Magnetization of carbonaceous asteroids by nebular fields and the origin
of CM chondrites
Abstract
Within the young solar system, a strong magnetic field permeated the
protoplanetary disc. The solar nebular magnetic field is likely the
source of magnetization for some meteorites like the CM and CV
chondrites, which underwent aqueous alternation on their parent bodies
before the solar nebular field dissipated. Since aqueous alteration
produced magnetic minerals (e.g. magnetite and pyrrhotite), the
meteorites could have acquired a chemical remanent magnetization from
the nebular field while part of their respective parent bodies. However,
questions about the formation history of the parent bodies that produced
magnetized CM and CV chondrites await answers—including whether the
parent bodies exhibit a detectable magnetic field today. Here, we use
thermal evolution models to show that a parent body of the CM chondrites
could record ancient magnetic fields and, perhaps, exhibit strong
present-day crustal remanent fields. An undisturbed planetesimal would
experience one of three thermal evolution cases with respect to the
lifetime of the nebular field. First, if a planetesimal formed too late
for 26Al-driven water ice melting to occur before the
solar nebula dissipates, then aqueous alteration would not occur in the
presence of the nebular field and result in no magnetization (Fig. panel
a). Second, if a planetesimal forms early enough to undergo alteration
before the nebula dissipates but not enough to heat beyond the blocking
temperature(s) of the magnetic mineral(s), then nearly the entire
planetesimal could be magnetized (Fig. panel b). Lastly, if a
planetesimal forms early enough to undergo alteration and subsequently
heats beyond the blocking temperature, then any magnetization would be
erased except for a thin shell near the surface (Fig. panel c). Our
thermal model results suggest that planetesimals that formed between
~2.7 and 3.7 Myr after CAIs could acquire large-scale
magnetization. Spacecraft missions could detect this magnetization if it
is at the strength recorded in CM chondrites and if it is coherent at
scales of tens of kilometers. In-situ magnetometer measurements of
chondritic asteroids could help link magnetized asteroids to magnetized
meteorites. Specifically, a spacecraft detection of remanent
magnetization at 2 Pallas would bolster the claim that 2 Pallas is a
parent body of CM chondrites.