The Top‐Down Solidification of Iron Asteroids Driving Dynamo Evolution

Abstract

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.

Ferrovolcanism: Iron Volcanism on Metallic Asteroids

“…This model uses the approach of Nimmo and Spencer () to numerically solve the Stefan problem with the surface pinned to 200 K, the liquid interior at 1,200 K, and other parameter values given in Table . Models taking core superheat and delamination into account (Neufeld et al, ) yield similar results.…”

“…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.…”

“…Delamination at the base of the crust can also modify the thermal evolution of the body by maintaining a thinner crust. However, although delamination is likely to occur, Neufeld et al () show that neither it nor core superheat significantly modify the overall thermal evolution of the body and that the Stefan assumption is generally justified.…”

“…Solid material tends to delaminate from the crust because it is more dense than the melt immediately below it, and at the base of the crust temperatures are high enough, and thus viscosities low enough, for material to flow. Delamination has been discussed on Io (Kirchoff & McKinnon, 2009), as well as on metallic asteroids (Neufeld et al, 2019), and in the latter case is expected to recur on timescales of a few tens of kiloyears. The stress will vary in and around the diapir, but Kirchoff and McKinnon (2009) point out that its magnitude is roughly ∼ Δ g where is the thickness of the layer being shed.…”

Variations in the magnetic properties of meteoritic cloudy zone

“…Furthermore, unlike Earth's core, the rapid cooling rate of the surface of the IVA parent core is expected to result in initial solidification at the asteroid surface, and the crystallisation front will then have advanced toward the centre of the core as cooling continued. The solidification of this core therefore represents a fundamentally different regime to that of our own planet and could provide crucial constraints on dynamo generation (Neufeld et al, 2019).…”