Magnetization of carbonaceous asteroids by nebular fields and the origin of CM chondrites

Planetesimals, the building blocks of planets, accreted from the solar nebula: the cloud of gas and dust surrounding the young Sun during its first few million years. Strong magnetic fields in the solar nebula may have aided planetesimal accretion. This nebular field might have imparted a chemical remanent magnetization (CRM) upon some carbonaceous chondrites, dark and primitive meteorites that formed with abundant water ice and at a distance beyond Jupiter. If so, the CRM captures a snapshot of the solar system’s magnetic field at time when the meteorite experienced chemical alteration from interaction with water. Using planetesimal thermal evolution models, we show how CRM in carbonaceous chondrites would be a natural consequence of water-rich planetesimals forming within the solar nebular magnetic field. We find that large carbonaceous asteroids—those without metallic cores that could never have hosted endogenous dynamo magnetic fields like Earth—could record ancient magnetic fields from the solar nebula in magnetic remanence that produces strong modern magnetic fields. Hence, future in-situ magnetometer measurements of C-type asteroids could reveal clues about the solar nebular field and the formation of planetesimals.