A Long-Lived Lunar Core Dynamo

Shea, Erin K.; Weiss, B. P.; Cassata, William S.; Shuster, David L.; Tikoo, S. M.; Gattacceca, J.; Grove, T. L.; Fuller, M.

doi:10.1126/science.1215359

Cited by 103 publications

(92 citation statements)

References 75 publications

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“…Although this event may have partially remagnetized or demagnetized low blocking temperature grains in this rock (depending on whether a field was present at this time), many of these grains would have subsequently been demagnetized during zero-field residence on the lunar surface over the intervening 3 Ga and during residence in our laboratory's shielded room. As has been inferred for many other Apollo 11 basalts (7,35), both 10017 and 10049 also apparently experienced modest gas loss due to solar heating over the last 304.7 ± 2.0 Ma and 17.5 ± 0.1 Ma, respectively. In particular, numerical models of simultaneous production and diffusion of both radiogenic 40 Ar and cosmogenic 38 Ar indicate that sample 10049 only experienced temperatures in excess of the ambient crustal conditions because it was exposed near the lunar surface.…”

supporting

confidence: 69%

Persistence and origin of the lunar core dynamo

Suavet

Weiss

Cassata

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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The lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation. Here, we report analyses of two 3.56-Gy-old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least ∼13 μT. These data extend the known lifetime of the lunar dynamo by ∼160 My and indicate that the field was likely continuously active until well after the final large basin-forming impact. This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history. Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo.high-K mare basalts | paleomagnetism

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supporting

confidence: 69%

Persistence and origin of the lunar core dynamo

Suavet

Weiss

Cassata

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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“…If DANG < MAD, then the component's decay is origin-trending to within the uncertainty of the component's direction (21,78). The rate of HC component decay over its coercivity range is similar to that of an ARM but not to that of an IRM (Fig.…”

Section: The Hc/ht Componentmentioning

confidence: 95%

“…As was the case for the IRM calibration factor a, the value of f has not been determined for Semarkona bulk material using TRM acquisition experiments. We adopt f=1.34 as a typical value for metal-bearing samples (78,79).…”

Section: Nrm Of Bulk Samplesmentioning

confidence: 99%

Solar nebula magnetic fields recorded in the Semarkona meteorite

Weiss

Lima

et al. 2014

Science

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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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“…Thermal diffusion calculations show that relatively thin basaltic lava flows cool efficiently by radiation and do not heat subsurface materials to temperatures sufficient to thermally demagnetize plausible anomaly sources at depth [see, e.g., Rumpf et al, 2013]. In contrast to Caloris, the lunar Imbrium and Orientale basins show no internal anomalies even though a core dynamo apparently existed when they formed [e.g., Shea et al, 2012;Suavet et al, 2013]. This may imply that the latter impacts did not produce impact melt containing sufficient magnetic carriers to produce detectable anomalies or that most impact melt was not retained in the basin interior due to the weak lunar gravity field.…”

Section: Discussionmentioning

confidence: 99%

Magnetic anomalies concentrated near and within Mercury's impact basins: Early mapping and interpretation

Hood

2016

JGR Planets

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Ninety‐five low‐altitude passes of MErcury Surface, Space ENvironment, GEochemistry, and Ranging magnetometer data from February, March, and April of 2015 have been applied to produce an approximate map of the crustal magnetic field at a constant altitude of 40 km covering latitudes of 35°–75°N and longitudes of 90°–270°E. Anomalies are concentrated near and within the Caloris impact basin. A smaller concentration occurs over and around Sobkou Planitia and an associated older large impact basin. The strongest anomalies are found within Caloris and are distributed in a semicircular arc that is roughly concentric with the basin rim. They imply the existence of a core dynamo at the time when Caloris formed (∼3.9 Gyr ago). Anomalies over high‐reflectance volcanic plains are relatively weak while anomalies over low‐reflectance material that has been reworked by impact processes are relatively strong. The latter characteristics are qualitatively consistent with the ejecta deposit model for anomaly sources.

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A Long-Lived Lunar Core Dynamo

Cited by 103 publications

References 75 publications

Persistence and origin of the lunar core dynamo

Persistence and origin of the lunar core dynamo

Solar nebula magnetic fields recorded in the Semarkona meteorite

Magnetic anomalies concentrated near and within Mercury's impact basins: Early mapping and interpretation

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