The paleomagnetic record of ryugu samples and a simple test for identifying potential magnetic contamination
Résumé
Introduction: The JAXA Hayabusa 2 mission returned 5.4 g of material from the C-type asteroid Ryugu [1]. A recent study reporting TIMS measurements of Cr isotopes in Ryugu samples indicates that this material experienced aqueous alteration 4.13!".$$ %".&' Myr after the formation of calcium-aluminum-rich inclusions (CAI) [2]. This time range may overlap with the estimated lifetime of the solar nebula magnetic field in the outer region of the solar system [3], where Ryugu’s parent body is thought to have formed (e.g., [4]). We analyzed three Ryugu particles to determine whether their secondary magnetite and/or pyrrhotite grains preserved a paleomagnetic record of the solar nebula field that may have been present at the time of aqueous alteration on Ryugu’s parent body [5]. Paleomagnetic results: We were allocated two samples by JAXA: sample C0005 (21.87 mg) and sample A0154 (broken in two particles of 0.82 and 0.97 mg). We find that none of these samples exhibits a stable natural remanent magnetization (NRM) upon alternating field demagnetization. Because most Ryugu samples are breccias, the weak, unstable NRM of C0005 (the 21.87-mg sample) could be explained by the random orientation of clasts of ~1 mg, each with a coherent magnetization acquired before brecciation. However, because neither of the A0154 pieces (of 0.82 and 0.97 mg) exhibit a stable remanence, we deem this explanation unlikely. Alternatively, by comparison with the demagnetization of anhysteretic remanent magnetizations of the same samples, and with the NRM of other carbonaceous chondrites, we estimate that the aqueous alteration of Ryugu’s parent body took place in a field no stronger than a few µT and possibly null. This result is consistent with a formation at a large heliocentric distance (> 5 au), akin to the paleomagnetic record of the ungrouped carbonaceous chondrite Tagish Lake [6]. It does not, however, provide a strong constraint on the lifetime or dissipation time of the solar nebula magnetic field in the outer solar system. Two other independent paleomagnetic investigations were conducted on different Ryugu particles of similar mass as A0154. Our results are consistent with one [7] but not the other, which argues for a recorded paleofield intensity of 40 to 390 μT [1,8]. Our interpretation of this discrepancy is that these samples may have been exposed to artificial magnetic fields (> mT) during experiments that were performed on these same samples before the paleomagnetic investigation. Testing for magnetic contamination: The typical ways of testing for magnetic contamination (either by a magnet or by spurious strong fields) require comparing the NRM to a laboratory magnetization applied to the same sample (e.g., [9]). These tests are therefore destructive to the NRM, and require demagnetizing it carefully prior to knowing whether the samples were contaminated. We show that the ratio of NRM over magnetic susceptibility, two quantities that can be easily and quickly measured, can be used to identify a potential contamination, without the need for demagnetizing the NRM and remagnetizing the sample. The utility of this totally non-destructive indicator is supported by a large database compiling the NRM over susceptibility ratios for various types of meteorites (falls and finds), returned samples (Apollo) and terrestrial samples. We recommend measuring this ratio routinely, possibly at the curation stage, before paleomagnetic investigations of meteorites and returned samples, to avoid conducting time-consuming measurements of most likely contaminated samples. We also stress the importance of conducting, as much as possible, the paleomagnetic investigations of returned samples (e.g., the forthcoming MSR samples) before any other experiment. References: [1] Nakamura et al. (2022) Science 379, eabn8671; [2] Tanaka et al. (2024) Astrophys. J. 965:52; [3] Weiss et al. (2021) Sci. Adv. 7, eaba5967; [4] Yokoyama et al. (2022) Science 379, eabn7850; [5] Maurel et al. (2024) Earth Planet. Sci. Lett. 627, 118559; [6] Bryson et al. (2020) Astrophys. J. 892:126; [7] Mansbach et al. (2024) Lunar Planet. Sci. Conf., abstract #1958; [8] Sato et al. (2022) J. Geophys. Res. Planets 127 , e2022JE007405; [9] Gattacceca and Rochette (2004) Earth Planet. Sci. Lett. 227, 377-393