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INTRODUCTION
The Hayabusa2 spacecraft made two landings on the asteroid (162173) Ryugu in 2019, during which it collected samples of the surface material. Those samples were delivered to Earth in December 2020. The colors, shapes, and morphologies of the returned samples are consistent with those observed on Ryugu by Hayabusa2, indicating that they are representative of the asteroid. Laboratory analysis of the samples can determine the chemical composition of Ryugu and provide information on its formation and history.
RATIONALE
We used laboratory analysis to inform the following questions: (i) What are the elemental abundances of Ryugu? (ii) What are the isotopic compositions of Ryugu? (iii) Does Ryugu consist of primary materials produced in the disk from which the Solar System formed or of secondary materials produced in the asteroid or on a parent asteroid? (iv) When were Ryugu’s constituent materials formed? (v) What, if any, relationship does Ryugu have with meteorites?
RESULTS
We quantified the abundances of 66 elements in the Ryugu samples: H, Li, Be, C, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Te, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Tl, Pb, Bi, Th, and U. There is a slight variation in chemical compositions between samples from the first and second touchdown sites, but the variations could be due to heterogeneity among the samples that were analyzed.
The Cr-Ti isotopes and abundance of volatile elements are similar to those of carbonaceous meteorites in the CI (Ivuna-like) chondrite group. The Ryugu samples consist of the minerals magnetite, breunnerite, dolomite, and pyrrhotite as grains embedded in a matrix composed of serpentine and saponite. This mineral assemblage and the texture are also similar to those of CI meteorites. Anhydrous silicates are almost absent, which indicates extensive liquid water–rock reactions (aqueous alteration) in the material. We conclude that the samples mainly consist of secondary materials that were formed by aqueous alteration in a parent body, from which Ryugu later formed.
The oxygen isotopes in the bulk Ryugu samples are also similar to those in CI chondrites. We used oxygen isotope thermometry to determine the temperature at which the dolomite and magnetite precipitated from an aqueous solution, which we found to be 37° ± 10°C. The 53Mn-53Cr isotopes date the aqueous alteration at
million (statistical) or
million (systematic) years after the birth of the Solar System.
Phyllosilicate minerals are the main host of water in the Ryugu samples. The amount of structural water in Ryugu is similar to that in CI chondrites, but interlayer water is largely absent in Ryugu, which suggests a loss of interlayer water to space. The abundance of structural water and results from dehydration experiments indicate that the Ryugu samples remained below ~100°C from the time of aqueous alteration until the present. We ascribe the removal of interlayer water to a combination of impact heating, solar heating, solar wind irradiation, and long-term exposure to the ultrahigh vacuum of space. The loss of interlayer water from phyllosilicates could be responsible for the comet-like activity of some carbonaceous asteroids and the ejection of solid material from the surface of asteroid Bennu.
CONCLUSION
The Ryugu samples are most similar to CI chondrite meteorites but are more chemically pristine. The chemical composition of the Ryugu samples is a closer match to the Sun’s photosphere than to the composition of any other natural samples studied in laboratories. CI chondrites appear to have been modified on Earth or during atmospheric entry. Such modification of CI chondrites could have included the alteration of the structures of organics and phyllosilicates, the adsorption of terrestrial water, and the formation of sulfates and ferrihydrites. Those issues do not affect the Ryugu samples. Those modifications might have changed the albedo, porosity, and density of the CI chondrites, causing the observed differences between CI meteorites, Hayabusa2 measurements of Ryugu’s surface, and the Ryugu samples returned to Earth.
