An Ancient Core Dynamo in Asteroid Vesta

Fu, R. R.; Weiss, B. P.; Shuster, David L.; Gattacceca, J.; Grove, T. L.; Suavet, C.; Lima, Eduardo A.; Li, Luyao; Kuan, Aaron T.

doi:10.1126/science.1225648

Cited by 90 publications

(83 citation statements)

References 97 publications

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“…61 However, our results suggest that mechanically-forced orbital systems can harvest significant amounts of spin-orbital energy to drive turbulent processes without the need for convection. Together with recent numerical findings showing that mechanically forced instabilities can drive dynamos in precessing spheres, 18 spheroids, 19 in longitudinally-librating systems, 20 and in tidal forced systems, 21 our results support the possibility that mechanical forcing leads to dynamo generation in smaller bodies, like the early Moon 62 and asteroids, 63,64 explaining data that do not presently fit into the standard model for convective dynamos. …”

Section: Discussion and Concluding Remarkssupporting

confidence: 63%

Experimental study of global-scale turbulence in a librating ellipsoid

et al. 2014

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We present laboratory experimental results demonstrating that librational forcing of an ellipsoidal container of water can produce intense motions through the mechanism of a libration driven elliptical instability (LDEI). These libration studies are conducted using an ellipsoidal acrylic container filled with water. A particle image velocimetry method is used to measure the 2D velocity field in the equatorial plane over hundreds libration cycles for a fixed Ekman number, E = 2 × 10 −5 . In doing so, we recover the libration induced base flow and a time averaged zonal flow. Further, we show that LDEI in non-axisymmetric container geometries is capable of driving both intermittent and saturated turbulent motions in the bulk fluid. Additionally, we measure the growth rate and amplitude of the LDEI induced excited flow in a fully ellipsoidal container at more extreme parameters than previously studied [Noir

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Section: Discussion and Concluding Remarkssupporting

confidence: 63%

Experimental study of global-scale turbulence in a librating ellipsoid

et al. 2014

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“…Both a and f are subject to uncertainties of a factor of 3-5 (21,80). Because the paleointensities of bulk samples are used only as an aid towards determining the origin of postaccretional overprint, uncertainties in the paleointensities within this range do not affect our conclusions.…”

Section: Nrm Of Bulk Samplesmentioning

confidence: 98%

“…We 75 conducted paleomagnetic studies on Semarkona, focusing in particular on dusty olivine-bearing 76 chondrules ( experienced by 21)]. Finally, the low porosities of the surrounding 88 olivine crystals have protected metal from aqueous alteration (Fig.…”

Section: Processes 74mentioning

confidence: 99%

Solar nebula magnetic fields recorded in the Semarkona meteorite

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Lima

et al. 2014

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24 25 26Accepted Manuscript. This version has not undergone final editing. Please refer to the complete version of record at http://www.sciencemag.org/.2 Magnetic fields are proposed to have played a critical role in some of the most enigmatic 26 processes of planetary formation by mediating the rapid accretion of disk material onto the 27 central star and the formation of the first solids. However, there have been no direct 28 experimental constraints on these fields. Here we show that dusty olivine-bearing 29 chondrules from the Semarkona meteorite were magnetized in a nebular field of 54±21 µT. 30This intensity supports chondrule formation by nebular shocks or planetesimal collisions 31 rather than by electric currents, the x-wind, or other mechanisms near the sun. This 32 implies that background magnetic fields in the terrestrial planet-forming region were likely 33 5-54 µT, which is sufficient to account for measured rates of mass and angular momentum 34 transport in protoplanetary disks. 35 36Astronomical observations of young stellar objects indicate that early planetary systems 37 evolve through a protoplanetary disk phase in <5 million years (My) following the collapse of 38 their parent molecular clouds (1, 2). Disk evolution on such short timescales requires highly 39 efficient inward transport of mass accompanied by outward angular momentum transfer, which 40 allows disk material to accrete onto the central star while delivering angular momentum out of 41 the protoplanetary system. 42The mechanism of this rapid mass and angular momentum redistribution remains unknown. 43Several proposed processes invoke a central role for nebular magnetic fields. Among these, the 44 magnetorotational instability (MRI) and magnetic braking predict magnetic fields with intensities 45 of ~100 µT at 1 AU in the active layers of the disk (3, 4). Alternatively, transport by 46 magnetocentrifugal wind (MCW) requires large-scale, ordered magnetic fields stronger than ~10 47 µT at 1 AU. Finally, non-magnetic effects such as the baroclinic and Goldreich-Schubert-Fricke 48 instabilities may be the dominant mechanism of angular momentum transport in the absence of 49 sufficiently strong magnetic fields (5). Direct measurement of magnetic fields in the planet-50 forming regions of the disk can potentially distinguish among and constrain these hypothesized 51 mechanisms. 52Although current astronomical observations cannot directly measure magnetic fields in 53 planet-forming regions [(6); supplementary text], paleomagnetic experiments on meteoritic 54 materials can potentially constrain the strength of nebular magnetic fields. Chondrules are 55 millimeter-sized lithic constituents of primitive meteorites that formed in transient heating events 56 in the solar nebula. If a stable field was present during cooling, they should have acquired a 57 thermoremanent magnetization (TRM), which can be characterized via paleomagnetic 58 experiments. Besides assessing the role of magnetic fields in disk evolution, such paleomagnetic 59 measure...

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“…Therefore, SQUID microscopy provides important evidence supporting the possibility that this meteorite could retain a record of an ancient magnetic field on its parent body. We show elsewhere [Fu et al, 2012] that such assemblages in ALHA81001 in fact have retained a record of the crustal magnetic field in the asteroid 4 Vesta at about 3.69 billion years ago, which was likely produced by an earlier dynamo. This application clearly demonstrates how scanning magnetic microscopy can provide invaluable information that is complementary to traditional paleo-and rock magnetic techniques on bulk samples.…”

Section: Geological Samplesmentioning

confidence: 99%

“…This sample not only provides an excellent opportunity to test the technique on a well-studied specimen, but also allows the comparison with results previously obtained using spatial-domain inversion techniques. We follow this demonstration of the technique with an inversion to identify the spatial distribution of high-coercivity grains in a 30 mm thin section of the eucrite meteorite ALHA81001 (a basaltic achondrite thought to be from asteroid Vesta) that highlights the power of scanning magnetic microscopy [Fu et al, 2012].…”

Section: Geological Samplesmentioning

confidence: 99%

Fast inversion of magnetic field maps of unidirectional planar geological magnetization

et al. 2013

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[1] Scanning magnetic microscopes are being increasingly utilized in paleomagnetic studies of geological samples. These instruments typically map a single component of the sample's magnetic field at close proximity with submillimeter horizontal spatial resolution. However, in most applications, an image of the magnetization distribution within the sample is desired rather than its external magnetic field. This requires carefully solving an ill-posed inverse problem to obtain solutions that are nearly free of artifacts and consistent with both natural and laboratory magnetization processes. We present a new, fast inversion technique based on classic methods developed for the Fourier domain that retrieves planar unidirectional magnetization distributions from magnetic field maps. Whereas our approach considers the subtle peculiarities of scanning magnetic microscopy which otherwise can complicate this technique, much of the formalism and algorithms described in this work can also be directly applied to province-scale magnetic field data from aeromagnetic surveys and may be used as an initial step in the modeling of magnetic sources with complex three-dimensional geometries. We discuss sources of inaccuracy observed in practical implementations of the technique and present strategies to improve the quality of inversions. Numerous examples of inversion of both synthetic and experimental data demonstrate the performance of the technique under different conditions. In particular, we retrieve magnetization distributions of a Hawaiian basalt and compare it to inversions calculated in a previous work. We conclude by showing a reconstructed magnetization for the eucrite meteorite ALHA81001 that displays in high resolution the spatial distribution of high-coercivity grains within the sample.

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An Ancient Core Dynamo in Asteroid Vesta

Cited by 90 publications

References 97 publications

Experimental study of global-scale turbulence in a librating ellipsoid

Experimental study of global-scale turbulence in a librating ellipsoid

Solar nebula magnetic fields recorded in the Semarkona meteorite

Fast inversion of magnetic field maps of unidirectional planar geological magnetization

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