Characteristics of natural remanence records in fine-grained particles returned from asteroid Ryugu

TL;DR

This study analyzed the magnetic remanence in 28 particles from asteroid Ryugu, revealing stable and unstable NRM components, and suggesting aqueous alteration as a key process in remanence acquisition, enhancing understanding of primitive solar system materials.

Contribution

It significantly expands the sample size for NRM analysis in Ryugu particles and provides new insights into the mechanisms of remanence acquisition in asteroid materials.

Findings

Some particles show stable NRM components, others do not.

NRM directions are spatially inhomogeneous within particles.

Remanence likely acquired before solidification via aqueous alteration.

Abstract

Particles collected from the asteroid Ryugu by the Hayabusa2 spacecraft offer a unique opportunity to investigate the magnetic record of the primitive solar system, as any terrestrial magnetic contamination is minimal and can be accounted for. In previous studies, stepwise alternating field demagnetization (AFD) measurements of natural remanent magnetization (NRM) records have been conducted on seven Ryugu particles. However, due to the limited number of samples, there is no consensus regarding the interpretation of the results of these measurements. To address this problem, we performed stepwise AFD measurements of the NRM on 28 Ryugu particles. Twenty-three of the particles exhibited one or two stable NRM components, whereas the remaining five did not. Isothermal remanent magnetization-based paleointensity values derived from stable NRM components varied by more than one order of magnitude. These NRM characteristics were consistent with those observed in previous studies. Therefore, as a reflection of the original nature of the NRM record, some Ryugu particles exhibited stable NRM components, whereas others did not. The Ryugu particles investigated in this study and those from a previous study exhibited spatially inhomogeneous NRM directions within individual particles, constraining the NRM acquisition time to before the final solidification of the current Ryugu particles. A mechanism of remanence acquisition that can explain the observed NRM characteristics is a chemical remanent magnetization associated with the growth of framboidal magnetite during aqueous alteration in Ryugu's parent body.