Implications of Philae Magnetometry Measurements at Comet 67P/Churyumov-Gerasimenko for the Nebular Field of the Outer Solar System

The remanent magnetization of solar system bodies reflects their accretion mechanism, the space environment in which they formed, and their subsequent geological evolution. In particular, it has been suggested that some primitive bodies may have formed large regions of coherent remanent magnetization as a consequence of their accretion in a background magnetic field. Measurements acquired by the Rosetta Magnetometer and Plasma Monitor have shown that comet 67P/Churyumov-Gerasimenko (67P) has a surface magnetic field of less than 0.9 nT. To constrain the spatial scale and intensity of remanent magnetization in 67P, we modeled its magnetic field assuming various characteristic spatial scales of uniform magnetization. We find that for regions of coherent magnetization with ≥10 cm radius, the specific magnetic moment is 5 × 10−6 . If 67P formed during the lifetime of the solar nebula and has not undergone significant subsequent collisional or aqueous alteration, this very low specific magnetization is inconsistent with its formation from the gentle gravitational collapse of a cloud of millimeter-sized pebbles in a background magnetic field 3 T. Given the evidence from other Rosetta instruments that 67P formed by pebble-pile processes, this would indicate that the nebular magnetic field was 3 T at 15-45 au from the young Sun. This constraint is consistent with theories of magnetically driven evolution of protoplanetary disks.