Paleomagnetic Evidence for Formation of Ryugu in the Distal Solar System
Elias N. Mansbach1, Eduardo A. Lima1, Michael Sowell2, Joseph L. Kirschvink2, Jodie B. Ream1, Roger R. Fu3, Saverio
Cambioni1, Xue-ning Bai4, Benjamin P. Weiss1
1

Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology; Cambridge, MA,USA.
2

Division of Geological and Planetary Sciences, California Institute of Technology; Pasadena, CA, USA
3

Department of Earth and Planetary Sciences, Harvard University; Cambridge, MA, USA

4

Institute for Advanced Study and Department of Astronomy, Tsinghua University; Beijing, China

Paleomagnetic studies of meteorites have demonstrated that the solar nebula generated a magnetic field that likely played an
important role in accretion during the early solar system [1]. The nebular field reached intensities of 54 ± 21 µT at heliocentric
distances of 1-3 AU [2] and 101 ± 48 µT at 3-7 AU [3], and likely decayed by ~3-4 million years (Ma) after the formation ofcalcium aluminum-rich inclusions (CAIs) [4-7]. However, with the possible exception of Tagish Lake and Wisconsin Range(WIS) 91600, these studies have been restricted to meteorites that formed at distances <7 AU [8, 9]. Tagish Lake is of particularinterest as its arrival to Earth from the asteroid belt implies that its parent body had been inwardly scattered, perhaps due to theformation and migration of the giant planets [9, 10].
The returned samples from asteroid (162173) Ryugu
by the Hayabusa2 mission currently provide the best
opportunity to study the nebular field in the distal solar system
as the parent body may have experienced aqueous alteration atdistances up to ~20 AU [11]. These particles are also highquality targets for paleomagnetic investigations as they are: 1)
in pristine condition since careful sample handling limits thepossibility of magnetic and terrestrial contamination, which
can lead to the creation of ferromagnetic minerals that mask
the primary record of the particles, and 2) contain single-vortex
framboidal magnetite which can retain magnetic records over
the age of the solar system [12]. If the magnetite shows
evidence of magnetization produced by an ancient field, thesource of the field could have been the nebular field or a
dynamo on the parent body. The latter case is of particularinterest since this would imply that the parent body was
Figure 1: Orthographic projection of endpoints of NRM vectors on
partially differentiated with a carbonaceous chondritic crust
the northeast (N-E) and up-east (Z-E) planes during alternating fielddemagnetization of particle A0397. The LC component is shown in[13]. Our current understanding of the lifetime of the nebula
red, the MC component in blue, and the HC range in black. Nosuggests that if the natural remanent magnetization (NRM)
components are observed >24 mT.
was acquired < 4 Ma, the source of the field was the nebula
while a younger age would be indicative of a parent body dynamo. Initial Mn-Cr dating on carbonates that are thought to haveformed at the same time as the magnetite yielded an age of 3.1 – 6.8 Ma after the formation of CAIs [14], but more recent datingon carbonates using the same dating system and improved calibrations suggests alteration occurred < 1.8 Ma after CAI formation[15].
An initial paleomagnetic study of two Ryugu particles concluded that the particles formed in a > ~40 - 400 µT nebular
field [14, 16]. However, this magnetization may be post-sampling contamination because: 1) magnetite can have coercivities up
to 300 mT, but the NRMs of previously studied samples were unblocked to only 30 mT; 2) the previously studied samples had
been analyzed with an electron microprobe which may have imparted a weak isothermal remanent magnetization (IRM); and 3)
initial analysis of MasMag magnetometer data suggested that a boulder on Ryugu did not possess a measurable magnetization(< 3 × 10-6Am2 kg-1 at the m-scale) [17], yet the moment per unit mass of the reported samples is well above this limit (2.14 ×10-5 Am2 kg-1).

To address the origin and nature of NRM in Ryugu, weconducted an additional paleomagnetic analysis on threepreviously unstudied Ryugu particles: A0397, C0085b, andC0006. These samples have not been previously studied using a
microprobe or instrument with a strong magnet prior to ourpaleomagnetic analysis.
Alternating field (AF) demagnetization of the NRM inA0397 to 400 mT in steps of 0.5-1 mT revealed two non-origin
trending components (Fig. 1), a low coercivity (LC) component
that unblocked between 0 and 10.5 mT and a medium coercivity(MCs) component that unblocked between 11 and 23.5 mT.
Above >24 mT is a high coercivity (HC) range in which noFigure 2: Predicted midplane nebular field assuming accretion
NRM components are observed. AF demagnetization of C0085b
around a solar mass star and that the nebular field and disk rotation
to 400 mT revealed three NRM components: an LC component
are aligned. Accretion rates are given at the right-hand side foreach curve. Colored lines represent constraints on heliocentricthat unblocked between 0 and 12.5 mT, and two MC
distance at which aqueous alteration occurred based on uppercomponents that unblocked between 13 and 19 mT and 19.5 andpaleointensity limits from particles C0085b and A0397.
23.5 mT, respectively. The second MC component was origintrending. As before, above >24 mT is a HC range containing no stable NRM. AF demagnetization of C0006 to 80 mT conductedin 5-10 mT steps revealed a non-origin trending LC component that unblocked between 0 and 20 mT with no apparent
components in the HC range above 20 mT.
Paleointensities estimated via the anhysteretic remanent magnetization (100 µT bias field and 260 mT AC field) method
for the LC components are 125.8 ± 28.9 (95% confidence interval, as for all below), 694.6 ± 145.5 and 44.8 ± 69.5 µT for A0397,
C0085b, and C006 respectively. The MC paleointensity for A0397 is 45.6 ± 31.6 µT, while the two MC paleointensities for
C0085b are 36.0 ± 44.4 and 62.3 ± 134.7. Importantly, the HC paleointensities are all also consistent with null paleofields: 26.4
± 30.6 µT for C0085b, 7.4 ± 9.2 µT for A0397, and 8.8 ± 16.4 µT for C0006. This indicates that there are magnetic recorders
with coercivities >24 mT in the particles that are capable of recording an ancient field but instead formed in the presence of anull or weak field.
Experiments conducted to determine the susceptibility of A0397 to viscous (VRM) remagnetization from prolonged
exposure to an Earth-strength field indicate that the total moment gained over 4.5 years (initial sampling to NRMdemagnetization) could account for 98% of the combined LC and MC components. We therefore suggest that the source of theLC and MC components in our samples are overprints, most likely a VRM from sitting in Earth’s field and the field producedby spacecraft engines (10s of µT [16]) since sampling. The non-origin trending nature of the two A0397 components further
indicate that they are not primary. We suggest that the previously reported strong paleointensities are due to a weak IRM fromthe electron microprobe as ratio of NRM to saturating IRM is >10% and the components are not origin trending as well.
Our results have profound implications for the history of the Ryugu parent body. Taking the alteration age to be < 1.8
Ma after CAI-formation, our most stringent upper paleointensity limit of 7.4 µT indicates that alteration occurred at heliocentric
distances >15 AU for typical accretion rates during the main lifetime of the nebula of 10-8 M☉ yr-1 (and >5 AU for the lower
range of accretion rates during the main lifetime of the nebula) (Fig. 2). Given Ryugu’s current location (~1 AU), the parent
body may have been scattered inward prior to catastrophic disruption, but after experiencing aqueous alteration, due to influence
from the giant planets in a similar manner to that proposed for the parent body of Tagish Lake. Our results therefore provide
further evidence that the formation and migration of the giant planets led to a major reconstructing of the organization of thesolar system.
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