Early and elongated epochs of planetesimal dynamo generationOPEN ACCESS 

Hannah R. Sanderson, James F. J. Bryson, Claire I. O. Nichols

This is a preliminary release in order to share the manuscript at a conference. This paper will be submitted to Earth and Planetary Science Letters within the next month


  • Thermal dynamos can begin before the dissipation of the nebula field.
  • Core solidification is not required for a second epoch of dynamo generation.
  • Planetesimal dynamos can last several 100 Ma longer than previously thought.
  • Planetesimal radius and reference viscosity have a strong effect on dynamo duration”

    “Accreting in the first few Ma after Solar System formation, planetesimals record conditions in the protoplanetary disc and are the remnants of planetary formation processes. The meteorite paleomagnetic record carries key insights into the thermal history of planetesimals and their extent of differentiation. The current paradigm splits the paleomagnetic record into three magnetic field generation epochs: an early nebula field (<5Ma after CAI formation), followed by thermal dynamos (5-34 Ma after CAI formation), then a gap in dynamo generation, before the onset of core solidification and compositional dynamos. The split between these epochs has been defined using thermal evolution and dynamo generation models of planetesimals. Here we demonstrate these epochs are not as distinct as previously thought based on our refined thermal evolution model that includes more realistic parametrisations for mantle convection, non-eutectic core solidification and radiogenic 60Fe in the core. Inclusion of 60Fe in the core brings forward the onset of dynamo generation to 1-2 Ma after CAI formation, which overlaps with the existence of the nebula field. The second epoch of dynamo generation begins prior to the onset of core solidification, suggesting this epoch is not purely compositionally driven. Planetesimal radius is the dominant control on dynamo generation, and the choice of reference viscosity can widen the gap between epochs of dynamo generation from 0-200 Ma. Overall, timings of different planetesimal magnetic field generation mechanisms are more variable. This alters the information we can glean from the meteorite paleomagnetic record about the early Solar System. Evidence for the nebula field requires more careful interpretation and young paleomagnetic remanences, for example in the pallasites, may not be evidence for planetesimal core solidification.”