A long-lived lunar dynamo powered by core crystallization

Laneuville, M.; Wieczorek, M. A.; Breuer, D.; Aubert, Julien; Morard, Guillaume; Rückriemen, Tina

doi:10.1016/j.epsl.2014.05.057

Cited by 122 publications

(146 citation statements)

References 45 publications

(80 reference statements)

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“…Alternatively, (2) the internal field may have been triggered only about 250 Ma after core formation. Second, the paleointensity between 3.56 and 3.85 Ga b.p., inferred for the magnetizing field from the measured magnetization, was about two orders of magnitude higher than suggested by estimates from magnetic field scaling laws for typical thermal or chemical dynamos (e.g., Evans et al 2014;Laneuville et al 2014;Weiss and Tikoo 2014). Finally, no lunar dynamo model exists to date that is capable of explaining the rapid decline of the field intensity with time until 3.3 Ga b.p.…”

Section: The Moonmentioning

confidence: 66%

“…Values for the critical heat flux that have been used in the literature vary between 5 and 20 mW m for Mars (Nimmo and Stevenson 2000) and Mercury (Stevenson et al 1983;Schubert et al 1988) and between 1 and 10 mW m −2 for the Moon (Zhang et al 2013;Laneuville et al 2014;Evans et al 2014) and Ganymede (Hauck et al 2006;Bland et al 2008;Kimura et al 2009;Rückriemen et al 2015).…”

Section: Thermal and Chemical Dynamosmentioning

confidence: 99%

“…It has also been suggested that Mars' core may enter the iron snow regime (Stewart et al 2007) and that the lunar core may have gone through an iron snow episode (Zhang et al 2013;Laneuville et al 2014). Although the thermodynamics of the iron snow regime seems to be reasonably well developed, the questions of how and where a magnetic field can be generated in an iron snow core remain debated.…”

Section: Iron Snow Regimementioning

confidence: 99%

“…In fact, a thermal dynamo is, in general, difficult to obtain for stagnant-lid planets. For these, superheating of the core with respect to the mantle must be postulated (e.g., Stevenson et al 1983;Breuer and Spohn 2003;Laneuville et al 2014). If superheating can be obtained (e.g., upon core formation), the thermal dynamo typically shuts off very early during the evolution, since the heat flow from the core decreases rapidly during the first few hundred million years.…”

Section: Scaling Laws and Power For Dynamo Actionmentioning

confidence: 99%

“…Most previous thermal and magnetic field evolution models have considered a thermally driven dynamo during the early evolution of the Moon (Konrad and Spohn 1997;Spohn et al 2001;Laneuville et al 2014). These models show that a thermally driven dynamo would have been active since core formation and may have lasted a maximum of 500 Myr, until 4 Ga b.p., if the core was superheated by a few hundred degrees with respect to the mantle.…”

Section: The Moonmentioning

confidence: 99%

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Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Breuer

Rueckriemen

Spohn

2015

Prog. in Earth and Planet. Sci.

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Recent planetary space missions, new experimental data, and advanced numerical techniques have helped to improve our understanding of the deep interiors of the terrestrial planets and moons. In the present review, we summarize recent insights into the state and composition of their iron (Fe)-rich cores, as well as recent findings about the magnetic field evolution of Mercury, the Moon, Mars, and Ganymede. Crystallizing processes in iron-rich cores that differ from the classical Earth case (i.e., Fe snow and iron sulfide (FeS) crystallization) have been identified and found to be important in the cores of terrestrial bodies. The Fe snow regime occurs at pressures lower than that in the Earth's core on the iron-rich side of the eutectic, where iron freezes first close to the core-mantle boundary rather than in the center. FeS crystallization, instead, occurs on the sulfur-rich side of the eutectic. Depending on the core temperature profile and the pressure range considered, FeS crystallizes either in the core center or close to the core-mantle boundary. The consequences of the various crystallizing mechanisms for core dynamics and magnetic field generation are discussed. For the Moon, revised paleomagnetic data obtained with advanced techniques suggest a peculiar history of its internal dynamo, with an early strong field persisting between 4.25 and 3.5 Ga, and subsequently a much weaker field. In addition, the long-lasting dynamo and the possible presence of an inner core, as inferred from a revised interpretation of Apollo seismic data, suggest core crystallization as a viable process of magnetic field generation for a substantial period during lunar evolution. The present-day magnetic fields of Mercury and Ganymede (if they occur on the iron-rich side of the Fe-FeS eutectic) and the related dynamo action are likely generated in the Fe snow regime and seem to be recent features. An earlier dynamo in Mercury would have been powered differently. For Mercury, MESSENGER data further suggest core formation under reducing conditions that may have resulted in an Fe-S-Si composition, further complicating the core crystallization process. Mars, with its early and strong paleo-field, likely has not yet started to freeze out an inner iron core.

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Section: The Moonmentioning

confidence: 66%

Section: Thermal and Chemical Dynamosmentioning

confidence: 99%

Section: Iron Snow Regimementioning

confidence: 99%

Section: Scaling Laws and Power For Dynamo Actionmentioning

confidence: 99%

Section: The Moonmentioning

confidence: 99%

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Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Breuer

Rueckriemen

Spohn

2015

Prog. in Earth and Planet. Sci.

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Geophysical evidence for melt in the deep lunar interior and implications for lunar evolution

Khan

Connolly

Pommier

et al. 2014

J. Geophys. Res. Planets

116

176

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Analysis of lunar laser ranging and seismic data has yielded evidence that has been interpreted to indicate a molten zone in the lowermost mantle overlying a fluid core. Such a zone provides strong constraints on models of lunar thermal evolution. Here we determine thermochemical and physical structure of the deep Moon by inverting lunar geophysical data (mean mass and moment of inertia, tidal Love number, and electromagnetic sounding data) in combination with phase-equilibrium computations. Specifically, we assess whether a molten layer is required by the geophysical data. The main conclusion drawn from this study is that a region with high dissipation located deep within the Moon is required to explain the geophysical data. This region is located within the mantle where the solidus is crossed at a depth of ∼1200 km (≥1600• C). Inverted compositions for the partially molten layer (150-200 km thick) are enriched in FeO and TiO 2 relative to the surrounding mantle. The melt phase is neutrally buoyant at pressures of ∼4.5-4.6 GPa but contains less TiO 2 (<15 wt %) than the Ti-rich (∼16 wt %) melts that produced a set of high-density primitive lunar magmas (density of 3.4 g/cm 3 ). Melt densities computed here range from 3.25 to 3.45 g/cm 3 bracketing the density of lunar magmas with moderate-to-high TiO 2 contents. Our results are consistent with a model of lunar evolution in which the cumulate pile formed from crystallization of the magma ocean as it overturned, trapping heat-producing elements in the lower mantle.

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Tides on the Moon: Theory and determination of dissipation

2015

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Solid body tides on the Moon vary by about ±0.1 m each month. In addition to changes in shape, the Moon's gravity field and orientation in space are affected by tides. The tidal expressions for an elastic sphere are compact, but dissipation introduces modifications that depend on the forcing period. Consequently, a Fourier representation of the tide-raising potential is needed. A mathematical model for the distortion-caused tidal potential may be used for the analysis of precise spacecraft tracking data. Since tides affect gravitational torques on the Moon from the Earth's attraction, the lunar orientation is also affected. Expressions for five periodic perturbations of orientation are presented. The rheological properties of lunar materials determine how the Moon responds to different tidal periods. New lunar laser ranging solutions for the tidal orientation terms are presented. The quality factor Q is 38 ± 4 at 1 month, 41 ± 9 at 1 year, ≥74 at 3 years, and ≥58 at 6 years. The ranging results can be matched with absorption band models that peak at 120 days and single relaxation time models that peak at~100 days. Combined models are possibilities. Dissipation can modify laser ranging solutions; previously reported core flattening is too uncertain to be useful. Strong lunar tidal dissipation, modeled to arise in the deep hot mantle, appears to be from a region with radius ≥535 km. Classical Maxwell-type dissipation is too weak to detect at 3 and 6 year periods.

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A long-lived lunar dynamo powered by core crystallization

Cited by 122 publications

References 45 publications

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Geophysical evidence for melt in the deep lunar interior and implications for lunar evolution

Tides on the Moon: Theory and determination of dissipation

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