Core solidification and dynamo evolution in a mantle‐stripped planetesimal

Abstract

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

The Top‐Down Solidification of Iron Asteroids Driving Dynamo Evolution

“…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.…”

“…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 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, ).…”

Lunar Cumulate Mantle Overturn: A Model Constrained by Ilmenite Rheology

“…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.…”

Ferrovolcanism: Iron Volcanism on Metallic Asteroids

“…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.…”

“…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.…”