Exposure Conditions of Samples Collected on Ryugu’s Two Touchdown Sites Determined byCosmogenic Nuclides
Kunihiko Nishiizumi1, Marc W. Caffee2, Keisuke Nagao3, Ryuji Okazaki4, Hisayoshi Yurimoto5, Tomoki Nakamura6, Takaaki
Noguchi7, Hiroshi Naraoka4, Hikaru Yabuta8, Kanako Sakamoto9, Shogo Tachibana9, 10, Sei-ichiro Watanabe9, 11, Yuichi
Tsuda9, and Hayabusa2 Initial Analysis Volatile Team1
Space Sci. Lab., Univ. of California, Berkeley, CA 94720-7450, USA., 2Dept. of Phys. & Astro., Purdue Univ., West Lafayette,IN 47907-2036, USA., 3KOPRI, Incheon 21990, Korea, 4Kyushu Univ., Fukuoka 819-0395, Japan, 5Hokkaido Univ.,
6TohokuUniv., 7Kyoto Univ., 8Hiroshima Univ., 9ISAS/JAXA, 10Univ. of Tokyo, 11Nagoya Univ.
Hayabusa2 arrived at the C-type asteroid 162173 Ryugu in Jun. 2018, and successfully collected surface samples from two
sampling sites, returning ~5.4 g of samples to Earth on Dec. 6, 2020. Surface samples stored in Chamber A were collected bythe 1st touchdown (TD) on Ryugu’s surface on Feb. 21, 2019. A crater (diameter of ~14 m) on Ryugu’s surface was madeusing a collision device - denoted “Small Carry-on Impactor (SCI)” - on Apr. 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 Jul. 11, 2019 [2].
Our studies are based on the measurement of those nuclides produced in asteroidal surface materials by cosmic rays - bothsolar and galactic cosmic rays. Cosmic-ray-produced (cosmogenic) nuclides are used to determine the duration and nature ofthe 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. They are also key to understanding the history of Ryugu’s surface
and asteroid-meteoroid evolutionary dynamics. For Hayabusa2 samples, there are several specific questions we aim to address:(1) are the Chamber C samples, collected during the 2nd touchdown ejecta deposits from the artificial crater, (2) if so, what is theoriginal depth of each recovered sample in the Ryugu regolith, and (3) what is the surface exposure time, mixing rate, anderosion/escape rate of Ryugu’s surface? To answer these questions, we were allocated and received 2 particles from Chamber
A (A0105-19 and -20) and 3 particles from Chamber C (C0106-09, -10, and -11) for measurements of cosmogenic radionuclides
and noble gases. Each sample is several hundred µm in size.
We transferred the individual grains to acid cleaned sapphire containers, with ~2 mm diameter hole, using a vacuum tweezer
at the JAXA curation facility. The five samples were hand carried to the Space Sciences Laboratory (SSL), University ofCalifornia, Berkeley. Each sample was gently crushed by a mortar and pestle made from sapphire and then divided into twofractions, one fraction for cosmogenic radionuclides and one for noble gases. The samples were individually transferred to asmall 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 from the Al boat and dissolved with a few drops of HF-HNO3 mixture in thepresence of clean Be, Al, Cl, and Mn carriers. After Cl was separated as AgCl, a small analysis aliquot was taken for chemicalanalysis by ICP-OES (Table 1). Beryllium and Al were separated by ion chromatography, using 1 mL anion and cation ion
exchange columns, and purified for accelerator mass spectrometry (AMS) measurements. To serve as a baseline comparison,
three grains of the Nogoya CM2 chondrite were analyzed using the same protocols.
Beryllium-10 (t1/2 = 1.36 x 106 yr) AMS analysis was performed at PRIME lab, Purdue University [3] and result was shownin Table 1. Analyses of 26Al (7.05 x 105 yr) and 36Cl (3.01 x 105 yr) as well as noble gases will be done in the near future.
Table 1. Chemical compositions and cosmogenic nuclide 10Be concentration in Hayabusa2 samples and Nogoya CM2 chondrite.
SampleHayabusa2 A0105-19
Hayabusa2 A0105-20
Hayabusa2 C0106-09
Hayabusa2 C0106-10
Hayabusa2 C0106-11
Nogoya CM2
Nogoya CM2
Nogoya CM2

Mass(mg)0.24290.20610.12280.15430.18980.45940.34370.2049Mg(%)
8.18
8.416.05
7.97
8.45
9.96
9.61
11.27

Ti
(ppm)
470
720
690
490
450
480
480
410

Mn(ppm)(2010)(2090)
1800
1680
1710

Fe(%)
17.6
18.0
19.5
21.0
21.5
20.0
19.9
19.8

Co(ppm)
590
460
580
440
430
580
540
380

Ni
(%)
1.00
1.03
1.36
1.43
1.29
1.18
1.17
1.20

10

Be(dpm/kg)
12.76 ± 0.37
12.75 ± 0.29
7.10 ± 0.30
7.48 ± 0.26
7.21 ± 0.43
2.09 ± 0.13
2.12 ± 0.09
2.00 ± 0.13

Mn concentrations in parentheses are large uncertainty.

Although the chemical compositions of all 5 Hayabusa2 particles are very similar, the concentrations of Fe and Ni of the 3Chamber C samples are ~10 % higher than that of Chamber A samples. Our measurements of the 5 Hayabusa2 samples indicatethat the chemical composition of Ryugu is closer to those of CI or CM carbonaceous chondrites, rather than CV or CO.

To validate our procedures we measured the 10Be concentrations in 3 individual grains from Nogoya. The concentrations
from the Nogoya grains are nearly identical and in good agreement with our previous measurement of 2.22 ± 0.10 dpm/kg. Themeasurements reported here used ~3 orders of magnitude less sample mass than the measurements made years ago, haveessentially the same uncertainties. For Ryugu, we were able to obtain high quality 10Be measurements using ~100 µg sample.
The surface of Ryugu is bombarded by cosmic ray in a 2π exposure geometry, similar to surface of the Moon. On the Moon,
the production profile of cosmogenic 10Be from the surface to a depth of over 400 g/cm2 is well established by measurements of10
Be concentrations in the Apollo 15 drill core and 15008/7 core, shown in Fig. 1 [4, 5]. The 10Be concentrations in A0105-19
and -20 are equivalent to the saturation value measured at the surface of the Moon. The simplest scenario is that both A010519 and -20 were collected from a depth of 0-30 g/cm2 on Ryugu. Both samples have been continuously exposed to cosmic ray
at a similar depth for more than several Myr. A more precise depth can be obtained by measurement of 26Al, which has adifferent production profile than 10Be, and is also produced by solar cosmic ray at shallow depth (≤ several g/cm2). Since the10Be activity is saturated, the total exposure age must be obtained by measurements of longer half-life (3.7 x 106 yr) 53Mn andcosmogenic noble gases. Beryllium-10 concentrations in three Chamber C samples are lower than that of the Chamber A
samples but nearly the same as each other. The sampling location of the 2nd TD is close to the ”ejecta ray 3” described by [1].
Assuming that the Chamber C samples were shielded during their exposure to cosmic rays and then were incorporated into theejecta deposits from the artificial crater by SCI impact, we can calculate the depth at which they were exposed. All threeChamber C samples were ejected from a depth of 150 – 180 g/cm2 from Ryugu (see Fig. 1). This depth corresponds to 1.3 –1.5 m, assuming the regolith density is the same as Ryugu’s bulk density of 1.2 g/cm3 [6]. Since the depth of the crater floor
from the initial surfaces was determined 1.7 m by a digital elevation map (DEM) [1], three Chamber C samples were ejectedfrom near bottom of the crater. Alternatively, if we assume these were not ejecta, but were resident on the surface, the lower
10Be activities would indicate that these grains have shorter exposure ages, ~ 1 Myr. Based on their location near the artificial
crater ejecta ray, we consider this scenario unlikely. Aluminum-26 and 36Cl measurements of these samples will further
constrain ejection depth and condition.
For our work, we obtained 5 similar sized, ~0.5 – 1 mm, grains. We don’t know whether the sizes we observe now are
representative of the grain’s original sizes on Ryugu’s surface; it is possible that they were broken during the collection process
or during sample handling. We are certain though that independent of their original size on Ryugu, the three Chamber C grains
were exposed to cosmic rays at the same depth. These grains had ample 10Be for measurement, 10Be measurements are possible
in smaller grain sizes. A logical next step would be the measurement of 10Be in grains of different sizes to investigate whether
any possible association of grain size and ejection depth.
Based on saturated 10Be in surface samples and from ejecta samples we conclude that the surface of Ryugu has been exposed
to cosmic rays for more than several Myr and that the upper 1 - 2 m of regolith have been relatively undisturbed for more thanseveral Myr. Our observation shows that the surface of Ryugu is more stable than the surface of Itokawa and the estimation bygeomorphological indicators [7]. Additional cosmogenic nuclide measurements, especially stable noble gases, will allow adetailed understanding of the surface evolution of Ryugu.
15

Figure 1.
10Be1010Be (dpm/kg)

1098BeThe depth profile of 10Be production on the Moon.

Black diamonds indicate 10Be concentration vs. depth on the Moon
measured in undisturbed Apollo 15 drill core and 15008/7 core [4,A0105

5]. Observed 10Be concentrations in Hayabusa2 samples are plot
at estimate depths on Ryugu and marked red circle (A0105) andC0106

square (C0106).

7References: [1] Arakawa M. et al. 2020. Science 368, 67-71. [2]6Tsuda Y. et al. 2020. Acta Astronautica 171, 42-54. [3] Sharma P.

5et al. 2000. NIM B172, 112-123. [4] Nishiizumi K. et al. 1984. EPSL70, 157-163. [5] Nishiizumi K. and Caffee M. W. 2019. MAPS 54,46087pdf. [6] Watanabe S. et al. 2019. Science 364, 268-272. [7]30Sugita S. et al. 2019. Science 364, eaaw0422.

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1502002Depth (g/cm )250300Acknowledgments: We thank Hayabusa2 project and initial analysis team, especially Hayabusa2 curation members. Z. Nett and K. C. Welten
helped a part of laboratory works. Nogoya was obtained from Field Muse. of Nat. His. This work was supported by NASA’s LARS program.