Acquisition and Preservation of Remanent Magnetization in Carbonaceous Asteroids

The solar nebula sustained a strong magnetic field that may have aided planetesimal accretion and imparted the chemical remanent magnetization (CRM) observed in some carbonaceous chondrite meteorites. The CRM thus provides a record of the magnetic field of the early Solar System at the time when carbonaceous chondrite parent bodies experienced aqueous alteration. However, the link between CRM recorded in carbonaceous chondrites and the geophysical evolution of carbonaceous chondrite parent bodies has not been thoroughly investigated. Using planetesimal thermal evolution models, we show that 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 chondrite parent bodies (>50 km radius), which never hosted endogenous dynamo-driven magnetic fields due to their lack of metallic cores, could have strong, present-day remanent magnetism from the ancient nebular magnetic field. In situ magnetometer measurements of large C-type asteroids could therefore validate models of carbonaceous chondrite magnetization by the solar nebular magnetic field. We suggest that 2 Pallas may be a good target for such a study. The residual magnetic field detected in some carbonaceous chondrite meteorites is a remanent of the primordial field of the early solar nebula, preserved via aqueous alteration processes that happened in large planetesimals formed around 4 Myr after CAI formation and just before the dissipation of the solar nebula.