Evidence for Solar Nebula Magnetic Fields from CO Chondrites

Abstract:

Theoretical studies have long suggested that the transport of angular momentum and mass of protoplanetary disks is mediated by large-scale magnetic fields in the nebula. Paleomagnetic studies of LL chondrules have found that a nebular field ~50 µT was likely present at 2-3 million years (Ma) after the formation of calcium-aluminum-rich inclusions (CAIs) in the inner solar system (1-3 AU). Angrites and other meteorites have provided evidence for near-zero field conditions in this region by 4 Ma after CAI formation, suggesting that the gas had locally dissipated by this time. However, there have been no well-dated and accurate paleointensity constraints on the magnetic field in the innermost solar system (<0.1 AU) and in the outer solar system (>3 AU). Such data could test whether magnetic fields were mediating accretion elsewhere in the solar system and test the hypothesis that the solar system passed through a brief transition disk phase in which following accretion, the gas cleared within <0.5 Ma from the inside out. To address this, we have studied the paleomagnetism of the pristine CO chondrites DOM 08006 (type 3.00) and ALHA 77307 (type 3.03). CAIs in these meteorites may have recorded fields at <0.1 Ma after CAI-formation likely within 0.1 AU of the Sun, while chondrules recorded fields at ~2.5 Ma after CAI-formation at 3-7 AU and bulk samples at ~5 Ma after CAI-formation at 3-7 AU. Our ongoing measurements of CAIs suggest that they contain pure Fe inclusions <0.5 µm in size that could have retained a magnetic record from their formation. Our measurements of 5 mutually-oriented chondrules found that they recorded a field of 42.3 ± 13.8 µT with random directions amongst chondrules and uniform directions within a chondrule, indicating that the magnetization was acquired before parent body accretion. This is the first robust identified paleointensity estimate of magnetic fields in the outer solar system and provides evidence for a nebular field globally mediating angular momentum and mass transfer to the early sun. Our paleomagnetic results of bulk samples found that they recorded a field <0.9 µT. Combined with the angrite data, this implies that the solar nebula dissipated almost within 0.45 Ma in the inner and outer solar system by ~4-5 Ma after CAI-formation. This is consistent with rapid passage through a transition disk phase.