The Thermal Evolution of Planetesimals During Accretion and Differentiation: Consequences for Dynamo Generation by Thermally‐Driven Convection

Abstract

Advances in rock magnetism and paleomagnetic techniques over the past 2 decades have revealed that many meteorites carry primary magnetic remanences imparted by magnetic fields generated in the first few 100 Myr after the formation of the solar system. This primary remanence has been found in both achondrites (e.g.,

The Fine‐Scale Magnetic History of the Allende Meteorite: Implications for the Structure of the Solar Nebula

“…With these refinements, the Dodds et al. (2021) conclusions remain similar to those of Bryson et al. (2019), suggesting that only a highly specific scenario with a very small body and a narrow range of intermediate accretion rates can achieve a core dynamo by 4 My after CAIs.…”

“…The models also consider a wide range of accretion timescales between instantaneous to several million years. With these refinements, the Dodds et al (2021) conclusions remain similar to those of Bryson et al (2019), suggesting that only a highly specific scenario with a very small body and a narrow range of intermediate accretion rates can achieve a core dynamo by 4 My after CAIs. In any case, bodies with a substantial unmolten chondritic lid that could have sourced the CV chondrites do not form dynamos.…”

“…This melt would enter the magma ocean with a composition corresponding to the lower temperature regions from which the melt was extracted, which would result in less Fe-rich melts than the core. As such, this mechanism is unlikely to change the conclusions of the Bryson et al (2019) and Dodds et al (2021) works, especially for the most relevant interval 3.0-4.2 My after CAIs. In parallel, several paleomagnetic studies have found possible experimental evidence for a delayed core dynamo in planetesimals.…”

“…An updated study by Dodds et al. (2021) includes a time‐resolved model of core segregation accounting for thermally driven changes in the density of newly descended Fe‐S melt. The models also consider a wide range of accretion timescales between instantaneous to several million years.…”

A Time‐Resolved Paleomagnetic Record of Main Group Pallasites: Evidence for a Large‐Cored, Thin‐Mantled Parent Body

“…Both studies retrieved paleointensities of ∼100 μT, interpreted as evidence for an active dynamo ∼140–240 Myr after parent body accretion. An active thermal dynamo is predicted to have only lasted for a maximum of ∼40 Myr after accretion (Bryson, Neufeld, et al., 2019; Dodds et al., 2021; Elkins‐Tanton et al., 2011). Therefore, this measured magnetic remanence has instead been attributed to compositional convection resulting from core solidification.…”