Revisiting the magnetization of comet 67P/Churyumov-Gerasimenko

Abstract

Context. The landing of the Philae probe as part of the ESA Rosetta mission made it possible to study the magnetization of comet 67P/Churyumov-Gerasimenko (67P) by combining observations from the lander and orbiter. In this work, we revisit the magnetic properties with information gained during the progression of the mission for a comprehensive understanding of the circumstances of Philae's descent and landing. Aims. The aim is to derive a limit for any possible magnetization of the cometary material on the surface of 67P. To achieve this, the surface contacts of Philae were analyzed. Combined with a more detailed understanding of the background magnetic field, this allows us to interpret the underlying magnetic measurements in detail. Methods. We combined magnetic field observations from the ROMAP magnetometer on board Philae with observations from the RPC-MAG instrument on board the Rosetta orbiter. To facilitate this, a correlation analysis was used to correct phase shifts between the observed signals. Additionally, in-flight calibration of the ROMAP offsets was performed using information about the dynamics of Philae during flight. These corrections made it possible to use the orbiter measurements as reference for the comet-based Philae observations. We assumed a simple dipole model and used the magnetic field observations to derive an upper limit for the magnetization of the cometary material. Results. An upper limit of 0.9 nT for the observed magnetic field on the surface of 67P was derived for any contribution from surface magnetization. For homogeneously magnetized pebbles with a size of typical aggregates in the range of ~5 cm, this translates into an upper limit of ~5 × 10-5 Am2 kg-1 for the specific magnetic moment. Depending on the exact history of formation, this results in an upper limit of 4 μT for the magnitude of the magnetic field in the solar nebula during the formation of comet 67P.