Evidence for Magnetically-Driven Accretion in the Distal Solar System

Paleomagnetic measurements of meteorites indicate that magnetic fields existed in the inner solar nebula capable of driving accretion at rates similar to those observed for young stellar objects with protoplanetary disks. However, the field strength in the solar system beyond ∼7 astronomical units (AU) and its role in accretion remain poorly constrained. Returned samples from asteroid (162173) Ryugu offer the possibility of determining the nebular field intensity in this distal region. Here, we report paleomagnetic studies of three Ryugu particles which reveal that alteration occurred in the presence of a null or relatively weak (<15.8 μT) field within 3 million years (Ma) after solar system formation. This resolves previously contrasting reports that Ryugu's parent body experienced alteration in the presence of a strong (>80 μT) magnetic field and weak or null field (<3 μT). In addition, we re-examine previous paleomagnetic and Mn-Cr chronometry studies of three other distally-sourced meteorites, Tagish Lake, Tarda, and Wisconsin Range 91600, which measured paleointensities of <0.9, <1.7 and 5.1 ± 4.5 μT respectively. While it was previously unclear whether these records were acquired while the nebula was present, our re-analysis suggests that their records are sufficiently old (i.e., <3.5 Ma after solar system formation) to be nebular in origin. Collectively, these data demonstrate that the distal solar system nebular field, while faint, was likely still strong enough to drive accretion at rates like those observed in the inner solar system.