Noble Gases of the 1st and 2nd AO Ryugu Samples Collected by the Hayabusa2 Spacecraft
Keisuke Nagao1*, Kunihiko Nishiizumi2, Marc W. Caffee3, Hwayoung Kim1, Changkun Park1, Jong Ik Lee1, Jozef Masarik4
KOPRI, Incheon 21990, Korea, 2Space Sciences Laboratory, Univ. of California, Berkeley, CA 94720-7450, USA.,3Dept. ofPhysics & Astronomy, Purdue Univ., West Lafayette, IN 47907-2036, USA, 4Comenius Univ., Bratislava, Slovakia, *present
address: Seki 2082, Maniwa-shi, Okayama 719-3156, Japan. knagao520@gmail.com
1Hayabusa2 arrived at the C-type asteroid 162173 Ryugu in June 2018, and successfully collected surface samples fromtwo sampling sites, returning ~5.4 g of samples to Earth on December 6, 2020. Surface samples stored in Chamber A werecollected during 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]. Samples in Chamber
C were collected proximal to this artificial crater and are possibly ejecta from the north side of the crater by the 2nd TD on July11, 2019 [2].
Our studies on the Hayabusa2 samples are based on the measurement of those nuclides produced in asteroidal surface
materials by both solar (SCR) and galactic cosmic rays (GCR). Cosmic-ray-produced (cosmogenic) nuclides are used todetermine the duration and nature of the exposure of materials to energetic particles [3]. Our goals are to understand both thefundamental processes on the asteroidal surface and the evolutionary history of its surface materials. These processes occur overtimescales spanning the present to 109 yrs into the past, and are important not only for understanding the history of Ryugu’s
surface but also for studies of asteroid-meteoroid evolutionary dynamics. 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 grain in the Ryugu regolith, and (3) what is the averagesurface exposure time, mixing rate of regolith materials, and erosion/escape rate of Ryugu’s surface? We investigate and utilize
cosmogenic radionuclides (10Be, 26Al, 36Cl, and 41Ca) and noble gases, especially cosmogenic 21Ne in Ryugu samples. Expected
maximum cosmogenic nuclide concentrations in millimeter-sized (~1 mg in mass) Ryugu sample are a few times 105 - 107 atoms
or lower. These values are higher than the present detection limits of accelerator mass spectrometry (AMS) and noble gas mass
spectrometry, but 2 orders of magnitude smaller than normal studies of cosmogenic nuclide measurements in meteoritic materials.
We present here the noble gas isotopic compositions of 6 samples (Table 1) allocated as the 1st and 2nd AO. Noble gases
were measured with the system of modified-VG5400 mass spectrometer at Korea Polar Research Institute (KOPRI). The noble
gas system had been used to measure the Itokawa samples at the University of Tokyo [4], and then moved to KOPRI afterward
in 2015. Noble gases were extracted from each sample using a miniature furnace specially designed for small samples, using aTa-tube with a Mo-crucible. A sample dropped into the crucible is heated from outside of the Ta-tube by a W-heater. Electriccurrent applied to the W-heater is controlled by a computer program monitoring the heating temperature with a W-Re
thermocouple. Samples were installed in a glass-made sample holder, which was set on the furnace and preheated for a day at
about 120°C to remove atmospheric noble gas contamination adsorbed on the samples. Stepwise heating was applied to extract
noble gases, i.e., 200, 900, and 1700°C for small samples (A0130, C0162, and C0182), and 200, 600, 900, 1200, and 1700°C forlarger samples (A0221, C0012, and C0212). The extracted noble gases from each sample were purified and separated into 5
fractions, i.e., He, Ne, Ar, Kr, and Xe, then measured isotope abundances and isotopic ratios for each fraction.
Total values of concentrations of 4He, 20Ne, 36Ar, 84Kr, and 132Xe, and isotope ratios of He, Ne, and Ar obtained by thestepwise heating method are presented in Table 1. Data of Orgueil CI chondrite measured in this work are also presented forcomparison. Solar He and Ne with very high concentrations of 4He (14.9 and 9.3×10-4 cm3STP/g) and 20Ne (2.0 and 1.5×10-3cm3STP/g) were observed for the aggregate samples A0221 and C0212, respectively. These samples showed 20Ne/22Ne ratios of
12.8–13.7 in the temperature range of 200–900°C. The ratios are close to the value of 13.78 for solar wind (SW) [5]. Releasepeaks of 4He for these samples were at the heating temperature of 600°C, for which 3He/4He was 3.9×10-4, slightly lower thanthe SW value of 4.64×10-4 [5]. Smaller amounts of SW He and Ne are also indicated in A0130. Neon isotopic compositions forother samples are a mixture of several primitive components, e.g., P1 (Ne-Q), P3 (presolar diamond), and Ne-E(H) (SiC) [6].
Xe isotopic ratios, e.g., 130Xe/132Xe, 129Xe/132Xe, and 136Xe/132Xe, indicate that trapped Xe is mostly Q-Xe component, with asmall or negligible contribution of cosmogenic and HL or fissiogenic Xe.
Concentrations of cosmogenic 3He were difficult to calculate, because only a small excess of cosmogenic 3He wasobserved at the low heating temperatures at 200–900℃ for some samples (3He/4He = 5.2, 7.3, 4.9, and 4.8×10 -4 for A0221,C0012, C0162, and C0212, respectively), and most 3He/4He ratios were between 1.23×10-4 for trapped Q-He [6] and 4.64×10-4
for SW [5]. Neon isotopic compositions are a mixture of several primitive components, e.g., SW [5], P1 (Ne-Q), P3 (presolardiamond), and Ne-E(H) (SiC) [6], to which a small contribution of cosmogenic 21Ne was detected. Calculated concentrations ofcosmogenic 21Ne in excess of the assumed trapped 21Ne/22Ne = 0.029 are presented in Table 1. The concentrations of cosmogenic
21
Ne are (4.06–11.9)×10-9 cm3STP/g. The concentrations of 11.9 and 9.83×10-9 cm3STP/g for aggregate samples A0221 andC0212, respectively, are higher than those observed for the initial analysis samples, (1.6–7.8)×10-9 cm3STP/g [7, 8]. Exposure
depths were estimated from preliminary measurements of 10Be and/or 26Al in same samples. Then, each production rate P21 wasobtained by new calculation using the MCNP Code System. Calculated cosmic ray exposure (CRE) ages are shown in Table 1.
CRE ages calculated for the samples from Chamber-A are roughly 4 and 11 Myr for A0130 and A0221, respectively. CRE agesfor 3 samples from Camber-C, C0012, C0162, and C0182, are in the range of 3–5 Myr, while C0212 has longer CRE age of 89 Myr. The CRE ages of 3–5 Myr for the samples from Chamber-A and -C are similar to the reported ages for initial analysis
samples, 3–5 Myr [7] and 5 Myr [8]. Contrary to these grain samples, two aggregate samples A0221 and C0212 with longerCRE ages are enriched in SW-Ne. Samples measured in the present work must have been irradiated heterogeneously by bothGCR and SW on the Ryugu’s surface before sampling by the Hayabusa2.
Table 1. Concentrations and isotopic ratios of noble gases 1) in 6 Ryugu samples and Orgueil CI chondrite.
4203684
321
StepwiseSample
MassHeNeAr
Kr 132Xe
He/4He 20Ne/22Ne 21Ne/22Ne 38Ar/36Ar 40Ar/36Ar
NecosmP 213)
mgheating2)
10-9cm3STP/g
10-4
10-9 cm3 STP/g 10-9 cm3 STP/g/MyrA0130
0.2086
3 steps
2190001230160015.9
19.3
3.84
10.54
0.06540.1858
1.79
4.41
1.10-1.20
A0221
1.66105 steps 1490000
20400
3430
16.9
18.5
3.54
12.85
0.03620.1842
0.485
11.9
1.20
C0012
2.21795 steps
97700
472
1530
15.9
19.6
3.59
8.27
0.09780.1861
0.419
4.06
1.12–1.20
C0162
0.1886
3 steps
118000
706
1430
15.3
17.7
4.46
8.68
0.09840.1867
1.34
5.83
1.13–1.17
C0182
0.27683 steps
93800
432
1370
20.5
17.0
3.80
8.09
0.12800.1876
1.18
5.45
1.15–1.20
C0212
0.82325 steps
932000
15200
2720
16.1
18.7
3.89
12.95
0.03710.18490.427
9.83
1.12–1.17
Orgueil(CI) 1.7205
3 steps672000
1130
1260
13.0
15.7
3.51
10.08
0.10990.18562.37
9.37
2.224)
1)
Isotopic ratios of Kr and Xe are not presented.
2)
Heating temperatures are 200, 900, and 1700°C for 3 steps, and 200, 600, 900, 1200, and 1700°C for 5 steps.
3)
Each depth was estimated based on prelininary measurements of cosmogenic radionuclides. The production rates and CRE ages were not included ±5 % uncertainty of our newcalculations and experimental errors.
4)

Prodction rate in 4π-geometry for CI chondrites [9].

Acknowledgment: We thank Hayabusa2 project team, especially the curation members. We also thank HMNH-Paris forproviding Orgueil sample.
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[4] Nagao K. et al. 2011. Science 26, 1128–1131.
[5] Heber V. S. et al. 2009 Geochim. Cosmochim. Acta 73, 7414–7432.
[6] Ott U. (2002) Noble gases in Geochem. and Cosmochem., Rev. in Mineral. Geochem. 47, 71–100.
[7] Nagao K. et al. 2022. Meteoritical Soc. Meeting, Abstract #6134.
[8] Okazaki R. et al. 2023. Science 379, 788.
[9] Eugster O. 1988. Earth Planet Sci. Lett. 52, 1649–1662.

T21 3)
Myr
3.7–4.0
9.9
3.4-3.6
5.0–5.2
4.5–4.7
8.4–8.8
4.22