433 Eros: Problems with the meteorite magnetism record in attempting an asteroid match

摘要

The magnetometer experiment (MAG) onboard the Near-Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2.1 * 10~(-6) Am~2 kg~(-1) for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S-class asteroids (Gaspra, ～2.4 * 10~(-2) Am~2 kg~(-1); Braille, ～2.8 * 10~(-2) Am~2 kg~(-1)) and is below published values for all types of ordinary chondrites. This includes the L/LL types considered to most closely match 433 Eros based on preliminary interpretations of NEAR remote geochemical experiments. The ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid-meteorite match based on magnetic property measurements. Natural remanent magnetization (NRM) intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the H, L, and LL groups, all well above the 2.1 * 10~(-6) Am!2 kg~(-1) level for 433 Eros. The REM values (ratio of the NRM to the SIRM (saturation remanent magnetization)) range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between NMR and the amount of magnetic material. Levels of magnetic noise in chondrite meteorites can be as much as 70% or more of the NRM. Consequently, published values of the NMR should be considered suspect unless careful evaluation of the noise sources is done. NASA Goddard SFC studies of per unit mass intensities in large (>10 000 g) and small (down to <1 g) samples from the same meteorite demonstrate magnetic intensity decreases as size increases. This would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples. This would then argue for some level of demagnetization of large objects such as an asteroid. The possibility that 433 Eros is an LL chondrite cannot be discounted.