Reports

Astronomical observations of young stellar objects indicate that early planetary systems evolve through a protoplanetary disk phase in <5 million years (My) following the collapse of their parent molecular clouds (1, 2). Disk evolution on such short timescales requires highly efficient inward transport of mass accompanied by outward angular momentum transfer, which allows disk material to accrete onto the central star while delivering angular momentum out of the protoplanetary system..

The mechanism of this rapid mass and angular momen-tum redistribution remains unknown. Several proposed processes invoke a central role for nebular magnetic fields. Among these, the magnetorotational instability (MRI) and magnetic braking predict magnetic fields with intensities of ~100 μT at 1 AU in the active layers of the disk (3, 4). Alter-natively, transport by magnetocentrifugal wind (MCW) re-quires large-scale, ordered magnetic fields stronger than ~10 μT at 1 AU. Finally, non-magnetic effects such as the baro-clinic and Goldreich-Schubert-Fricke instabilities may be the dominant mechanism of angular momentum transport in the absence of sufficiently strong magnetic fields (5). Di-rect measurement of magnetic fields in the planet-forming regions of the disk can potentially distinguish among and

constrain these hypothesized mechanisms.

Although current astronomi-cal observations cannot directly measure magnetic field strengths in planet-forming re-gions [(6, 7); supplementary text], paleomagnetic experiments on meteoritic materials can po-tentially constrain the strength of nebular magnetic fields. Chondrules are millimeter-sized lithic constituents of primitive meteorites that formed in transi-ent heating events in the solar nebula. If a stable field was pre-sent during cooling, they should have acquired a ther-moremanent magnetization (TRM), which can be character-ized via paleomagnetic experi-ments. Besides assessing the role of magnetic fields in disk evolu-tion, such paleomagnetic meas-urements would constrain the currently unknown mechanism of chondrule formation.

Chondrules likely constituted a significant fraction of the mass of asteroids and terrestrial plan-et precursors and may have facil-itated the accretion of the first

planetesimals (8, 9). The formation of chondrules therefore very likely represents a key stage in the evolution of the ear-ly solar system. The ambient magnetic field strength is a distinguishing characteristic among chondrule formation models. The x-wind model implies strong stellar fields of >80-400 μT (10). In contrast, magnetic fields in the nebular shock and planetesimal collision models are likely signifi-cantly lower than 100 μT (11, 12).

Previous paleomagnetic measurements of individual chondrules have focused mostly on the Allende CV chon-drite (13). However, due to extensive aqueous alteration on the CV parent body, magnetic phases in Allende chondrules are secondary and do not retain pre-accretional magnetiza-tion [i.e., magnetization acquired after the last heating of chondrules in the nebula and before the accretion of the meteorite's parent body; (14)]. Reliable recovery of pre-accretional magnetization requires samples that have avoid-ed significant post-accretional remagnetization processes.

Among the most pristine known meteorites is the Semarkona LL3.00 ordinary chondrite. We conducted pale-omagnetic studies on Semarkona, focusing in particular on dusty olivine-bearing chondrules (Fig. 1). Dusty olivine crys-tals consist of sub-micrometer sized grains of nearly pure

Solar nebula magnetic fields recorded in the Semarkona meteorite Roger R. Fu,1* Benjamin P. Weiss,1 Eduardo A. Lima,1 Richard J. Harrison,2 Xue-Ning Bai,3,4 Steven J. Desch,5 Denton S. Ebel,6 Clement Suavet,1 Huapei Wang,1 David Glenn,3 David Le Sage,7 Takeshi Kasama,8 Ronald L. Walsworth,3,7 Aaron T. Kuan9 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology (MIT), Cambridge, MA, USA. 2Department of Earth Sciences, University of Cambridge, Cambridge, UK. 3Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA. 4Hubble Fellow. 5School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA. 6Department of Earth and Planetary Sciences, American Museum of Natural History (AMNH), New York, NY, USA. 7Department of Physics, Harvard University, Cambridge, MA, USA. 8Center for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark. 9School of Engineering and Applied Science, Harvard University, Cambridge, MA, USA.

*Corresponding author. E-mail: rogerfu@mit.edu

Magnetic fields are proposed to have played a critical role in some of the most enigmatic processes of planetary formation by mediating the rapid accretion of disk material onto the central star and the formation of the first solids. However, there have been no experimental constraints on the intensity of these fields. Here we show that dusty olivine-bearing chondrules from the Semarkona meteorite were magnetized in a nebular field of 54 ± 21 μT. This intensity supports chondrule formation by nebular shocks or planetesimal collisions rather than by electric currents, the x-wind, or other mechanisms near the sun. This implies that background magnetic fields in the terrestrial planet-forming region were likely 5-54 μT, which is sufficient to account for measured rates of mass and angular momentum transport in protoplanetary disks..

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body centered cubic (bcc) Fe (kamacite) embedded in for-steritic olivine (15). Such olivine grains are found in approx-imately one in ten chondrules in ordinary chondrites.

Due to their unique compositional and magnetic proper-ties, dusty olivine grains are expected to retain pre-accretional magnetization. The small grain size of dusty oli-vine metal implies that most are in the single domain (SD) or single vortex (SV) states, which can retain stable magnet-ization over the history of the solar system (16–19). Further, the Ni-poor composition (Ni <2 wt%) of dusty olivine metal precludes metamorphic recrystallization (15, 20). The do-main states of dusty olivine metals imply very high coercivi-ties ranging up to >200 mT, as confirmed by our demagnetization experiments (see below). The magnetiza-tion of grains with such high coercivities should not have been significantly altered by the low shock pressures likely experienced by Semarkona [4-10 GPa (21, 22)]. Finally, the low porosities of the surrounding olivine crystals have pro-tected metal from aqueous alteration (Fig. 1B).

The distinctive, high coercivities of dusty olivine grains allow for the isolation of their remanent magnetization dur-ing alternating field (AF) demagnetization as larger (10-100 μm) mesostasis metal grains are expected to demagnetize at AF levels <50 mT (23). Furthermore, since the post-accretional peak metamorphic temperature of Semarkona was likely only 200-260°C (24, 25), pre-accretional rema-nence in dusty olivine metals should be isolated upon labor-atory (1 hour duration) thermal demagnetization to <450°C assuming that metamorphism lasted ~5 My (16, 19). In summary, no known post-accretional process is likely to have compromised pre-accretional remanent magnetization in Semarkona dusty olivines; strong field AF demagnetiza-tion or thermal demagnetization above ~450°C is expected to isolate the pre-accretional component of magnetization.

We isolated eight dusty olivine-bearing chondrules from two 15 mm × 10 mm × 150 μm thick sections of Semarkona provided by the American Museum of Natural History (AMNH). Both sections contain fusion crust along one edge. We also extracted five non-dusty olivine-bearing chondrules and twenty-nine bulk (i.e., mixed matrix and chondrule) samples to characterize any post-accretional overprints. All extracted samples are mutually oriented to within 5°. Due to their weak moments [natural remanent magnetization (NRM) ranging between 10−10 and 3×10−12 Am2 prior to de-magnetization], chondrules were measured using the Super-conducting Quantum Interference Device (SQUID) Microscope (26, 27) at the MIT Paleomagnetism Laboratory (Fig. 2). Supporting magnetic imaging measurements with higher spatial resolution were performed using a nitrogen-vacancy (NV) quantum diamond microscope (supplemen-tary text).

Bulk samples were subjected to AF demagnetization up to 85-145 mT or thermal demagnetization up to 580°C. We identified three unidirectional post-accretional overprints in our Semarkona samples: two low coercivity (LCa and LCb)

and one medium coercivity (MCa) components. Twenty bulk samples carry the LCa overprint blocked up to between 4.5 and 13 mT (Fig. 3). This component is present in both fusion crust material and meteorite interior samples, indicating that it was acquired after arrival on Earth. Removal of the LCa magnetization during thermal demagnetization to only 70°C indicates that it is likely a viscous remanent magneti-zation (VRM) acquired during long-term exposure to the Earth’s field.

In contrast, the MCa overprint is only present in samples within 4.7 mm of the fusion crust. The mean paleointensity of this component based on the isothermal remanent mag-netization (IRM) and anhysteretic remanent magnetization (ARM) normalization methods (76 μT) is within uncertainty of the Earth’s magnetic field. We conclude that MCa com-ponent was acquired during heating from atmospheric pas-sage. Finally, a small subset of seven samples from one edge of our Semarkona section carry the LCb overprint, which is completely removed by AF demagnetization up to 10.5 to 30 mT. The high intensity of the LCb component (NRM to sat-uration IRM ratio of 0.23) suggests that it was acquired dur-ing exposure to artificial magnetic fields. Only two dusty olivine-bearing chondrules carried the LCb overprint, which was fully removed upon AF application to 20 mT. To sum-marize, all samples >1.0 mm from the fusion crust, which include all dusty olivine-bearing chondrules, do not carry any post-accretional remagnetization other than the LCa, MCa, and LCb components. These overprints, if present, are readily identified and removed via AF cleaning.

Eight dusty olivine-bearing chondrules were subjected to AF and thermal demagnetization up to 290-420 mT or 780°C. Six of these were found to carry a high coercivity (HC) or high temperature (HT) component of magnetiza-tion. We argue based on seven lines of evidence that these HC/HT components are pre-accretional TRMs (supplemen-tary text). First, HC/HT components decay to the origin up-on demagnetization, which is the expected behavior of primary magnetization (Fig. 2). Second, the HC/HT magnet-ization directions in the six chondrules are collectively ran-dom, passing the conglomerate test at the 95% confidence level [Fig. 3; (28)]. No HC/HT component is oriented in the direction of any of the post-accretional overprints. Third, the HC magnetizations in chondrules DOC3 and DOC4, which were each partitioned into two subsamples, are uni-directional within each chondrule and inconsistent with random magnetizations at the 99% confidence level. Such uniformity is expected of a TRM acquired by individual chondrules cooling in the solar nebula because the field should be uniform across the submillimeter scale of each sample (14). Fourth, the blocking temperature range of the HT component (350-750°C) agrees closely with that ex-pected of a pre-accretional magnetization that was partially demagnetized for ~5 My at ~200°C [Fig. 2B; (16, 19)], which is the estimated metamorphic temperature for Semarkona on the LL parent body (16, 25). Fifth, magnetic field maps of

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dusty olivine-bearing chondrules confirm that the HC mag-netization is carried by dusty olivines, which formed in the nebula and are expected to retain pre-accretional magneti-zation as outlined above. Sixth, the magnetization direction acquired during cooling for a spinning chondrule is ex-pected to be parallel to its rotation axis (29). The close alignment between the HC directions and the short axes of our chondrules, which are likely related to the rotation axis (supplementary text), are non-random at the 98% confi-dence level, suggesting that HC/HT magnetizations were indeed acquired parallel to the spin axis. Seventh, the coer-civity spectrum of the HC component of dusty olivine-bearing chondrules is very similar to that of an ARM and dissimilar to that of an IRM, which suggests that the HC component was acquired as a TRM (30). We therefore con-clude with high confidence that the HC/HT magnetizations observed in dusty olivine-bearing chondrules are TRMs ac-quired in the solar nebula.

TRM acquisition experiments on analogs of dusty olivine chondrules have shown that the ARM normalization meth-od potentially produces paleointensities accurate to ~40% [2σ; (30)]. Assuming that the five chondrules with recovered paleointensities formed in similar magnetic field conditions in the nebula, our nominal ARM paleointensities for six dusty olivines yielded a mean value of 27 μT. The morpholo-gy of chondrules (31) and the apparent correspondence be-tween HC magnetization directions and the short axes of our chondrule samples (see above paragraph) strongly sug-gest that chondrules were rotating during remanence acqui-sition in the solar nebula. In the case of rotation around a single axis, which is the expected motion inherited from cooling of a viscous droplet, the true mean field intensity should have been greater than our experimental paleointen-sity by a factor of 2 (supplementary text). Meanwhile, pre-cession of the chondrule's rotation axis would imply a multiplicative factor of up to 4. However, given the high inferred rotation rates of chondrules (>50 s−1), the magni-tudes of effects that may lead to precession are comparative-ly small. Therefore, we adopt non-precessing rotation as the most likely state of cooling chondrules and recommend a value of 54 ± 21 μT (2σ) for the ambient nebular field strength. Although unlikely, if chondrules did not rotate or precessed strongly during remanence acquisition, the corre-sponding estimated ambient field strength would be 27 ± 8 μT or 108 ± 42 μT (2σ), respectively.

This paleointensity constrains the magnetic field envi-ronment during the last time the chondrule cooled through the 765-350°C blocking temperature range of the HC/HT component, which likely was the chondrule forming event. Our recommended paleointensity is significantly lower than the >80-400 μT expected for chondrules purportedly formed in the x-wind model (10). Furthermore, mechanisms that invoke intense electric currents such as magnetic reconnec-tion flares and current sheets predict strong fields in excess of 500 μT during chondrule heating (32). The short circuit

instability may also imply similarly strong fields at high temperature (33), although the decay of field strength below 765°C has not been studied in detail. In contrast, given that magnetic fields inherited from the collapsing molecular cloud were likely on the order of 10 μT (12), nebular shocks, which may enhance the ambient magnetic field by a factor of <10, would result in paleointensities of <100 μT (11). Meanwhile planetesimal collisions would likely not perturb the background field. Therefore, nebular shocks and plane-tesimal collisions are the chondrule formation models most consistent with our measured paleointensities.

Adopting nebular shocks and planetesimal collisions as the most likely origins of chondrules, background magnetic fields in the nebula may have been amplified by a factor between 1 and 10 during chondrule formation. We therefore infer that background magnetic fields in the solar nebula were between 5 and 54 μT (11). Assuming Semarkona chon-drules formed near 2.5 AU, which is the present day location of S-type asteroids (34), the vertical distribution of dust in the nebula strongly suggests that chondrule formation took place in the weakly ionized “dead zone”, which contains gas poorly coupled to local magnetic fields and occurs within ~3 gas scale heights of the midplane (35). Our measurements therefore indicate that a substantial magnetic field [yet still well below the ~400 μT equipartition field strength (3)] ex-isted in the dead zone, potentially due to fields inherited from the collapse of the solar system's parent molecular cloud. Given our measured field strengths, mass accretion driven by the MRI or magnetic braking at 2.5 AU would have been <0.04-3.5×10−8 Msun yr−1, where Msun is the Sun’s mass (supplementary text). Meanwhile, the MCW model would predict mass accretion rates of 0.3-30×10−7 Msun yr−1 or less. The inferred age of Semarkona chondrules is 2-3 My after the first calcium alunimum-rich inclusions (36). Given that protoplanetary disks are observed to have accretion rates of 10−9-10−7 Msun yr−1 at 2-3 My after collapse of their parent molecular clouds (2), both magnetic mechanism could fully account for the expected accretion rates. This suggests that magnetic fields govern the observed rapid transformation of protoplanetary disks into planetary sys-tems around sun-like stars.

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ACKNOWLEDGMENTS

We thank A. J. Brearley, H. C. Connolly, A. M. Hughes, B. C. Johnson, J. L. Kirschvink, M. Mac Low, G. J. MacPherson, M. I. Petaev, D. D. Sasselov, H. E. Schlichting, J. B. Simon, N. Turner, and B. Zanda for a range of discussions that improved the manuscript. We also thank J. Gross, S. Wallace, and Z.I. Balogh for help with SEM and STEM sample analyses and acknowledge S.-C.L.L. Lappe, N.S. Church, S. Russell, M. Uehara, and N. Nakamura for pioneering work on the magnetism of dusty olivines. We thank Thomas F. Peterson for supporting critical instrumentation and analysis costs. R.R.F., B.P.W., E.A.L., S.J.D., and C.S. thank the NASA Origins Program while R.R.F. and B.P.W. thank the U.S. Rosetta

Project, Jet Propulsion Laboratory for support. R.R.F. thanks the NSF Graduate Research Fellowship Program, and C.S. thanks the NASA Lunar Science Institute and the NASA Solar System Exploration and Research Virtual Institute for support. R.J.H. and T.K. thank the European Research Council under the European Union’s Seventh Framework Programme and the Leverhulme Trust for support. X.N.B. acknowledges support from NASA through the Hubble Fellowship. D.G., D.L.S., and R.L.W. thank the DARPA QuASAR program and the NSF for support..

SUPPLEMENTARY MATERIALS www.sciencemag.org/cgi/content/full/science.1258022/DC1 Supplementary Text Figs. S1 to S12 Tables S1 to S4 References (37–125) 30 June 2014; accepted 31 October 2014 Published online 13 November 2014 10.1126/science.1258022

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Fig. 1. Dusty olivine-bearing chondrules from the Semarkona meteorite. (A) Optical photomicrograph of chondrule DOC4 showing the location of dusty olivine grains. Image taken in reflected light with crossed polarizers. (B) Annular dark field scanning transmission electron microscope (STEM) image of four dusty olivine Fe grains from chondrule DOC5. Brightness in image reflects column-averaged atomic number; darker grains are smaller in size, implying a higher relative abundance of olivine at their location and hence a lower mean atomic number. Note the euhedral morphology and chemical homogeneity of the Fe grains, which indicate the lack of secondary recrystallization and alteration. Such Fe grains are the primary carriers of pre-accretional magnetization in Semarkona chondrules.

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Fig. 2. AF and thermal demagnetization of single dusty olivine-bearing chondrules measured using SQUID microscopy. Orthogonal projection diagrams showing the evolution of the natural remanent magnetization (NRM) of two chondrules upon progressive demagnetization. Open and solid circles indicate the projection of the NRM vector onto the vertical (up-east) and horizontal (north-east) planes, respectively. (A) AF demagnetization of DOC1 reveals a low coercivity (LC) overprint removed by 20 mT and higher coercivity (HC) magnetization that persists to >290 mT while decaying in magnitude toward the origin. Insets show associated magnetic field maps measured with the SQUID microscope at the indicated demagnetization levels where positive (red) field values are in the up direction. Note the stable directionality and steady decay of the magnetization during AF application above 20 mT. (B) Mixed AF and thermal demagnetization of DOC8 shows the removal of the post-accretional LCb overprint by 20.0 mT (green points), a stationary moment between room temperature and ~400°C (gray), and an origin-trending high temperature (HT) component removed by 780°C (red). Steps above 40 mT or 575°C have been averaged to suppress noise.

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Fig. 3. Magnetization directions in Semarkona chondrules and bulk samples. Equal area stereonet projection diagram where colored points and circles denote high coercivity or high temperature (HC/HT) magnetization in individual dusty olivine-bearing chondrule samples or subsamples and the associated maximum angular deviation (MAD) obtained from principle components analysis. Each color represents a single chondrule with chondrules DOC3 and DOC4 having two subsamples each. Black stars and associated ovals represent the mean directions of the three post-accretional overprints identified from bulk samples and their 95% confidence intervals. Open symbols represent the upper hemisphere; solid symbols represent the lower hemisphere. The wide scatter of the HC/HT magnetizations, the unidirectionality of subsamples from DOC3 and DOC4, and their non-correspondence to the directions of post-accretional overprints provide strong evidence for a pre-accretional origin of the HC/HT magnetizations in dusty olivine-bearing chondrules.

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