The Early Solar System — Benjamin P. Weiss

NEBULAR MAGNETIC FIELDS

Our paleomagnetic studies of chondrules from the Semarkona meteorite provided the first measurements of the intensity of nebular magnetic fields in the terrestrial-planet forming region of the early solar system.

DUSTY OLIVINE CHONDRULE

The Semarkona meteorite contains chondrules with dusty olivine crystals. This transmission electron microscopy image shows how the olivine (dark grey) hosts fine pure iron metal grains (white) with excellent magnetic recording properties. These metal grains are formed by reduction of an iron-bearing olivine precursor, which also produces excess silica (black phase at tips of metal grains)

LIFETIME OF THE SOLAR NEBULA

We found that volcanic angrites are not magnetized, indicating that the solar nebular field and gas likely had dissipated by just 3.8 million years after the formation of the oldest known solids.

Magnetic fields are proposed to have played a critical role in some of the most enigmatic processes of planetary formation by mediating the rapid accretion of disk material onto the central star and the formation of the first solids. However, there have been no experimental constraints on the intensity of these fields. We have been studying meteoritic materials from the early solar system to infer the existence intensity of nebular magnetic fields (Weiss et al. 2021). We showed that chondrules in two meteorites were magnetized in a nebular field of ~50-100 μT (Fu et al. 2014, Mai et al. 2018, Borlina et al. 2021) at ~2 million years after solar system formation. This field strength is sufficient to account for the rates of stellar accretion observed in protoplanetary disks. We have also shown that, under certain assumptions, the weak magnetic field of comet 67P indicates the nebular field was below 3 μT at 15-25 AU from the young Sun, consistent with theoretical expectations (Biersteker et al. 2019).

Because nebular gas is required for the sustenance of nebular magnetism, paleomagnetic methods can also be used to trace the lifetime of the nebula itself. We have shown that several meteorite groups are essentially not magnetized (Wang et al. 2017, Weiss et al. 2017, Borlina et al. 2022). This indicates the solar nebular field and gas had likely dissipated by just 4 million years after solar system formation. This times the birthdate of the Sun, Jupiter and Saturn to less than 4 million years after the collapse of the molecular cloud.