GP33A-01
Timing of Solar Nebula Dispersal Constrained by Early Solar System Paleomagnetism
Wednesday, 16 December 2015: 13:40
300 (Moscone South)
Huapei Wang1, Benjamin P Weiss1, Brynna G Downey1, Xue-Ning Bai2, Jun Wang3, Jiajun Wang3, Clement R Suavet1, Roger R Fu1, Eduardo A. Lima1 and Maria E Zucolotto4, (1)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (2)Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States, (3)Brookhaven National Laboratory, Photon Sciences Directorate, Upton, NY, United States, (4)Universidade Federal do Rio de Janeiro, Museu Nacional, Rio de Janeiro, Brazil
Abstract:
The formation of the solar system/extrasolar planets largely takes place in the gas-rich solar nebular/protoplanetary disks. Nebular magnetic fields are thought to play a dominant role in global disk evolution by driving angular momentum transport via the magneto-rotational instability and/or magnetized disk winds, with the magnetically-driven accretion rate proportional to the square of the field strength. Previous paleomagnetic analyses of the Semarkona meteorite found evidence for a ~5-50 µT solar nebular field at ~2-3 My after the formation of calcium-aluminum-rich inclusions (CAIs), which consist of the first solids condensed from the cooling protoplanetary disk. These field strengths are consistent with stellar accretion rates of ~10-8 Msun/yr as typically observed for Sun-like stars. A key remaining question is the time when the nebular magnetic field and solar nebula itself dispersed. To address this, we analyzed the paleomagnetism of angrites, a class of exceptionally well-preserved igneous rocks that should retain magnetic records beginning just ~4 My after CAI formation. Here we present paleomagnetic, rock magnetic, and synchrotron-based transmission X-ray microscopic analyses of the quenched angrites D’Orbigny, Sahara 99555 and Asuka 881371. Our data show that the magnetic field at the angrite parent body region was < ~0.1 µT at ~4 My after CAI formation. This indicates that the nebular magnetic field had rapidly declined by at least a factor of ~50 by that time, such that the magnetically driven solar accretion rate was well below 10-11 Msun/yr. Because a strong nebular magnetic field was likely present throughout most of the gaseous disk lifetime, our results suggest that the solar nebula itself had probably already dispersed by ~4 My after CAI formation. This dispersal time agrees with typical protoplanetary disk lifetimes inferred from infrared excesses for Sun-like protostars. Our results suggest that the formation of the solar system giant planets, as well as the gas phase planetary migration, had largely completed by ~4 My after CAI formation. Furthermore, the formation of chondrules after this period would have not required substantial magnetic fields or nebular gas, thereby favoring planetesimal collision models over nebular shocks and magnetic reconnection.
