Cosmogenic Radionuclide Records of Hayabusa Aggregate and Particle Samples
Kunihiko Nishiizumi1, Marc W. Caffee2, Keisuke Nagao3, Kees C. Welten1, Jozef Masarik4
Space Sciences Laboratory, Univ. of California, Berkeley, CA 94720-7450, USA., 2Dept. of Physics & Astronomy, Purdue
Univ., West Lafayette, IN 47907-2036, USA., 3Div. of Polar Earth-System Sci., KOPRI, Incheon 21990, Korea, 4Faculty ofMath. & Physics, Comenius Univ., SK-842 15, Bratislava, Slovakia

1Hayabusa2 arrived at the C-type asteroid 162173 Ryugu in June 2018, and successfully collected surface samples from twosampling sites, returning ~5.4 g of material to Earth on December 6, 2020. The surface samples stored in Chamber A werecollected by the 1st touchdown (TD) on Ryugu’s surface on February 21, 2019. A crater (diameter of ~14 m) on Ryugu’s
surface was made using a collision device - denoted “Small Carry-on Impactor (SCI)” - on April 5, 2019 [1]. The samples inChamber C were collected proximal to the north side of this artificial crater by the 2nd TD on July 11, 2019 and possibly containejecta from the crater [2]. Distance between TD1 and TD2 is ~820 m.
Our studies are based on the measurement of those nuclides produced in asteroidal surface materials by cosmic rays - bothsolar (SCR) and galactic cosmic rays (GCR). Cosmic-ray-produced (cosmogenic) nuclides are used to determine the durationand nature of the exposure of materials to energetic particles. Our goals are to understand both the fundamental processes on
the asteroidal surface and the evolutionary history of its surface materials. For Hayabusa2 samples, there are several specificquestions we aim to address: (1) are the Chamber C samples, collected during the 2nd TD ejecta deposits from the artificial crater,(2) if so, what is the original depth of each recovered sample in the Ryugu regolith, and (3) what is the surface exposure time,
mixing rate, and erosion/escape rate of Ryugu’s surface? To answer these questions, we were allocated and received particles
from Chamber A and Chamber C for measurements of cosmogenic radionuclides and noble gases as a part of initial analysis ofRyugu. We have measured cosmogenic 10Be (t1/2 = 1.36 x 106 yr), 26Al (7.05 x 105 yr), 36Cl (3.01 x 105 yr), and stable noble
gases in those samples [3-5]. Based on noble gas analysis of ~2 dozen Ryugu samples, we found that cosmic ray exposure(CRE) ages of Chamber A (surface) samples, A0105, range from 2.9 to 7.5 Myr and Chamber C (sub-surface) samples, C0106,
range from 1.6 to 5.8 Myr based on 21Ne concentrations [3, 6]. We also found that four C0106 particles were exposed at depthof ~50, 110, 130, and 145 g/cm2 respectively and had different CRE ages based on cosmogenic nuclide measurements [3, 5].
To further study of the exposure condition of Ryugu surface materials, we obtained one grain, A0130, from Chamber A and3 grains, C0012, 0162, and 0182 from Chamber C. In addition, we obtained two aggregate samples, A0221 and C0212, inorder to determine average exposure ages and depths for both Chamber A and C samples. As shown above, each grain haddifferent exposure histories (age and depth). This is similar to what we found for exposure histories of individual grains oflunar core samples. While cosmogenic radionuclides of lunar bulk soils show smooth depth profiles, the measured radionuclide
concentrations of individual rock fragments at the same depth show deviations from the smooth depth profiles [7].
Each sample was gently crushed by a mortar and pestle made from sapphire and then divided into two fractions, one fraction
for cosmogenic radionuclides (this work) and one for noble gases measurements [8]. The samples were individually transferred
to a small Al weighing boat and the masses were determined using an ultra-micro balance. For cosmogenic radionuclide
analysis, the sample was transferred to a Teflon bomb and dissolved with HF and HNO3 mixture in the presence of Be and Clcarriers. After Cl was separated as AgCl for 36Cl measurements, a small analysis aliquot was taken for chemical analysis byICP-OES. Beryllium, Al, and Mn were separated by ion chromatography, using anion and cation ion exchange columns, andBe and Al were purified for AMS measurements. To serve as a baseline comparison, a few grains of the Orgueil CI chondrite
were analyzed using the same protocols. Beryllium-10 and 26Al AMS analyses were performed at PRIME Lab, Purdue
University [9]. Measured isotopic ratios were normalized by AMS standards [10, 11] and converted to radionuclide
concentrations (dpm/kg).
Chemical compositions in Ryugu samples measured by ICP-OES are shown in Table 1. Since sample sizes are slightly
larger than previous Ryugu initial analysis, data show less scatter. Bulk chemical compositions of samples from TD1 and TD2
determined using ICP-MS [12] are also shown in the table. Our analysis of A0221 and C0212 are in good agreement withanalysis of TD1 and TD2 samples determined by [12]. Noticeably, both our two samples and published analysis of TD1 andTD2 samples [12] were obtained from aggregate samples and not individual grains. Although [12] concluded that are no
systematic differences in chemical composition between the samples from TD1 and TD2 sites, our limited measurements indicatethat Ca is slightly higher and Fe is lower in TD1 samples than that of TD2.

Table 1. Chemical compositions in Ryugu samples and Orgueil CI chondrite.
Sample

Mg(%)
9.32
10.84
10.54
10.74
10.81
9.36
10.77
10.69
10.42
7.37
9.63

A0130
A0221
C0012
C0012
C0162
C0182
C0212
Ryugu (TD1)*
Ryugu (TD2)*
Orgueil CI
Orgueil CI#Al
(%)
0.94
0.90
0.91
0.91
0.90
0.84
0.98
0.86
0.90
0.64
0.60

K(ppm)
720
520
600
580
570
400
550
520
600
440
530

Ca(%)
1.45
1.69
1.09
1.18
1.08
0.95
1.18
1.63
1.37
0.60
0.76

Ti
(ppm)
520
530
460
490
570
510
510
480
470
340
-Mn(ppm)
1990
3040
2020
2340
2120
1510
2370
30002490
1380
1680

Fe(%)
18.39
19.76
21.15
20.11
20.75
18.63
20.53
19.49
20.22
14.23
17.76

Co(ppm)
540
560
600
560
560
520
590
550
590
400
540

Ni(%)
1.13
1.20
1.31
1.21
1.26
1.13
1.26
1.18
1.22
1.18
1.04

*[12], #Previous measurement (unpublishd 2003)

Cosmogenic nuclide 10Be and 26Al concentrations in Ryugu samples are shown in Table 2 along with that of Orgueil CI
chondrite. Using the MCNP Code System [13], we calculated the GCR production rates of 10Be and 26Al by spallation reactions
for a body having a 2π geometry with Ryugu’s chemical composition [12]. Based on the comparison of the observed
radionuclide concentrations with model calculated 10Be and 26Al production rate depth profiles, assuming the radionuclide
concentrations in Ryugu are saturated, we obtained the first order estimation of exposure depth of each sample and shown inTable 2. Based on those depths we calculated 21Ne production rates and calculated 21Ne CRE ages in each sample [8]. The
calculated exposure depths and CRE ages of each grain obtained by TD2 scatter, in similar fashion as previous works [3, 5].
The preliminary estimation of average exposure depth of aggregate sample A0221 is somewhat deeper than those of each grain
obtained from TD1. The depth range of aggregate sample C0212 collected by TD2 is estimated to be 110 - 140 g/cm2 basedon 10Be concentration. The 26Al concentration of C0212 indicates much shallow depth of 10 – 90 g/cm2. One possible
explanation is that C0212 contains a small amount of surface material that contains high SCR produced 26Al. The measurementof thermal neutron-capture produced 41Ca (t1/2 = 1.0 x 105 yr) – in progress - is critical to further constrain the exposure depth ofeach sample.
Table 2. Cosmogenic nuclide 10Be and 26Al concentrations in Ryugu samples and Orgueil CI chondrite.
1026Sample
MassBe
Al
Depth
(mg)
(dpm/kg)(dpm/kg)(g/cm2)A0130
0.676
9.94 ± 0.29
29.2 ± 1.3
10 - 70
A0221
10.718
11.23 ± 0.10
27.4 ± 0.6
70 - 100
C0012
4.200
7.50 ± 0.10
24.3 ± 0.8
90 - 140
C0012
7.274
7.43 ± 0.15
23.3 ± 0.8
110 - 150C0162
1.531
7.58 ± 0.21
24.6 ± 1.0
95 - 145
C0182
0.983
6.28 ± 0.15
25.3 ± 1.3
85 - 135
C0212
8.406
9.06 ± 0.09
29.3 ± 0.6
110 -140
Orgueil CI
6.000
14.91 ± 0.31
35.9 ± 1.2
Orgueil CI#
52.60
21.19 ± 0.36
40.4 ± 0.8#

Previous measurement (unpublished 2003)

Acknowledgments: We thank Hayabusa2 project team and Hayabusa2 curation members. Orgueil was obtained from
MNHN-Paris. This work was supported by NASA’s LARS program.
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