Nucleosynthetic s-Process Depletion in Mo from Ryugu samples returned by Hayabusa2
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Keywords: Hayabusa2, Ryugu, Mo isotopes, presolar grains
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Abstract
Figures and Tables
 Figure 1 Molybdenum isotopic compositions of Ryugu and Allende analysed in this study (solid lines). The dotted lines represent comparison bulk meteorite data for Allende (green) and Orgueil (blue) from previous studies (Table S-1). Note that 100Mo was not measured in this study. |  Figure 2 Molybdenum isotope diagrams of bulk meteorites and the Ryugu sample. (a) The solid lines are the regression lines of CC (blue, from Budde et al., 2019) and NC (red, from Spitzer et al., 2020) meteorites. (b), (c), (d) Reference data of bulk carbonaceous chondrites as listed in Table S-1. Black lines are extensions of the mixing lines between a representative terrestrial sample (origin) and the theoretical s-process Mo isotopic composition calculated by Stephan et al. (2019). |  Table 1 Molybdenum isotopic compositions for single analyses of bulk samples of Ryugu and the CV3 carbonaceous chondrite Allende.Mo isotope ratios are normalized to 98Mo/96Mo =1.453173. Uncertainties reported for measured ɛiMo values represent the external reproducibility (2 s.d.) obtained from repeated analyses of Highland Valley molybdenite (Table S-4). |
| Figure 1 | Figure 2 | Table 1 |
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Introduction
The Hayabusa2 mission by JAXA performed two sampling sequences on the asteroid (162173) Ryugu, and returned 5.4 g of the asteroid materials back to Earth (Tachibana et al., 2022
Tachibana, S., Sawada, H., Okazaki, R., Takano, Y., Sakamoto, K., et al. (2022) Pebbles and sand on asteroid (162173) Ryugu: In situ observation and particles returned to Earth. Science 375, 1011–1016. https://doi.org/10.1126/science.abj8624
). Spectral observations led to the classification of the asteroid Ryugu as Cb-type, which has long been assumed to be related to carbonaceous chondrite meteorites (Bus and Binzel, 2002Bus, S.J., Binzel, R.P. (2002) Phase II of the small main-belt asteroid spectroscopic survey. A feature-based taxonomy. Icarus 158, 146–177. https://doi.org/10.1006/icar.2002.6856
). This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022Hopp, T., Dauphas, N., Abe, Y., Aléon, J., Alexander, C.M.O.’D., et al. (2022) Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Science Advances 8, 1–10. https://doi.org/10.1126/sciadv.add8141
; Moynier et al., 2022Moynier, F., Dai, W., Yokoyama, T., Hu, Y., Paquet, M., et al. (2022) The Solar System calcium isotopic composition inferred from Ryugu samples. Geochemical Perspectives Letters 24, 1–6. https://doi.org/10.7185/geochemlet.2238
; Nakamura et al., 2022Nakamura, E., Kobayashi, K., Tanaka, R., Kunihiro, T., Kitagawa, H., et al. (2022) On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective. Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 98, 227–282. https://doi.org/10.2183/pjab.98.015
, 2023Nakamura, T., Matsumoto, M., Amano, K., Enokido, Y., Zolensky, M.E., et al. (2023) Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science 379, eabn8671. https://doi.org/10.1126/science.abn8671
; Paquet et al., 2023Paquet, M., Moynier, F., Yokoyama, T., Dai, W., Hu, Y., et al. (2023) Contribution of Ryugu-like material to Earth’s volatile inventory by Cu and Zn isotopic analysis. Nature Astronomy 7, 182–189. https://doi.org/10.1038/s41550-022-01846-1
; Yokoyama et al., 2023aYokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
). Chemical and physical properties of the returned samples suggest that the parent asteroid of Ryugu was accreted in the outer Solar System where water and CO2 were present as ices, followed by melting of the accreted ice due to the radioactive decay of 26Al that heated the parent asteroid to ∼40 °C (Nakamura et al., 2023Nakamura, T., Matsumoto, M., Amano, K., Enokido, Y., Zolensky, M.E., et al. (2023) Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science 379, eabn8671. https://doi.org/10.1126/science.abn8671
). The melting of the ice led to the hydration of the inner portion of the parent body, causing pervasive aqueous alteration that resulted in the precipitation of secondary minerals, including phyllosilicates, carbonates, oxides, sulfides, and phosphates (Nakamura et al., 2022Nakamura, E., Kobayashi, K., Tanaka, R., Kunihiro, T., Kitagawa, H., et al. (2022) On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective. Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 98, 227–282. https://doi.org/10.2183/pjab.98.015
, 2023Nakamura, T., Matsumoto, M., Amano, K., Enokido, Y., Zolensky, M.E., et al. (2023) Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science 379, eabn8671. https://doi.org/10.1126/science.abn8671
; Yokoyama et al., 2023aYokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
).Nucleosynthetic isotopic anomalies of meteorites provide robust information that can be used to constrain the origin of materials that contributed to the accretion of their parent bodies (Dauphas and Schauble, 2016
Dauphas, N., Schauble, E.A. (2016) Mass Fractionation Laws, Mass-Independent Effects, and Isotopic Anomalies. Annual Review of Earth and Planetary Sciences 44, 709–783. https://doi.org/10.1146/annurev-earth-060115-012157
; Yokoyama and Walker, 2016Yokoyama, T., Walker, R.J. (2016) Nucleosynthetic Isotope Variations of Siderophile and Chalcophile Elements in the Solar System. Reviews in Mineralogy & Geochemistry 81, 107–160. https://doi.org/10.2138/rmg.2016.81.03
). Isotopic compositions of Cr, Ti, Fe, and Zn have been reported for Ryugu samples, all of which are generally consistent with those of CI chondrites (Hopp et al., 2022Hopp, T., Dauphas, N., Abe, Y., Aléon, J., Alexander, C.M.O.’D., et al. (2022) Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Science Advances 8, 1–10. https://doi.org/10.1126/sciadv.add8141
; Paquet et al., 2023Paquet, M., Moynier, F., Yokoyama, T., Dai, W., Hu, Y., et al. (2023) Contribution of Ryugu-like material to Earth’s volatile inventory by Cu and Zn isotopic analysis. Nature Astronomy 7, 182–189. https://doi.org/10.1038/s41550-022-01846-1
; Yokoyama et al., 2023aYokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
). These observations suggest that the parent bodies of Ryugu and CI chondrites formed in a common source reservoir in the outer Solar System. Remarkably, both Ryugu materials and CI chondrites show variable ɛ54Cr values (ɛ54Cr = [(54Cr/52Cr)sample/(54Cr/52Cr)standard – 1] × 104; ranging from +1.22 ± 0.06 to +2.22 ± 0.13 for Ryugu and from +0.84 ± 0.14 to +1.94 ± 0.12 for CI chondrites), which exceed the range of analytical uncertainties (Kadlag et al., 2019Kadlag, Y., Becker, H., Harbott, A. (2019) Cr isotopes in physically separated components of the Allende CV3 and Murchison CM2 chondrites: Implications for isotopic heterogeneity in the solar nebula and parent body processes. Meteoritics & Planetary Science 54, 2116–2131. https://doi.org/10.1111/maps.13375
; Williams et al., 2020Williams, C.D., Sanborn, M.E., Defouilloy, C., Yin, Q.Z., Kita, N.T., Ebel, D.S., Yamakawa, A., Yamashita, K. (2020) Chondrules reveal large-scale outward transport of inner Solar System materials in the protoplanetary disk. Proceedings of the National Academy of Sciences of the United States of America 117, 23426–23435. https://doi.org/10.1073/pnas.2005235117
; Nakamura et al., 2022Nakamura, E., Kobayashi, K., Tanaka, R., Kunihiro, T., Kitagawa, H., et al. (2022) On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective. Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 98, 227–282. https://doi.org/10.2183/pjab.98.015
; Yokoyama et al., 2023aYokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
, bYokoyama T., Wadhwa, M., Iizuka, T., Abe Y., Aléon J., et al. (2023b) Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample. Science Advances 9, eadi7048. https://doi.org/10.1126/sciadv.adi7048
). The isotopic heterogeneity was most likely caused by physicochemical fractionation between 54Cr-rich presolar grains and Cr-bearing secondary minerals during aqueous alteration in the parent bodies (Yokoyama et al., 2023bYokoyama T., Wadhwa, M., Iizuka, T., Abe Y., Aléon J., et al. (2023b) Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample. Science Advances 9, eadi7048. https://doi.org/10.1126/sciadv.adi7048
). By contrast, the mass independent isotopic compositions of Ti, Fe, and Zn of Ryugu samples are homogeneous, and are within the range of CI meteorite compositions. Given the contrast between Cr and other isotopic tracers, it is important to assess whether there are other elements characterised by isotopic heterogeneities in Ryugu materials, and between Ryugu samples and CI chondrites. The nature of any differences may provide important further insights to the causes of the heterogeneities.Molybdenum is a moderately siderophile element with seven stable isotopes synthesised by a combination of the s-process (trace 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, and trace 100Mo), the r-process (95Mo, 97Mo, 98Mo, and 100Mo), and the processes that produce proton-rich nuclides (92Mo and 94Mo). Thus, the mass independent isotopic compositions of Mo in meteorites vary in accordance with differences in the proportions of various nucleosynthetic components present in bulk meteorite samples. High precision measurements of Mo isotopes in bulk chondrites and differentiated meteorites have revealed considerable variation among early Solar System materials (e.g., Dauphas et al., 2002
Dauphas, N., Marty, B., Reisberg, L. (2002) Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites. The Astrophysical Journal 569, L139–L142. https://doi.org/10.1086/340580
; Budde et al., 2016Budde, G., Burkhardt, C., Brennecka, G.A., Fischer-gödde, M., Kruijer, T.S., Kleine, T. (2016) Molybdenum isotopic evidence for the origin of chondrules and a distinct genetic heritage of carbonaceous and non-carbonaceous meteorites. Earth and Planetary Science Letters 454, 293–303. https://doi.org/10.1016/j.epsl.2016.09.020
; Spitzer et al., 2020Spitzer, F., Burkhardt, C., Budde, G., Kruijer, T. S., Morbidelli, A., Kleine, T. (2020) Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites. The Astrophysical Journal 898, L2. https://doi.org/10.3847/2041-8213/ab9e6a
). In particular, an internal isotopic dichotomy of ɛ94Mo-ɛ95Mo (ɛiMo =[(iMo/96Mo)sample/(iMo/96Mo)standard – 1] × 104) between non-carbonaceous meteorites (NC) and carbonaceous meteorites (CC) has been shown to be particularly sensitive for evaluating genetic differences of source materials present in bulk meteorite samples (Budde et al., 2016Budde, G., Burkhardt, C., Brennecka, G.A., Fischer-gödde, M., Kruijer, T.S., Kleine, T. (2016) Molybdenum isotopic evidence for the origin of chondrules and a distinct genetic heritage of carbonaceous and non-carbonaceous meteorites. Earth and Planetary Science Letters 454, 293–303. https://doi.org/10.1016/j.epsl.2016.09.020
; Poole et al., 2017Poole, G.M., Rehkämper, M., Coles, B.J., Goldberg, T., Smith, C.L. (2017) Nucleosynthetic molybdenum isotope anomalies in iron meteorites – new evidence for thermal processing of solar nebula material. Earth and Planetary Science Letters 473, 215–226. https://doi.org/10.1016/j.epsl.2017.05.001
; Worsham et al., 2017Worsham, E.A., Bermingham, K.R., Walker, R.J. (2017) Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex. Earth and Planetary Science Letters 467, 157–166. https://doi.org/10.1016/j.epsl.2017.02.044
). Additionally, the Mo isotope compositions of meteorites can be modified by parent body processes, including thermal metamorphism (Yokoyama et al., 2019Yokoyama, T., Nagai, Y., Fukai, R., Hirata, T. (2019) Origin and Evolution of Distinct Molybdenum Isotopic Variabilities within Carbonaceous and Noncarbonaceous Reservoirs. The Astrophysical Journal 883, 62. https://doi.org/10.3847/1538-4357/ab39e7
), and presumably aqueous alteration, as has been observed for the isotopic composition of the highly siderophile element Os (Yokoyama et al., 2011Yokoyama, T., Alexander, C.M.O.’D., Walker, R.J. (2011) Assessment of nebular versus parent body processes on presolar components present in chondrites: Evidence from osmium isotopes. Earth and Planetary Science Letters 305, 115–123. https://doi.org/10.1016/j.epsl.2011.02.046
). To further investigate the origin of the materials that accreted to form Ryugu, and chemical processes that affected the asteroidal materials after accretion, we have examined the mass independent (nucleosynthetic) Mo isotopic composition of the returned materials.top
Results
Experimental details are provided in the Supplementary Information (Experiments). We analysed a 73.5 mg composite sample of four bulk Ryugu samples (A0106, A0106-A0107, C0107, and C0108). Because of the limited quantity of Mo present in the samples, two samples from each of the two Hayabusa2 sampling locations were combined for a single measurement of the mass independent isotopic composition of Mo. In addition to the Ryugu sample, an equivalent mass of the well characterised CV3 carbonaceous chondrite Allende (Smithsonian Allende powder USNM 2359, Split 20, Position 31; ∼50 mg in total) was processed at the same time using the same methods in order to validate the analytical methods. The ɛiMo values of the Ryugu composite sample and Allende investigated in this study are provided in Table 1. The comparison to reference data is provided in the Supplementary Information.
Table 1 Molybdenum isotopic compositions for single analyses of bulk samples of Ryugu and the CV3 carbonaceous chondrite Allende.
| ɛ92Mo | ɛ94Mo | ɛ95Mo | ɛ97Mo | |
| Ryugu | 8.8 ± 0.6 | 7.2 ± 0.3 | 4.5 ± 0.3 | 2.2 ± 0.1 |
| Allende | 4.5 ± 0.6 | 3.0 ± 0.3 | 2.5 ± 0.3 | 0.6 ± 0.1 |
Mo isotope ratios are normalized to 98Mo/96Mo =1.453173. Uncertainties reported for measured ɛiMo values represent the external reproducibility (2 s.d.) obtained from repeated analyses of Highland Valley molybdenite (Table S-4).
The composite sample is characterised by positive ɛiMo values for 92Mo, 94Mo, 95Mo, and 97Mo, all of which are >2 times larger than those for Allende (Fig. 1). The ɛiMo values in the Ryugu composite decrease in the order of 92Mo > 94Mo > 95Mo > 97Mo, suggesting a deficit of s-process Mo isotopes compared to terrestrial values. The Ryugu sample plots on the CC line on the ɛ94Mo-ɛ95Mo diagram (Fig. 2a), which is consistent with the observation that Ryugu samples are characterised by CC-type ɛ50Ti-ɛ54Cr isotopic systematics (Yokoyama et al., 2023a
Yokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
, bYokoyama T., Wadhwa, M., Iizuka, T., Abe Y., Aléon J., et al. (2023b) Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample. Science Advances 9, eadi7048. https://doi.org/10.1126/sciadv.adi7048
). On a plot of ɛ97Mo vs. ɛ92Mo, the Ryugu sample plots off the mixing line between a representative terrestrial composition and the theoretical s-process composition. This may reflect an underestimate of external uncertainties for these isotopes (details are in Supplementary Information Experiments).Figure 1 Molybdenum isotopic compositions of Ryugu and Allende analysed in this study (solid lines). The dotted lines represent comparison bulk meteorite data for Allende (green) and Orgueil (blue) from previous studies (Table S-1). Note that 100Mo was not measured in this study.
Figure 2 Molybdenum isotope diagrams of bulk meteorites and the Ryugu sample. (a) The solid lines are the regression lines of CC (blue, from Budde et al., 2019
Budde, G., Burkhardt, C., Kleine, T. (2019) Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. Nature Astronomy 3, 736–741. https://doi.org/10.1038/s41550-019-0779-y
) and NC (red, from Spitzer et al., 2020Spitzer, F., Burkhardt, C., Budde, G., Kruijer, T. S., Morbidelli, A., Kleine, T. (2020) Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites. The Astrophysical Journal 898, L2. https://doi.org/10.3847/2041-8213/ab9e6a
) meteorites. (b), (c), (d) Reference data of bulk carbonaceous chondrites as listed in Table S-1. Black lines are extensions of the mixing lines between a representative terrestrial sample (origin) and the theoretical s-process Mo isotopic composition calculated by Stephan et al. (2019)Stephan, T., Trappitsch, R., Hoppe, P., Davis, A.M., Pellin, M.J., Pardo, O.S. (2019) Molybdenum Isotopes in Presolar Silicon Carbide Grains: Details of s-process Nucleosynthesis in Parent Stars and Implications for r- and p-processes. The Astrophysical Journal 877, 101. https://doi.org/10.3847/1538-4357/ab1c60
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Discussion
While the Ryugu composite plots on the ɛ94Mo-ɛ95Mo CC trend, its ɛ92Mo, ɛ94Mo, ɛ95Mo, and ɛ97Mo values are all greater than any other “bulk” samples of meteorites previously analysed (Fig. 2b-d), including the CI chondrite Orgueil, which is characterised as having the smallest s-process deficit among carbonaceous chondrites (Fig. 2, Dauphas et al., 2002
Dauphas, N., Marty, B., Reisberg, L. (2002) Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites. The Astrophysical Journal 569, L139–L142. https://doi.org/10.1086/340580
; Burkhardt et al., 2011Burkhardt, C., Kleine, T., Oberli, F., Pack, A., Bourdon, B., Wieler, R. (2011) Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth. Earth and Planetary Science Letters 312, 390–400. https://doi.org/10.1016/j.epsl.2011.10.010
). The values are also larger than any known iron meteorites (e.g., Kruijer et al., 2017Kruijer, T.S., Burkhardt, C., Budde, G., Kleine, T. (2017) Age of Jupiter inferred from the distinct genetics and formation times of meteorites. Proceedings of the National Academy of Sciences 114, 201704461. https://doi.org/10.1073/pnas.1704461114
; Bermingham et al., 2018Bermingham, K.R., Worsham, E.A., Walker, R.J. (2018) New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics. Earth and Planetary Science Letters 487, 221–229. https://doi.org/10.1016/j.epsl.2018.01.017
). Determining the cause of the extreme Mo isotope anomaly is an important aspect of fully understanding the origin and internal evolution of Ryugu. As follows, we consider four possible explanations for the large s-process deficits in the Ryugu composite sample: 1) biased sampling of chondritic components incorporated in the Ryugu composite sample, 2) the observed isotopic composition represents the original bulk composition of the asteroid Ryugu, 3) incomplete dissolution of presolar SiC grains during the acid digestion steps, and 4) heterogeneous distribution of Mo isotopes in the Ryugu body driven by aqueous alteration processes.The first potential explanation is that, in the small amount of material analysed, we over-sampled a chondritic component with strong deficits in s-process Mo isotopes. This could lead to a bias in the composition of a presumed “bulk” sample of Ryugu. Although Mo isotope data are limited for chondrite components, no known components provide a viable explanation for the Ryugu data. For instance, s-process-depleted Mo isotopic compositions have been reported for Allende chondrules (Budde et al., 2016
Budde, G., Burkhardt, C., Brennecka, G.A., Fischer-gödde, M., Kruijer, T.S., Kleine, T. (2016) Molybdenum isotopic evidence for the origin of chondrules and a distinct genetic heritage of carbonaceous and non-carbonaceous meteorites. Earth and Planetary Science Letters 454, 293–303. https://doi.org/10.1016/j.epsl.2016.09.020
), however, the Allende chondrules have ɛ94Mo values ranging only from +1.9 to +6.1, all of which are lower than the Ryugu composite (ɛ94Mo = +7.2 ± 0.3). Most calcium-aluminum inclusions (CAIs) have even smaller positive ɛMo values (ɛ94Mo < +2) than chondrules (Burkhardt et al., 2011Burkhardt, C., Kleine, T., Oberli, F., Pack, A., Bourdon, B., Wieler, R. (2011) Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth. Earth and Planetary Science Letters 312, 390–400. https://doi.org/10.1016/j.epsl.2011.10.010
; Shollenberger et al., 2018Shollenberger, Q.R., Borg, L.E., Render, J., Ebert, S., Bischoff, A., Russell, S.S., Brennecka, G.A. (2018) Isotopic coherence of refractory inclusions from CV and CK meteorites: Evidence from multiple isotope systems. Geochimica et Cosmochimica Acta 228, 62–80. https://doi.org/10.1016/j.gca.2018.02.006
; Brennecka et al., 2020Brennecka, G.A., Burkhardt, C., Budde, G., Kruijer, T.S., Nimmo, F., Kleine, T. (2020) Astronomical context of Solar System formation from molybdenum isotopes in meteorite inclusions. Science 370, 837–840. https://doi.org/10.1126/science.aaz8482
). Only a single CAI examined by Burkhardt et al. (2011)Burkhardt, C., Kleine, T., Oberli, F., Pack, A., Bourdon, B., Wieler, R. (2011) Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth. Earth and Planetary Science Letters 312, 390–400. https://doi.org/10.1016/j.epsl.2011.10.010
showed strong deficits of s-process Mo isotopes (ɛ94Mo = +16.8 ± 0.4), however, ∼10 % of the analysed Ryugu sample would have to consist of such exotic material to explain the measured Ryugu composition. In addition to the isotopic mismatch, the chemical data for equivalent bulk samples provide no evidence for over-sampling of chondrules or refractory inclusions (Yokoyama et al., 2023aYokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
). Further, petrographic observation shows that Ryugu samples, as with all CI chondrites, lack chondrules, CAIs, and metals (e.g., Kawasaki et al., 2022Kawasaki, N., Nagashima, K., Sakamoto, N., Matsumoto, T., Bajo, K.I., et al. (2022) Oxygen isotopes of anhydrous primary minerals show kinship between asteroid Ryugu and comet 81P/Wild2. Science Advances 8, eade2067. https://doi.org/10.1126/sciadv.ade2067
), hence, the components necessary to cause a biased analysis are absent. We conclude that it is unlikely that the observed extreme Mo isotopic anomalies were due to the preferential sampling of precursor chondrite components.It is also possible that the bulk asteroid Ryugu is characterised by strong s-process depletion in certain elements. This could mean that Ryugu formed from proportions of nebular materials that were significantly different from those incorporated in CI meteorites. As noted, Ryugu is characterised by CI-like nucleosynthetic isotope anomalies in Cr, Ti, Fe, and Zn (e.g., Hopp et al., 2022
Hopp, T., Dauphas, N., Abe, Y., Aléon, J., Alexander, C.M.O.’D., et al. (2022) Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Science Advances 8, 1–10. https://doi.org/10.1126/sciadv.add8141
). The inconsistency between these elements and Mo may suggest that lithophile and siderophile element-rich presolar materials were either differentially distributed within the nebula, or differentially extracted from the nebula during planetesimal accretion. Hopp et al., (2022)Hopp, T., Dauphas, N., Abe, Y., Aléon, J., Alexander, C.M.O.’D., et al. (2022) Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Science Advances 8, 1–10. https://doi.org/10.1126/sciadv.add8141
found that Ryugu and CI data have identical μ54Fe-μ50Ti values, but the values are distinct from other CC and NC meteorites. These authors suggested that Ryugu and CI parent bodies formed in a nebular region that was different from the source of other carbonaceous asteroids. If however the measured Mo isotopic composition accurately reflects the composition of the bulk Ryugu asteroid, the formation region(s) of Ryugu and CI parent bodies would have to have been heterogeneous with respect to Mo isotopic compositions. Given the strong genetic similarities between Ryugu and CI, we consider it unlikely that bulk Ryugu has a unique Mo isotopic composition.Certain types of presolar grains may be resistant to the acid digestion methods employed here (Nittler, 2003
Nittler, L.R. (2003) Presolar stardust in meteorites: recent advances and scientific frontiers. Earth and Planetary Science Letters 209, 259–273. https://doi.org/10.1016/S0012-821X(02)01153-6
). Hence, one possible explanation for the extreme s-process depletion is incomplete dissolution of presolar SiC grains during the sample digestion steps. This follows from the observation that the majority of presolar SiC grains are characterised by strongly s-process-enriched compositions. A sequential acid leaching study of the CI chondrite Orgueil showed that a phase accessed by leaching with 3 M HCl-13.5 M HF was strongly enriched in s-process Mo isotopes (ɛ94Mo = −31.76 ± 0.89), which was most likely contributed by the partial dissolution of presolar SiC (Dauphas et al., 2002Dauphas, N., Marty, B., Reisberg, L. (2002) Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites. The Astrophysical Journal 569, L139–L142. https://doi.org/10.1086/340580
). Stephan et al. (2019)Stephan, T., Trappitsch, R., Hoppe, P., Davis, A.M., Pellin, M.J., Pardo, O.S. (2019) Molybdenum Isotopes in Presolar Silicon Carbide Grains: Details of s-process Nucleosynthesis in Parent Stars and Implications for r- and p-processes. The Astrophysical Journal 877, 101. https://doi.org/10.3847/1538-4357/ab1c60
and Liu et al. (2019)Liu, N., Stephan, T., Cristallo, S., Gallino, R., Boehnke, P., et al. (2019) Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins. The Astrophysical Journal 881, 28. https://doi.org/10.3847/1538-4357/ab2d27
reported Mo isotopic anomalies directly measured in single SiC grains, with ɛ94Mo values ranging from −228 to −9138.In order to assess the possible effects of incomplete digestion of presolar SiC, we conducted a mass balance calculation assuming that the true bulk Ryugu sample had CI-like Mo isotopic composition. The detailed calculations and assumptions are described in Supplementary Information (mass balance calculation). It should be noted that the Mo isotopic composition of the CI chondrite component used in the calculation is taken from the value for bulk Orgueil (CI) determined by the laser fusion method (Burkhardt et al., 2011
Burkhardt, C., Kleine, T., Oberli, F., Pack, A., Bourdon, B., Wieler, R. (2011) Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth. Earth and Planetary Science Letters 312, 390–400. https://doi.org/10.1016/j.epsl.2011.10.010
). Of note, Dauphas et al. (2002)Dauphas, N., Marty, B., Reisberg, L. (2002) Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites. The Astrophysical Journal 569, L139–L142. https://doi.org/10.1086/340580
also reported Mo isotopic data for a bulk sample of Orgueil using an acid digestion method. Both laser fusion and acid digestion methods gave generally consistent isotopic compositions (Fig. 1).Mass balance calculations indicate that, depending on the Mo abundance present in the presolar SiC, anywhere from ∼20 to 100 % of the presolar SiC present in the Ryugu sample would have to remain undissolved in order to reduce the s-process deficits in Mo to the level of CI chondrites. Given the concordance of the prior analyses of bulk CI meteorites using two different digestion methods, this seems unlikely. Nevertheless, considering the large uncertainties on the assumptions, we cannot rule out this possibility. Although for this study the sample powders were treated with multiple acid digestion steps, including an HF-HNO3 heating step at 180 °C, a complete digestion technique applied to Ryugu materials (e.g., alkaline fusion) will ultimately be required to further assess this possibility.
Finally, we consider a scenario in which aqueous alteration and deposition of secondary minerals caused the redistribution of s-process-depleted Mo within the Ryugu parent body. Yokoyama et al. (2023b)
Yokoyama T., Wadhwa, M., Iizuka, T., Abe Y., Aléon J., et al. (2023b) Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample. Science Advances 9, eadi7048. https://doi.org/10.1126/sciadv.adi7048
reported that Ryugu materials and CI chondrites show variable ɛ54Cr values and suggested that fluid driven decoupling via parent body aqueous alteration between Cr in chemically labile phases with a slightly negative ɛ54Cr value, and 54Cr-rich Cr oxide nanoparticles, resulted in mm scale ɛ54Cr variability in the bulk Ryugu samples and CI chondrites. If this scenario explains the Mo isotopic variation between the Ryugu samples and CI chondrites, the analysed composite sample, consisting of the ∼70 mg of Ryugu material, does not represent the bulk Ryugu parent body, but would be dominated by materials depleted in s-process-rich presolar grains, likely including SiC, and enriched in the secondary minerals. Sequential acid leaching experiments on CI and CM chondrites revealed that the early stage leaching fractions with mild acids (e.g., CH3COOH, 4M HNO3) are characterised by positive ɛMo values up to e.g., ɛ94Mo = +24 (Dauphas et al., 2002Dauphas, N., Marty, B., Reisberg, L. (2002) Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites. The Astrophysical Journal 569, L139–L142. https://doi.org/10.1086/340580
; Burkhardt et al., 2011Burkhardt, C., Kleine, T., Oberli, F., Pack, A., Bourdon, B., Wieler, R. (2011) Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth. Earth and Planetary Science Letters 312, 390–400. https://doi.org/10.1016/j.epsl.2011.10.010
). These experimental results demonstrate that easily leachable phases in CI and CM chondrites, possibly aqueously formed secondary carbonates and sulfides, are depleted in s-process Mo isotopes. It is, therefore, conceivable that the pervasive aqueous fluid present in the Ryugu parent body, as suggested by Nakamura et al. (2023)Nakamura, T., Matsumoto, M., Amano, K., Enokido, Y., Zolensky, M.E., et al. (2023) Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science 379, eabn8671. https://doi.org/10.1126/science.abn8671
, preferentially dissolved s-process depleted, chemically labile phases in the protolith (e.g., amorphous silicates), releasing this Mo into the fluid for transport and deposition elsewhere in the body. One Ryugu sample (C0002) examined by Yokoyama et al. (2023b)Yokoyama T., Wadhwa, M., Iizuka, T., Abe Y., Aléon J., et al. (2023b) Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample. Science Advances 9, eadi7048. https://doi.org/10.1126/sciadv.adi7048
was characterised by a substantially higher ɛ54Cr value compared to four other Ryugu samples (A0106, A0106-A0107, C0107, and C0108), presumably due to the low abundance of the secondary minerals (e.g., dolomite) in the sample. This suggests that the isotopic composition of Cr may be correlated with the abundance of secondary minerals. Although C0002 was not analysed for Mo in this study, it might be predicted that this sample was also enriched in s-process Mo isotopes compared to the Ryugu materials examined in this study. It should be noted that Barosch et al. (2022)Barosch, J., Nittler, L.R., Wang, J., Alexander, C.M.O.’D., De Gregorio, B.T., et al. (2022) Presolar Stardust in Asteroid Ryugu. The Astrophysical Journal Letters 935, L3. https://doi.org/10.3847/2041-8213/ac83bd
reported the average SiC abundance of 25 ppm in Ryugu samples is consistent with the average value of 23 ppm in CI chondrites, however, given the limited area analysed, future additional analyses will be required to assess the heterogeneity of presolar materials in the Ryugu samples. A detailed mechanism that would result in the separation of complementary s-process depleted Mo from domains with s-process-rich presolar grains and the secondary minerals remains unclear.We conclude that the most likely cause for the extreme mass independent Mo isotopic composition of a composite sample of Ryugu is fluid redistribution of s-process depleted Mo in the asteroid. Alternatively, incomplete digestion of presolar SiC is also possible. None of the four options, however, can be fully dismissed at this time. Additional analyses using more rigorous digestion methods and/or acid leaching methods might reveal whether resistance of SiC grains to dissolution led to the strong s-process depletion. Analyses of different Ryugu samples would also provide important constraints on whether the Ryugu body was formed from CI-like materials, but with heterogeneous Mo isotopic composition at the ∼70 mg level, or the Ryugu body was formed from highly s-process depleted materials. In the case of the former, isotopic variations might be correlated with variations in bulk chemical compositions of individual samples (e.g., proxies for aqueous alteration or redistribution). In the case of the latter, isotopic homogeneity would be expected for multiple samples. If homogeneity of multiple samples was demonstrated, it might therefore, suggest a different origin for the siderophile Mo in Ryugu relative to the lithophile elements that have been measured for nucleosynthetic isotopic compositions (Cr, Ti, Fe, and Zn). Analysis of the isotopic compositions of Ru and W would then be useful given their similar, siderophile element chemical characteristics to Mo.
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Acknowledgments
Hayabusa2 was developed and built under the leadership of Japan Aerospace Exploration Agency (JAXA), with contributions from the German Aerospace Center (DLR) and the Centre National d’Études Spatiales (CNES), and in collaboration with NASA, and other universities, institutes, and companies in Japan. The curation system was developed by JAXA in collaboration with companies in Japan. The Smithsonian National Museum of Natural History is also acknowledged for providing the Allende powder. We thank three anonymous reviewers for their helpful comments. We also thank H.A. Tornabene, and K.R. Bermingham for the chemical processing of Highland Valley molybdenite, and R.D. Ash, and J.L. Hellmann for help with the MC-ICP-MS measurements. This research was supported by NASA Emerging Worlds grant (80NSSC20K0335 to RJW) and JSPS KAKENHI grants (18H01258, 19H00715, 20H04609, and 21KK0058 to TY).
Editor: Francis McCubbin
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References
Barosch, J., Nittler, L.R., Wang, J., Alexander, C.M.O.’D., De Gregorio, B.T., et al. (2022) Presolar Stardust in Asteroid Ryugu. The Astrophysical Journal Letters 935, L3. https://doi.org/10.3847/2041-8213/ac83bd
Show in context It should be noted that Barosch et al. (2022) reported the average SiC abundance of 25 ppm in Ryugu samples is consistent with the average value of 23 ppm in CI chondrites, however, given the limited area analysed, future additional analyses will be required to assess the heterogeneity of presolar materials in the Ryugu samples.
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Bermingham, K.R., Worsham, E.A., Walker, R.J. (2018) New insights into Mo and Ru isotope variation in the nebula and terrestrial planet accretionary genetics. Earth and Planetary Science Letters 487, 221–229. https://doi.org/10.1016/j.epsl.2018.01.017
Show in context The values are also larger than any known iron meteorites (e.g., Kruijer et al., 2017; Bermingham et al., 2018).
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Brennecka, G.A., Burkhardt, C., Budde, G., Kruijer, T.S., Nimmo, F., Kleine, T. (2020) Astronomical context of Solar System formation from molybdenum isotopes in meteorite inclusions. Science 370, 837–840. https://doi.org/10.1126/science.aaz8482
Show in context Most calcium-aluminum inclusions (CAIs) have even smaller positive ɛMo values (ɛ94Mo < +2) than chondrules (Burkhardt et al., 2011; Shollenberger et al., 2018; Brennecka et al., 2020).
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Budde, G., Burkhardt, C., Brennecka, G.A., Fischer-gödde, M., Kruijer, T.S., Kleine, T. (2016) Molybdenum isotopic evidence for the origin of chondrules and a distinct genetic heritage of carbonaceous and non-carbonaceous meteorites. Earth and Planetary Science Letters 454, 293–303. https://doi.org/10.1016/j.epsl.2016.09.020
Show in context High precision measurements of Mo isotopes in bulk chondrites and differentiated meteorites have revealed considerable variation among early Solar System materials (e.g., Dauphas et al., 2002; Budde et al., 2016; Spitzer et al., 2020).
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In particular, an internal isotopic dichotomy of ɛ94Mo-ɛ95Mo (ɛiMo =[(iMo/96Mo)sample/(iMo/96Mo)standard – 1] × 104) between non-carbonaceous meteorites (NC) and carbonaceous meteorites (CC) has been shown to be particularly sensitive for evaluating genetic differences of source materials present in bulk meteorite samples (Budde et al., 2016; Poole et al., 2017; Worsham et al., 2017).
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For instance, s-process-depleted Mo isotopic compositions have been reported for Allende chondrules (Budde et al., 2016), however, the Allende chondrules have ɛ94Mo values ranging only from +1.9 to +6.1, all of which are lower than the Ryugu composite (ɛ94Mo = +7.2 ± 0.3).
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Budde, G., Burkhardt, C., Kleine, T. (2019) Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. Nature Astronomy 3, 736–741. https://doi.org/10.1038/s41550-019-0779-y
Show in context The solid lines are the regression lines of CC (blue, from Budde et al., 2019) and NC (red, from Spitzer et al., 2020) meteorites.
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Burkhardt, C., Kleine, T., Oberli, F., Pack, A., Bourdon, B., Wieler, R. (2011) Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth. Earth and Planetary Science Letters 312, 390–400. https://doi.org/10.1016/j.epsl.2011.10.010
Show in context While the Ryugu composite plots on the ɛ94Mo-ɛ95Mo CC trend, its ɛ92Mo, ɛ94Mo, ɛ95Mo, and ɛ97Mo values are all greater than any other “bulk” samples of meteorites previously analysed (Fig. 2b-d), including the CI chondrite Orgueil, which is characterised as having the smallest s-process deficit among carbonaceous chondrites (Fig. 2, Dauphas et al., 2002; Burkhardt et al., 2011).
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Most calcium-aluminum inclusions (CAIs) have even smaller positive ɛMo values (ɛ94Mo < +2) than chondrules (Burkhardt et al., 2011; Shollenberger et al., 2018; Brennecka et al., 2020).
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Only a single CAI examined by Burkhardt et al. (2011) showed strong deficits of s-process Mo isotopes (ɛ94Mo = +16.8 ± 0.4), however, ∼10 % of the analysed Ryugu sample would have to consist of such exotic material to explain the measured Ryugu composition.
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It should be noted that the Mo isotopic composition of the CI chondrite component used in the calculation is taken from the value for bulk Orgueil (CI) determined by the laser fusion method (Burkhardt et al., 2011).
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Bus, S.J., Binzel, R.P. (2002) Phase II of the small main-belt asteroid spectroscopic survey. A feature-based taxonomy. Icarus 158, 146–177. https://doi.org/10.1006/icar.2002.6856
Show in context Spectral observations led to the classification of the asteroid Ryugu as Cb-type, which has long been assumed to be related to carbonaceous chondrite meteorites (Bus and Binzel, 2002).
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Dauphas, N., Schauble, E.A. (2016) Mass Fractionation Laws, Mass-Independent Effects, and Isotopic Anomalies. Annual Review of Earth and Planetary Sciences 44, 709–783. https://doi.org/10.1146/annurev-earth-060115-012157
Show in context Nucleosynthetic isotopic anomalies of meteorites provide robust information that can be used to constrain the origin of materials that contributed to the accretion of their parent bodies (Dauphas and Schauble, 2016; Yokoyama and Walker, 2016).
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Dauphas, N., Marty, B., Reisberg, L. (2002) Molybdenum Nucleosynthetic Dichotomy Revealed in Primitive Meteorites. The Astrophysical Journal 569, L139–L142. https://doi.org/10.1086/340580
Show in context High precision measurements of Mo isotopes in bulk chondrites and differentiated meteorites have revealed considerable variation among early Solar System materials (e.g., Dauphas et al., 2002; Budde et al., 2016; Spitzer et al., 2020).
View in article
While the Ryugu composite plots on the ɛ94Mo-ɛ95Mo CC trend, its ɛ92Mo, ɛ94Mo, ɛ95Mo, and ɛ97Mo values are all greater than any other “bulk” samples of meteorites previously analysed (Fig. 2b-d), including the CI chondrite Orgueil, which is characterised as having the smallest s-process deficit among carbonaceous chondrites (Fig. 2, Dauphas et al., 2002; Burkhardt et al., 2011).
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A sequential acid leaching study of the CI chondrite Orgueil showed that a phase accessed by leaching with 3 M HCl-13.5 M HF was strongly enriched in s-process Mo isotopes (ɛ94Mo = −31.76 ± 0.89), which was most likely contributed by the partial dissolution of presolar SiC (Dauphas et al., 2002). Stephan et al. (2019) and Liu et al. (2019) reported Mo isotopic anomalies directly measured in single SiC grains, with ɛ94Mo values ranging from −228 to −9138.
View in article
Of note, Dauphas et al. (2002) also reported Mo isotopic data for a bulk sample of Orgueil using an acid digestion method.
View in article
Sequential acid leaching experiments on CI and CM chondrites revealed that the early stage leaching fractions with mild acids (e.g., CH3COOH, 4M HNO3) are characterised by positive ɛMo values up to e.g., ɛ94Mo = +24 (Dauphas et al., 2002; Burkhardt et al., 2011).
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Hopp, T., Dauphas, N., Abe, Y., Aléon, J., Alexander, C.M.O.’D., et al. (2022) Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Science Advances 8, 1–10. https://doi.org/10.1126/sciadv.add8141
Show in context This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2023; Nakamura et al., 2023; Yokoyama et al., 2023a).
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Isotopic compositions of Cr, Ti, Fe, and Zn have been reported for Ryugu samples, all of which are generally consistent with those of CI chondrites (Hopp et al., 2022; Paquet et al., 2023; Yokoyama et al., 2023a).
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As noted, Ryugu is characterised by CI-like nucleosynthetic isotope anomalies in Cr, Ti, Fe, and Zn (e.g., Hopp et al., 2022).
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Hopp et al., (2022) found that Ryugu and CI data have identical μ54Fe-μ50Ti values, but the values are distinct from other CC and NC meteorites.
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Kadlag, Y., Becker, H., Harbott, A. (2019) Cr isotopes in physically separated components of the Allende CV3 and Murchison CM2 chondrites: Implications for isotopic heterogeneity in the solar nebula and parent body processes. Meteoritics & Planetary Science 54, 2116–2131. https://doi.org/10.1111/maps.13375
Show in context These observations suggest that the parent bodies of Ryugu and CI chondrites formed in a common source reservoir in the outer Solar System. Remarkably, both Ryugu materials and CI chondrites show variable ɛ54Cr values (ɛ54Cr = [(54Cr/52Cr)sample/(54Cr/52Cr)standard – 1] × 104; ranging from +1.22 ± 0.06 to +2.22 ± 0.13 for Ryugu and from +0.84 ± 0.14 to +1.94 ± 0.12 for CI chondrites), which exceed the range of analytical uncertainties (Kadlag et al., 2019; Williams et al., 2020; Nakamura et al., 2022; Yokoyama et al., 2023a, b).
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Kawasaki, N., Nagashima, K., Sakamoto, N., Matsumoto, T., Bajo, K.I., et al. (2022) Oxygen isotopes of anhydrous primary minerals show kinship between asteroid Ryugu and comet 81P/Wild2. Science Advances 8, eade2067. https://doi.org/10.1126/sciadv.ade2067
Show in context Further, petrographic observation shows that Ryugu samples, as with all CI chondrites, lack chondrules, CAIs, and metals (e.g., Kawasaki et al., 2022), hence, the components necessary to cause a biased analysis are absent.
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Kruijer, T.S., Burkhardt, C., Budde, G., Kleine, T. (2017) Age of Jupiter inferred from the distinct genetics and formation times of meteorites. Proceedings of the National Academy of Sciences 114, 201704461. https://doi.org/10.1073/pnas.1704461114
Show in context The values are also larger than any known iron meteorites (e.g., Kruijer et al., 2017; Bermingham et al., 2018).
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Liu, N., Stephan, T., Cristallo, S., Gallino, R., Boehnke, P., et al. (2019) Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins. The Astrophysical Journal 881, 28. https://doi.org/10.3847/1538-4357/ab2d27
Show in context A sequential acid leaching study of the CI chondrite Orgueil showed that a phase accessed by leaching with 3 M HCl-13.5 M HF was strongly enriched in s-process Mo isotopes (ɛ94Mo = −31.76 ± 0.89), which was most likely contributed by the partial dissolution of presolar SiC (Dauphas et al., 2002). Stephan et al. (2019) and Liu et al. (2019) reported Mo isotopic anomalies directly measured in single SiC grains, with ɛ94Mo values ranging from −228 to −9138.
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Moynier, F., Dai, W., Yokoyama, T., Hu, Y., Paquet, M., et al. (2022) The Solar System calcium isotopic composition inferred from Ryugu samples. Geochemical Perspectives Letters 24, 1–6. https://doi.org/10.7185/geochemlet.2238
Show in context This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2023; Nakamura et al., 2023; Yokoyama et al., 2023a).
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Nakamura, E., Kobayashi, K., Tanaka, R., Kunihiro, T., Kitagawa, H., et al. (2022) On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective. Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 98, 227–282. https://doi.org/10.2183/pjab.98.015
Show in context This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2023; Nakamura et al., 2023; Yokoyama et al., 2023a).
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The melting of the ice led to the hydration of the inner portion of the parent body, causing pervasive aqueous alteration that resulted in the precipitation of secondary minerals, including phyllosilicates, carbonates, oxides, sulfides, and phosphates (Nakamura et al., 2022; Nakamura et al., 2023; Yokoyama et al., 2023a).
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These observations suggest that the parent bodies of Ryugu and CI chondrites formed in a common source reservoir in the outer Solar System. Remarkably, both Ryugu materials and CI chondrites show variable ɛ54Cr values (ɛ54Cr = [(54Cr/52Cr)sample/(54Cr/52Cr)standard – 1] × 104; ranging from +1.22 ± 0.06 to +2.22 ± 0.13 for Ryugu and from +0.84 ± 0.14 to +1.94 ± 0.12 for CI chondrites), which exceed the range of analytical uncertainties (Kadlag et al., 2019; Williams et al., 2020; Nakamura et al., 2022; Yokoyama et al., 2023a, b).
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Nakamura, T., Matsumoto, M., Amano, K., Enokido, Y., Zolensky, M.E., et al. (2023) Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples. Science 379, eabn8671. https://doi.org/10.1126/science.abn8671
Show in context This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2023; Nakamura et al., 2023; Yokoyama et al., 2023a).
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Chemical and physical properties of the returned samples suggest that the parent asteroid of Ryugu was accreted in the outer Solar System where water and CO2 were present as ices, followed by melting of the accreted ice due to the radioactive decay of 26Al that heated the parent asteroid to ∼40 °C (Nakamura et al., 2023).
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The melting of the ice led to the hydration of the inner portion of the parent body, causing pervasive aqueous alteration that resulted in the precipitation of secondary minerals, including phyllosilicates, carbonates, oxides, sulfides, and phosphates (Nakamura et al., 2022; Nakamura et al., 2023; Yokoyama et al., 2023a).
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It is, therefore, conceivable that the pervasive aqueous fluid present in the Ryugu parent body, as suggested by Nakamura et al. (2023), preferentially dissolved s-process depleted, chemically labile phases in the protolith (e.g., amorphous silicates), releasing this Mo into the fluid for transport and deposition elsewhere in the body.
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Nittler, L.R. (2003) Presolar stardust in meteorites: recent advances and scientific frontiers. Earth and Planetary Science Letters 209, 259–273. https://doi.org/10.1016/S0012-821X(02)01153-6
Show in context Certain types of presolar grains may be resistant to the acid digestion methods employed here (Nittler, 2003).
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Paquet, M., Moynier, F., Yokoyama, T., Dai, W., Hu, Y., et al. (2023) Contribution of Ryugu-like material to Earth’s volatile inventory by Cu and Zn isotopic analysis. Nature Astronomy 7, 182–189. https://doi.org/10.1038/s41550-022-01846-1
Show in context This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2023; Nakamura et al., 2023; Yokoyama et al., 2023a).
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Isotopic compositions of Cr, Ti, Fe, and Zn have been reported for Ryugu samples, all of which are generally consistent with those of CI chondrites (Hopp et al., 2022; Paquet et al., 2023; Yokoyama et al., 2023a).
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Poole, G.M., Rehkämper, M., Coles, B.J., Goldberg, T., Smith, C.L. (2017) Nucleosynthetic molybdenum isotope anomalies in iron meteorites – new evidence for thermal processing of solar nebula material. Earth and Planetary Science Letters 473, 215–226. https://doi.org/10.1016/j.epsl.2017.05.001
Show in context In particular, an internal isotopic dichotomy of ɛ94Mo-ɛ95Mo (ɛiMo =[(iMo/96Mo)sample/(iMo/96Mo)standard – 1] × 104) between non-carbonaceous meteorites (NC) and carbonaceous meteorites (CC) has been shown to be particularly sensitive for evaluating genetic differences of source materials present in bulk meteorite samples (Budde et al., 2016; Poole et al., 2017; Worsham et al., 2017).
View in article
Render, J., Fischer-Gödde, M., Burkhardt, C., Kleine, T. (2017) The cosmic molybdenum-neodymium isotope correlation and the building material of the Earth. Geochemical Perspectives Letters 3, 170–178. https://doi.org/10.7185/geochemlet.1720
Shollenberger, Q.R., Borg, L.E., Render, J., Ebert, S., Bischoff, A., Russell, S.S., Brennecka, G.A. (2018) Isotopic coherence of refractory inclusions from CV and CK meteorites: Evidence from multiple isotope systems. Geochimica et Cosmochimica Acta 228, 62–80. https://doi.org/10.1016/j.gca.2018.02.006
Show in context Most calcium-aluminum inclusions (CAIs) have even smaller positive ɛMo values (ɛ94Mo < +2) than chondrules (Burkhardt et al., 2011; Shollenberger et al., 2018; Brennecka et al., 2020).
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Spitzer, F., Burkhardt, C., Budde, G., Kruijer, T. S., Morbidelli, A., Kleine, T. (2020) Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites. The Astrophysical Journal 898, L2. https://doi.org/10.3847/2041-8213/ab9e6a
Show in context High precision measurements of Mo isotopes in bulk chondrites and differentiated meteorites have revealed considerable variation among early Solar System materials (e.g., Dauphas et al., 2002; Budde et al., 2016; Spitzer et al., 2020).
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Molybdenum isotope diagrams of bulk meteorites and the Ryugu sample. (a) The solid lines are the regression lines of CC (blue, from Budde et al., 2019) and NC (red, from Spitzer et al., 2020) meteorites.
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Stephan, T., Trappitsch, R., Hoppe, P., Davis, A.M., Pellin, M.J., Pardo, O.S. (2019) Molybdenum Isotopes in Presolar Silicon Carbide Grains: Details of s-process Nucleosynthesis in Parent Stars and Implications for r- and p-processes. The Astrophysical Journal 877, 101. https://doi.org/10.3847/1538-4357/ab1c60
Show in context Black lines are extensions of the mixing lines between a representative terrestrial sample (origin) and the theoretical s-process Mo isotopic composition calculated by Stephan et al. (2019).
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A sequential acid leaching study of the CI chondrite Orgueil showed that a phase accessed by leaching with 3 M HCl-13.5 M HF was strongly enriched in s-process Mo isotopes (ɛ94Mo = −31.76 ± 0.89), which was most likely contributed by the partial dissolution of presolar SiC (Dauphas et al., 2002). Stephan et al. (2019) and Liu et al. (2019) reported Mo isotopic anomalies directly measured in single SiC grains, with ɛ94Mo values ranging from −228 to −9138.
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Tachibana, S., Sawada, H., Okazaki, R., Takano, Y., Sakamoto, K., et al. (2022) Pebbles and sand on asteroid (162173) Ryugu: In situ observation and particles returned to Earth. Science 375, 1011–1016. https://doi.org/10.1126/science.abj8624
Show in context The Hayabusa2 mission by JAXA performed two sampling sequences on the asteroid (162173) Ryugu, and returned 5.4 g of the asteroid materials back to Earth (Tachibana et al., 2022).
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Williams, C.D., Sanborn, M.E., Defouilloy, C., Yin, Q.Z., Kita, N.T., Ebel, D.S., Yamakawa, A., Yamashita, K. (2020) Chondrules reveal large-scale outward transport of inner Solar System materials in the protoplanetary disk. Proceedings of the National Academy of Sciences of the United States of America 117, 23426–23435. https://doi.org/10.1073/pnas.2005235117
Show in context These observations suggest that the parent bodies of Ryugu and CI chondrites formed in a common source reservoir in the outer Solar System. Remarkably, both Ryugu materials and CI chondrites show variable ɛ54Cr values (ɛ54Cr = [(54Cr/52Cr)sample/(54Cr/52Cr)standard – 1] × 104; ranging from +1.22 ± 0.06 to +2.22 ± 0.13 for Ryugu and from +0.84 ± 0.14 to +1.94 ± 0.12 for CI chondrites), which exceed the range of analytical uncertainties (Kadlag et al., 2019; Williams et al., 2020; Nakamura et al., 2022; Yokoyama et al., 2023a, b).
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Worsham, E.A., Bermingham, K.R., Walker, R.J. (2017) Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex. Earth and Planetary Science Letters 467, 157–166. https://doi.org/10.1016/j.epsl.2017.02.044
Show in context In particular, an internal isotopic dichotomy of ɛ94Mo-ɛ95Mo (ɛiMo =[(iMo/96Mo)sample/(iMo/96Mo)standard – 1] × 104) between non-carbonaceous meteorites (NC) and carbonaceous meteorites (CC) has been shown to be particularly sensitive for evaluating genetic differences of source materials present in bulk meteorite samples (Budde et al., 2016; Poole et al., 2017; Worsham et al., 2017).
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Yokoyama, T., Walker, R.J. (2016) Nucleosynthetic Isotope Variations of Siderophile and Chalcophile Elements in the Solar System. Reviews in Mineralogy & Geochemistry 81, 107–160. https://doi.org/10.2138/rmg.2016.81.03
Show in context Nucleosynthetic isotopic anomalies of meteorites provide robust information that can be used to constrain the origin of materials that contributed to the accretion of their parent bodies (Dauphas and Schauble, 2016; Yokoyama and Walker, 2016).
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Yokoyama, T., Alexander, C.M.O.’D., Walker, R.J. (2011) Assessment of nebular versus parent body processes on presolar components present in chondrites: Evidence from osmium isotopes. Earth and Planetary Science Letters 305, 115–123. https://doi.org/10.1016/j.epsl.2011.02.046
Show in context Additionally, the Mo isotope compositions of meteorites can be modified by parent body processes, including thermal metamorphism (Yokoyama et al., 2019), and presumably aqueous alteration, as has been observed for the isotopic composition of the highly siderophile element Os (Yokoyama et al., 2011).
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Yokoyama, T., Nagai, Y., Fukai, R., Hirata, T. (2019) Origin and Evolution of Distinct Molybdenum Isotopic Variabilities within Carbonaceous and Noncarbonaceous Reservoirs. The Astrophysical Journal 883, 62. https://doi.org/10.3847/1538-4357/ab39e7
Show in context Additionally, the Mo isotope compositions of meteorites can be modified by parent body processes, including thermal metamorphism (Yokoyama et al., 2019), and presumably aqueous alteration, as has been observed for the isotopic composition of the highly siderophile element Os (Yokoyama et al., 2011).
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Yokoyama, T., Nagashima, K., Nakai, I., Young, E.D., Abe, Y., et al. (2023a) Samples returned from the asteroid Ryugu are similar to Ivuna-type carbonaceous meteorites. Science 379, eabn7850. https://doi.org/10.1126/science.abn7850
Show in context This observation was confirmed by the initial analyses of returned samples; the mineralogical, chemical, and isotopic characteristics of the Ryugu samples showed similarities to carbonaceous chondrites, in particular the Ivuna-type (CI) group (Hopp et al., 2022; Moynier et al., 2022; Nakamura et al., 2022; Paquet et al., 2023; Nakamura et al., 2023; Yokoyama et al., 2023a).
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The melting of the ice led to the hydration of the inner portion of the parent body, causing pervasive aqueous alteration that resulted in the precipitation of secondary minerals, including phyllosilicates, carbonates, oxides, sulfides, and phosphates (Nakamura et al., 2022; Nakamura et al., 2023; Yokoyama et al., 2023a).
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Isotopic compositions of Cr, Ti, Fe, and Zn have been reported for Ryugu samples, all of which are generally consistent with those of CI chondrites (Hopp et al., 2022; Paquet et al., 2023; Yokoyama et al., 2023a).
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These observations suggest that the parent bodies of Ryugu and CI chondrites formed in a common source reservoir in the outer Solar System. Remarkably, both Ryugu materials and CI chondrites show variable ɛ54Cr values (ɛ54Cr = [(54Cr/52Cr)sample/(54Cr/52Cr)standard – 1] × 104; ranging from +1.22 ± 0.06 to +2.22 ± 0.13 for Ryugu and from +0.84 ± 0.14 to +1.94 ± 0.12 for CI chondrites), which exceed the range of analytical uncertainties (Kadlag et al., 2019; Williams et al., 2020; Nakamura et al., 2022; Yokoyama et al., 2023a, b).
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The Ryugu sample plots on the CC line on the ɛ94Mo-ɛ95Mo diagram (Fig. 2a), which is consistent with the observation that Ryugu samples are characterised by CC-type ɛ50Ti-ɛ54Cr isotopic systematics (Yokoyama et al., 2023a, b).
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In addition to the isotopic mismatch, the chemical data for equivalent bulk samples provide no evidence for over-sampling of chondrules or refractory inclusions (Yokoyama et al., 2023a).
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Yokoyama T., Wadhwa, M., Iizuka, T., Abe Y., Aléon J., et al. (2023b) Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample. Science Advances 9, eadi7048. https://doi.org/10.1126/sciadv.adi7048
Show in context These observations suggest that the parent bodies of Ryugu and CI chondrites formed in a common source reservoir in the outer Solar System. Remarkably, both Ryugu materials and CI chondrites show variable ɛ54Cr values (ɛ54Cr = [(54Cr/52Cr)sample/(54Cr/52Cr)standard – 1] × 104; ranging from +1.22 ± 0.06 to +2.22 ± 0.13 for Ryugu and from +0.84 ± 0.14 to +1.94 ± 0.12 for CI chondrites), which exceed the range of analytical uncertainties (Kadlag et al., 2019; Williams et al., 2020; Nakamura et al., 2022; Yokoyama et al., 2023a, b).
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The isotopic heterogeneity was most likely caused by physicochemical fractionation between 54Cr-rich presolar grains and Cr-bearing secondary minerals during aqueous alteration in the parent bodies (Yokoyama et al., 2023b).
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The Ryugu sample plots on the CC line on the ɛ94Mo-ɛ95Mo diagram (Fig. 2a), which is consistent with the observation that Ryugu samples are characterised by CC-type ɛ50Ti-ɛ54Cr isotopic systematics (Yokoyama et al., 2023a, b).
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Yokoyama et al. (2023b) reported that Ryugu materials and CI chondrites show variable ɛ54Cr values and suggested that fluid driven decoupling via parent body aqueous alteration between Cr in chemically labile phases with a slightly negative ɛ54Cr value, and 54Cr-rich Cr oxide nanoparticles, resulted in mm scale ɛ54Cr variability in the bulk Ryugu samples and CI chondrites.
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One Ryugu sample (C0002) examined by Yokoyama et al. (2023b) was characterised by a substantially higher ɛ54Cr value compared to four other Ryugu samples (A0106, A0106-A0107, C0107, and C0108), presumably due to the low abundance of the secondary minerals (e.g., dolomite) in the sample.
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Supplementary Information
The Supplementary Information includes:
- Experiments
- Results of Allende
- Mass balance calculation
- Table S-1 to S-4
- Supplementary Information References
Download the Supplementary Information (PDF)
Figures
Figure 1 Molybdenum isotopic compositions of Ryugu and Allende analysed in this study (solid lines). The dotted lines represent comparison bulk meteorite data for Allende (green) and Orgueil (blue) from previous studies (Table S-1). Note that 100Mo was not measured in this study.
Figure 2 Molybdenum isotope diagrams of bulk meteorites and the Ryugu sample. (a) The solid lines are the regression lines of CC (blue, from Budde et al., 2019
Budde, G., Burkhardt, C., Kleine, T. (2019) Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth. Nature Astronomy 3, 736–741. https://doi.org/10.1038/s41550-019-0779-y
) and NC (red, from Spitzer et al., 2020Spitzer, F., Burkhardt, C., Budde, G., Kruijer, T. S., Morbidelli, A., Kleine, T. (2020) Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites. The Astrophysical Journal 898, L2. https://doi.org/10.3847/2041-8213/ab9e6a
) meteorites. (b), (c), (d) Reference data of bulk carbonaceous chondrites as listed in Table S-1. Black lines are extensions of the mixing lines between a representative terrestrial sample (origin) and the theoretical s-process Mo isotopic composition calculated by Stephan et al. (2019)Stephan, T., Trappitsch, R., Hoppe, P., Davis, A.M., Pellin, M.J., Pardo, O.S. (2019) Molybdenum Isotopes in Presolar Silicon Carbide Grains: Details of s-process Nucleosynthesis in Parent Stars and Implications for r- and p-processes. The Astrophysical Journal 877, 101. https://doi.org/10.3847/1538-4357/ab1c60
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