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

Abstract

Abstract-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 x 10-6 Am 2 kg-l for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S-class asteroids (Gaspra,~2.4 x 10-2 Am 2 kg-I; Braille, 2.8 x 10-2 Am 2 kg-I) 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 x 10-6 Am 2 kg-I 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 NRM and the amount of magnetic material. Levels of magnetic noise in chondrite meteorites can be as much as 70% or more ofthe NRM. Consequently, published values of the NRM should be considered suspect unless careful evaluation ofthe 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.

Paleomagnetic Records of Meteorites and Early Planetesimal Differentiation

“…However the relationship of tetrataenite's NRM with that of the precursor taenite is largely unknown, which makes the interpretation of tetrataenite-carried remanent magnetization tentative. In fact, tetrataenite formation is a candidate explanation for the small-scale randomness of NRM directions in some meteorites (Wasilewski et al 2002;Gattacceca et al 2003). …”

“…Kesen (Kessen) H4 Nagata and Sugiura (1977), , Kukkonen and Pesonen (1983) Lancon H6 Brecher and Ranganayaki (1975) Leighton H5 Larson et al (1973) Markovka H4 Guskova (1988) Metsäkylä H4 Kukkonen and Pesonen (1983) Mooresfort H5 Westphal and Whitechurch (1983) Morland H6 Larson et al (1973) Mount Browne H6 Stacey and Lovering (1959), Stacey et al (1961) Oakley Stone H6 Westphal (1986), Westphal and Whitechurch (1983) Westphal and Whitechurch (1983), Guskova (1963); Brecher and Ranganayaki (1975), Guskova (1976a), Brecher and Leung (1979) Orlovka H5 Guskova and Pochtarev (1969) Petropavlovka H4 Guskova (1983) Plainview (1917) H5 , Wasilewski et al (2002) Prairie Dog Creek H3 Westphal (1986), Westphal and Whitechurch (1983) Pultusk H5 Guskova (1963), Guskova (1976a), Brecher and Leung (1979) Westphal and Whitechurch (1983), Westphal (1986), Guskova (1965a) Quenggouk H4 Brecher and Ranganayaki (1975) Raguli H3 Guskova (1988) Richardton H5 Yu et al (2009) Rose City H5 Brecher and Ranganayaki (1975) Saline H5 Brecher and Leung (1979) Seminole H4 Nagata and Sugiura (1977) Severny Kolchim H3 Guskova (1988) Ställdalen H5 Guskova (1963), Kukkonen and Pesonen (1983) Sverdlovsk...…”

“…As a consequence, it seems unavoidable that small scale NRM scatter has a different origin in the different meteorites in which it was observed. This scatter not only has important implications for the mode of origin and age of the NRM, but also for the accuracy of paleointensity methods: measurements of bulk samples with primary but nonunidirectional TRM (e.g., due to cold brecciation), would only place lower limits on the true fine-scale NRM and paleointensity (Wasilewski et al 2002;Weiss et al 2008b). …”

“…For fast cooling, there are six different possible composition-invariant transformations in the Fe-Ni system (Wilson 1994) including the common meteoritic mineral martensite (α 2 Fe). How any of these phases magnetize is very poorly understood (e.g., Wasilewski 1974aWasilewski , 1974bWasilewski et al 2002).…”

Physical properties of meteorites—Applications in space missions to asteroids

“…Magnetic properties of ordinary chondrite clan (Wasilewski et al 2002) are mostly related to Fe-Ni compounds, namely α-kamacite (<7% Ni), γ-taenite (>7% Ni), and γ-tetrataenite (43-52% Ni) capable of carrying remanent magnetization (Morden and Collinson 1992;Wasilewski 1988). The susceptibility of the ordinary chondrites (Fig.…”