The (Non-)Magnetization of 433 Eros: Possible Mechanisms for the Lack of Magnetism as Measured by NEAR

Abstract

The Near Earth Asteroid Rendezvous-Shoemaker ("NEAR") spacecraft orbited and ultimately landed on the near-Earth asteroid 433 Eros. One of the primary science objectives of NEAR was the MAG experiment, which measured the magnetic field in the vicinity of Eros during orbit and after landing. Eros is therefore at the present the best characterized asteroid using in situ measurement of magnetism. MAG results suggested that Eros was very unmagnetized-with an upper bound on the natural remanent magnetism (NRM) placed at $1.9\times 10^{-6}\mathrm{A\cdot m^2 \cdot kg^{-1}}$-especially when compared to meteorite samples of analogous composition. Since meteorites and asteroids are typically believed to represent the remnants of disrupted parent bodies, the ramifications of the low level of magnetization of Eros are considerable, since it could imply disparate origins for objects of similar composition. In this paper, we explore whether there are any systematic effects related to the actual process of measurement and derivation of the Erotian NRM, and whether such effects played a role in the low levels of NRM derived for Eros. By simulating the orbit of NEAR around Eros and using the field strength values measured by NEAR, we find that we are able to place a higher bound on the NRM of Eros by a factor of at least an order of magnitude higher than that originally suggested by Acu\~{n}a, et al. (2002). We find that if we suppose Eros to be made up of constituents that have roughly uniform magnetization directionally, that it is possible to infer an L or LL chondrite-type composition for Eros within the bounds of values for remanent magnetism reported in the meteorite record. The results provide a more rigorous confirmation of the suggestion by Wasilewski, et al. (2002) that Eros cannot be ruled out as an L- or LL-type analogue.