The Fe snow regime in Ganymede's core: A deep‐seated dynamo below a stable snow zone

Rückriemen, Tina; Breuer, D.; Spohn, Tilman

doi:10.1002/2014je004781

Cited by 61 publications

(85 citation statements)

References 84 publications

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“…The inverse will happen near the CMB: denser Fe will snow downwards in the core and leave the outermost liquid richer in S. If convection is not vigorous enough, the stratification could stabilize the core into compositionally distinct layers, in which convection occurs only in the steadily shrinking wellmixed central zone. This might quell dynamo activity and end Mars' magnetic era (Weiss et al 2002;Williams and Nimmo 2004), like what may be occurring in Ganymede (Rückriemen et al 2015).…”

Section: Compositionmentioning

confidence: 91%

Mars core structure—concise review and anticipated insights from InSight

Helffrich

2017

Prog Earth Planet Sci

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This review summarizes the knowledge of Mars' interior structure, its inferred composition, and the anticipated seismological properties arising from its composition with particular focus on Mars' core. The emphasis on the core stems from the unusual morphology of the liquidus diagram of iron at moderate pressures when enriched in sulfur. From a fairly detailed liquidus diagram constructed from experimental studies, I identify a set of processes that could act within Mars' core: an iron "snow" from the core-mantle boundary's surface and a Fe 3−x S 2 "ground fog" forming at the base of the core. Depending on temperature and bulk sulfur composition, these could form an inner core or could stratify the outer core by enriching it in sulfur, or both. Core stratification could be one explanation for the extinction of Mars' magnetic field early in the planet's history, and I demonstrate the feasibility of this mechanism. The crystallization processes in the core could be observable in the seismic data that the future Mars geophysical mission, InSight, is planned to provide. The core size, the presence of an inner core, and the wavespeed profile of the outer core, whose radial derivative provides a proxy for changes in composition, are key observables to seek.

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Section: Compositionmentioning

confidence: 91%

Mars core structure—concise review and anticipated insights from InSight

Helffrich

2017

Prog Earth Planet Sci

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“…This so-called iron snow regime has already been suggested to apply to the present cores of Ganymede (Hauck et al 2006;Christensen 2015;Rückriemen et al 2015) and Mercury (Chen et al 2008;Vilim et al 2010;Wicht and Heyner 2014). 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).…”

Section: Iron Snow Regimementioning

confidence: 91%

“…Thus, in the snow zone, the liquidus becomes collinear with the core temperature and reverses its slope, so that the liquidus temperature increases with increasing depth. The increase of the liquidus with depth in the snow zone is inevitably accompanied by a decrease in the sulfur concentration with depth, which implies the presence of a stable chemical gradient across the snow zone (Hauck et al 2006;Williams 2009;Rückriemen et al 2015). Usselman (1975), Boehler (1993), Boehler (1996), Fei et al (1997Fei et al ( , 2000, Shen et al (1998), Alfè et al (2002b), Campbell et al (2007), Chudinovskikh and Boehler (2007), Morard et al (2007), and Stewart et al (2007).…”

Section: Iron Snow Regimementioning

confidence: 99%

“…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%

“…The eutectic melting temperature is also given Boehler (1992) and Fe-FeS eutectic from Fei et al (1997Fei et al ( , 2000. for Ganymede, FeS starts to crystallize in the core or at the CMB and for the Moon, FeS starts to crystallize in the core or in the core center addition, Rückriemen et al (2015) argued that the nature of the convection induced by sedimentation in an otherwise chemically stably stratified layer is likely of small scale and thus lacks an important criterion (i.e., large-scale convection) that is necessary for dynamo action . Instead, it is argued that the dynamo may be located in the iron-rich layer below the snow zone.…”

Section: Iron Snow Regimementioning

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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Core solidification and dynamo evolution in a mantle‐stripped planetesimal

et al. 2016

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The physical processes active during the crystallization of a low-pressure, low-gravity planetesimal core are poorly understood but have implications for asteroidal magnetic fields and large-scale asteroidal structure. We consider a core with only a thin silicate shell, which could be analogous to some M-type asteroids including Psyche, and use a parameterized thermal model to predict a solidification timeline and the resulting chemical profile upon complete solidification. We then explore the potential strength and longevity of a dynamo in the planetesimal's early history. We find that cumulate inner core solidification would be capable of sustaining a dynamo during solidification, but less power would be available for a dynamo in an inward dendritic solidification scenario. We also model and suggest limits on crystal settling and compaction of a possible cumulate inner core.Iron meteorites can also provide information on the cooling rate experienced after solidification. Yang et al. [2008] examined several irons in the IVA meteorite family and found a large range of cooling rates within Key Points: • The rate and structure of inward fractional crystallization are constrained • Crystal settling could form a cumulate inner core with significant trapped melt • Constraints are placed on dynamo strength and longevity during solidification

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The Fe snow regime in Ganymede's core: A deep‐seated dynamo below a stable snow zone

Cited by 61 publications

References 84 publications

Mars core structure—concise review and anticipated insights from InSight

Mars core structure—concise review and anticipated insights from InSight

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

Core solidification and dynamo evolution in a mantle‐stripped planetesimal

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