have recorded and orientation of the a magnetic field. If this meteorite does field experienced by indeed originate from a core, the CZ does each meteorite can be

not form and record magnetic activity until extracted from these http://astrogeo.oxfordjournals.org/ temperatures have fallen well below that images. required for the core to be liquid. This is not to say that all iron meteorite groups are expected to lack a magnetic remanence. Some iron meteorite groups (e.g. IIE) are thought to originate from surface metal pools (McDermott et al. 2014) resulting from a similar collision to that proposed to generate the pallasites. In this case, these meteorites could have recorded at University of California, Santa Cruz on September 2, 2015 the magnetic field generated by their parent body, and paleomagnetic studies of their CZ could provide time-resolved records of magnetic activity. IVA iron meteorites are another group that could be of interest. These meteorites cooled uncharacteristi- cally quickly, which has led to suggestions that they originate from a core that had its mantle stripped from it by planetary colli- sion (Asphaug 2009, Yang et al. 2007). The resulting liquid body would have cooled at the high rates observed in the IVA meteor- ites and, as a result of its direct contact with space, is also expected to have undergone top-down solidification. It is feasible that meteorites from the upper part of the solid metal shell could have recorded the field 5 Field intensity generated while the deeper liquid was experienced by (a) still convecting. The IVA iron meteorites the Imilac and (b) the represent a unique opportunity to study Esquel pallasite over the mechanisms of field generation in a time (Bryson et al. top-down solidifying core. 2015). Each point cor- responds to ~1–2 Myr. Implications The Imilac meteorite The majority of measured rocky and stony- is thought to have iron meteorite groups carry a magnetic recorded a dipolar remanence consistent with a planetary field, while the Esquel magnetic field. This suggests that many meteorite captured small planetary bodies generated magnetic a dipolar–multipolar activity during the early solar system. This transition, multipolar is in stark contrast to ideas from 10–15 years regime and the end ago when there was little to no accurate of dynamo activity paleomagnetic evidence that these bodies with complete core could generate magnetic fields. solidification.

A &G • August 2015 • Vol. 56 • www.astrongeo.com 4.41 Magnetic meteorites

Generally, both paleomagnetic measure- within carbonaceous chondrite lids than continuous magnetic field would require ments and geochemical dating of rocky ordinary chondrite lids. The mantle water a near-constant bombardment, which is meteorites (both achondrites and carbon­ content of the Moon has been linked to the unlikely. We therefore predict a period of aceous chondrites) suggest that many heat flux through the lunar mantle and the largely quiescent dynamo activity after the asteroids were capable of generating early longevity of the lunar dynamo (Evans et thermally driven activity. Once cooling had magnetic activity. These observations are al. 2014); extending this idea to chondritic reached the point where the core started to consistent with thermally driven magnetic parent bodies, it could be the composition solidify, compositional convection could activity, which is supported by measure- of the carbonaceous chondrites themselves have generated magnetic activity. This is ments and models of lunar dynamo activ- that allowed their parent bodies to generate certainly the case for cores solidifying from ity. This deduction is consistent with ideas magnetic activity. the bottom up; evidence is still required to about achondritic parent bodies, which Paleomagnetic measurements of show whether the same is true for top- are thought to have formed a core, but is in stony-iron meteorites strongly suggest down solidifying cores. The first, thermally direct disagreement with current theories that asteroids were capable of generating driven, epoch of magnetic activity is pre- of the development of chondritic parent compositionally driven dynamo fields. dicted to have been relatively short-lived bodies. These bodies are not thought to Whether this mechanism of field genera- and weak, whereas the second, composi- have a core and hence would be incapable tion could act on asteroids was uncertain tionally driven, epoch was widespread, of generating a magnetic field. These recent for some time but, now there is experi- intense and long-lived. Downloaded from paleomagnetic measurements suggest mental evidence of their existence in the that partially differentiated parent bodies past, we can consider how likely they may Testable hypotheses (Elkins-Tanton et al. 2011, Weiss & Elkins- have been to exist in the early solar system. This picture leads to testable hypotheses. Tanton 2013), consisting of a chondritic lid Compositional convection is much more First, there should be meteorites within the

above differentiated material, could have efficient than thermally driven flow and same group that capture different epochs http://astrogeo.oxfordjournals.org/ been present during the early could generate fields for of activity (thermal, quiescence and com- solar system. “The thermally driven liquid-core cooling rates positional). The cooling rate of a meteorite The presence of this type magnetic activity era as low as 0.001 K/Myr. It is may correlate with its original depth in the of body means that accretion is predicted to have likely to have occurred on the parent body, and hence the time it cooled to form planetary bodies been short and weak” vast majority of differenti- to the remanence acquisition temperature. would have been a slow and ated small bodies that solidi- Therefore, by studying meteorites from the potentially multistage process. In recently fied from the bottom up, whereas thermally same family with a range of cooling rates, proposed models of the formation of these driven dynamos were probably rare. The a long time-resolved record of magnetic asteroids (Elkins-Tanton et al. 2011), differ- compositionally driven magnetic activity activity can be obtained from the same entiation occurs during the earliest stages would have been relatively late, because parent body, which could be used to argue at University of California, Santa Cruz on September 2, 2015 of the solar system, and the chondritic lid it could only have generated a field once for either the presence or absence of these is acquired slowly over millions of years as bodies had cooled enough for their cores to different epochs of activity. Secondly, pre-accretionary material from the solar start solidifying, and relatively long-lived, meteorite groups other than the pallasites nebula gradually rains down. A decreasing because it appears to have lasted for most should have captured compositionally thermal gradient through the chondritic lid of the time it took for the core to solidify. driven magnetic activity. Demonstration of can explain the varying degrees of thermal These are opposite traits to thermally this activity in other meteorites would sup- alteration observed within chondrites. driven fields, and suggest that there could port its proposed widespread occurrence. Currently, there is evidence of post- have been a widespread epoch of magnetic A final piece of future work that would accretionary magnetic activity in multiple activity among differentiated small bodies illuminate our understanding of these carbonaceous chondrite parent bodies, in the early solar system (Bryson et al. 2015). processes is further study of top-down core but no firm evidence for it in ordinary By combining observations from all solidification (Williams 2009). Modelling chondrite parent bodies. This trend could meteorite groups, we can start to build up work is underway for Ganymede, the Moon be a result of the difficulties in performing a picture of convection through the history and Mercury, in combination with obser- accurate paleomagnetic measurements on of the core of a small body. Initially, the vational data. If this mechanism is found ordinary chondrites, but it is also inter- thermal structure of an asteroid allowed for to generate magnetic activity, the cover- esting to consider the interpretation that thermally driven magnetic activity; models age of the compositionally driven epoch planetary lids composed of carbonaceous suggest that this ceased ~10–50 Myr after of magnetic activity could be extended to material may allow for magnetic activity, the formation of the solar system. After essentially all differentiated small bodies, while ordinary chondritic lids do not. Car- this time, dynamo activity could result implying that magnetic activity was wide- bonaceous chondrites show larger degrees from impacts. However, this mechanism spread in the early solar system. ● of aqueous alteration than ordinary is only capable of producing stochastic chondrites, implying there was more water and extremely short-lived core motion. A

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