Core solidification and dynamo evolution in a mantle‐stripped planetesimal

Scheinberg, Aaron; Elkins-Tanton, Linda T.; Schubert, G.; Bercovici, David

doi:10.1002/2015je004843

Cited by 46 publications

(73 citation statements)

References 48 publications

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“…This scenario most closely resembles the modeling of Scheinberg et al () who posited the detachment of dendrites and rate of growth and melting of iron crystals in the liquid core as the most potent driver of dynamo activity and magnetic fields in unmantled cores. In that study, while the possibility of delamination was raised, the detailed numerical modeling instead considered cumulate formation of unattached dendrites within the liquid core, which rapidly descended to form the inner core.…”

Section: Resultssupporting

confidence: 79%

“…The model developed in the preceding sections can be adapted to incorporate the distribution of sulfur, a relatively abundant light element within most planetesimal cores. As with the previous study of Scheinberg et al (), we use the simplified iron‐sulfur phase diagram of Ehlers (), approximating the depression of the melting temperature along the liquidus as

T_{L} false(C false) = T_{m} - m C,

for moderate sulfur concentration, C , where we take T m = 1810 K as before and the slope of the liquidus as m = 18 K/wt%. The rejection of a light impurity, such as sulfur, leads to constitutional supercooling at the solid‐liquid interface, R − a , and the formation of a partially solid crust, often referred to as a mushy layer (Worster, ).…”

Section: The Effects Of Compositionmentioning

confidence: 99%

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The Top‐Down Solidification of Iron Asteroids Driving Dynamo Evolution

2019

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The cores of some small planetesimals, such as asteroid (16) Psyche, are thought to have been exposed through collisions during the early solar system that removed their mantles. These small bodies likely solidified from the top down representing a fundamentally different solidification regime to that of Earth's core. Here we derive simplified models of the downward solidification of the metallic crust and consider thermal convection and the potential for viscous delamination of the weak, warm base of the crust to provide a buoyancy flux sufficient to drive a dynamo. Thermal buoyancy is very short lived (∼1,000 years) and therefore cannot be the source of measured paleomagnetic remanence. In contrast, viscous delamination is found to provide a long‐lasting buoyancy flux sufficient to generate an intense, multipolar magnetic field, while not greatly affecting the crustal solidification time. Our results suggest that a Psyche‐sized (150‐km radius) body solidified in roughly 6.7–20 Myr and that delamination produced a strong magnetic field over much of this time. Finally, including light, insoluble impurities, such as sulfur, results in a partially solid mushy zone at the base of the crust. This further weakens the base of the crust and results in smaller‐scale delamination events. Despite a significant change in the dynamics of delamination, the time to total solidification and the predicted properties of the magnetic field are broadly comparable to the sulfur‐free case, though we argue this may result in observable compositional stratification of the body.

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Section: Resultssupporting

confidence: 79%

T_{L} false(C false) = T_{m} - m C,

Section: The Effects Of Compositionmentioning

confidence: 99%

The Top‐Down Solidification of Iron Asteroids Driving Dynamo Evolution

2019

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“…While the inferred strength of the lunar magnetic field from paleomagnetism of Apollo samples remains difficult to explain with convective cooling of the core (cf. Scheinberg et al, ), it is important to understand the CMB flux implied by our models.…”

Section: Discussionmentioning

confidence: 99%

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

Zhang

Liang

et al. 2019

JGR Planets

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Lunar cumulate mantle overturn has been proposed to explain the abundances of TiO2 and heat‐producing elements (U, Th, and K) in the source region of lunar basalts. Ilmenite‐bearing cumulates (IBCs) that were formed near the end of lunar magma ocean solidification are the driving force for overturn. IBCs are enriched with dense TiO2 and FeO contents and have lower viscosity and solidus than those of the underlying lunar cumulate mantle. We investigate the effects of temperature‐ and ilmenite‐dependent mantle rheology on the dynamic process of lunar cumulate mantle overturn and conditions for long‐wavelength downwellings of an IBC layer in a 3‐D spherical geometry. Our results show that the ilmenite‐induced rheological weakening is necessary to decouple the IBC layer from the top stagnant lid and facilitate overturn. Models with IBC viscosity derived from the experimental scaling can only produce short‐wavelength downwellings (spherical harmonic degree >3) and show an overturn timescale more than 100 Ma. A viscosity of the IBC layer at least 10−4 lower than that of the ambient mantle can produce the long‐wavelength (spherical harmonic degree ≤3) downwellings in ~10 Ma and even a hemispheric downwelling. Such low IBC viscosity requires additional weakening mechanisms, such as remelting or/and water enrichment. During the overturn, the cold downwellings displace upward the materials from hot lower mantle and produce partial melting in upper mantle, which may serve as a viable mechanism for early lunar magmatisms. The settling of cold downwellings on the core‐mantle boundary stimulates a transient high heat flux, which may contribute to generating an early lunar dynamo event.

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“…These observations show that within the IVA meteorite family, the samples with the fastest cooling rates have the lowest incompatible element contents, implying that the shallowest material crystallizes first, and the parent body solidified from the top-down. While this explanation is not universally accepted (Albarède et al, 2013), for the purpose of this paper we assume it is correct; further aspects of solidification are addressed in Scheinberg et al (2016) and Neufeld et al (2019). We predict that in such bodies the buoyant melt beneath the crust will periodically be able to erupt, influencing their cooling and creating volcanic features on the surface.…”

Section: Introductionmentioning

confidence: 92%

“…For these very approximate values, the time to form a 20-km crust is roughly 1 Myr. This evolution can be more complicated if the simple top-down conductive cooling of the Stefan problem does not apply (Scheinberg et al, 2016). Advection is one way to modify this picture, but as discussed later fluid eruption is not expected to significantly alter the thermal behavior.…”

Section: Thermal Evolution After Disruptionmentioning

confidence: 99%

Ferrovolcanism: Iron Volcanism on Metallic Asteroids

Abrahams

Nimmo

2019

Geophysical Research Letters

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Metallic asteroids, the exposed cores of disrupted planetesimals, are expected to have been exposed while still molten. Some would have cooled from the outside in, crystallizing a surface crust which would then grow inward. Because the growing crust is expected to be more dense than the underlying melt, this melt will tend to migrate toward the surface whenever it is able. Compressional stresses produced in the crust while it cools will be relieved locally by thrust faulting, which will also provide potential conduits for melt to reach the surface. We predict iron volcanism to have occurred on metallic asteroids as they cooled and discuss the implications of this process for both the evolution and the modern appearance of these bodies.

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Core solidification and dynamo evolution in a mantle‐stripped planetesimal

Cited by 46 publications

References 48 publications

The Top‐Down Solidification of Iron Asteroids Driving Dynamo Evolution

The Top‐Down Solidification of Iron Asteroids Driving Dynamo Evolution

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

Ferrovolcanism: Iron Volcanism on Metallic Asteroids

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