New view on the paleomagnetic record of samples from asteroid Ryugu
1C. Maurel1, J. Gattacceca1, M. Uehara1
CNRS, Aix Marseille Université, IRD, INRAE, CEREGE, Aix-en-Provence, France

The JAXA Hayabusa 2 mission returned ~5 g of material from the C-type asteroid Ryugu. Ryugu
material contains abundant magnetite and pyrrhotite, formed during aqueous alteration on its
parent body. The timing of aqueous alteration is not yet well constrained, with estimates ranging
between 1.8 and 6.8 Myr after CAI formation (Yokoyama et al., 2022; McCain et al., 2023). Given
these uncertainties, aqueous alteration on Ryugu’s parent body may have occurred during thelifetime of the solar nebula magnetic field (i.e., up to ~5 Myr after CAI formation; Weiss et al.,2017). Magnetite and pyrrhotite may therefore carry a chemical remanent magnetization reflecting
the intensity of this field.
Rock magnetic and paleomagnetic data of Ryugu samples were recently published (Nakamura etal. 2022; Sato et al., 2022). The authors measured the NRM of two samples (0.425 mg and 1.556mg) and demagnetized the NRM using alternating fields (AF). They identified stable remanencecomponents in both samples up to 24 mT and 32 mT, respectively, which they interpreted as
evidence that Ryugu material experienced a 41- to 390-µT solar nebula field.
We conducted a new paleomagnetic investigation of two other Ryugu samples with mass 0.8 mg
and 20.8 mg. Data are very consistent among our two samples, but their mass-normalized NRM
intensities are 20 to 100 times weaker than the published ones. NRM demagnetization data exhibit
a poorly defined low-coercivity component up to ~8 mT, but become completely erratic at higher
AF field steps. A similar demagnetization behavior is found after applying anhysteretic remanent
magnetizations (ARM) acquired in bias fields up to 10 µT. For ARM bias fields of 20 µT andgreater, we can identify a component of the magnetization in the coercivity range 10-50 mT.
Although further analyses are being conducted, this favors the idea that the samples experienced
magnetic fields < 17 µT in intensity, assuming the sample acquired a chemical remanent
magnetization. This upper limit is compatible with both (i) the nebula field intensity estimatedfrom other meteorites with similar age of the NRM, and (ii) the absence of field.
We suspect that the samples used by Nakamura, Sato and colleagues may have been contaminatedby exposure to strong magnetic fields up to ~20 mT. We will discuss how such a contamination
can be identified using measurements that are non-destructive for the NRM. This highlights theneed for extreme caution to avoid magnetic contamination during handling of returned
extraterrestrial material. Our data show, however, that the samples were not remagnetized en masse
during sampling on the asteroid, in transit in the spacecraft or during curation preceding anymeasurements.
We are very grateful to JAXA for the allocation of the Ryugu samples A0154 and C0005.

References
Yokoyama, T., et al. (2023) Science 379, eabn7850; McCain, K. A., et al. (2023) Nat. Astro. 7, 309-317;
Weiss, B. P., et al. (2017) Earth Planet. Sci. Lett. 468, 119-132; Nakamura, T., et al. (2022) Science 379,eabn8671; Sato M., et al. (2022) J. Geophys. Res. Planets 127, e2022JE007405