Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide–bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu’s parent body formed ~2 million years after the beginning of Solar System formation. Description INTRODUCTION Observations of asteroid Ryugu by the Hayabusa2 spacecraft found that it is a rubble pile, formed from fragments of a parent asteroid. Samples retrieved from Ryugu by the spacecraft were expected to contain a record of this history, including the formation and early evolution of the parent body, the subsequent impact destruction and partial reaccretion, and later space weathering. The composition of Ryugu was expected to be similar to that of Ivuna-type carbonaceous chondrite meteorites (CI chondrites). RATIONALE We investigated the formation history of Ryugu through laboratory analysis of the samples. Specifically, we sought to determine (i) when and where in the Solar System the parent asteroid formed; (ii) the original mineralogy, elemental abundances as a whole, and chemical compositions of the accreted materials, including their ice content; (iii) how these materials evolved through chemical reactions; and (iv) how the material was ejected from the parent body in an impact. To address these issues, we analyzed 17 Ryugu particles of 1 to ~8 mm in size. RESULTS We found carbon dioxide (CO2)–bearing water in an iron-nickel (Fe–Ni) sulfide crystal, indicating that the parent body formed in the outer Solar System. Remanent magnetization was detected, implying that the solar nebula might still have been present when magnetite crystals formed on the parent body. We used muon analysis to determine the abundances of light elements, including carbon (C), nitrogen (N), sodium (Na), and magnesium (Mg), whose abundances relative to silicon (Si) are similar to those in CI chondrites, whereas oxygen (O) is deficient compared with that in CI chondrites. X-ray computed tomography analysis shows that all our Ryugu samples consist of fine-grained material. There are only rare objects of high-temperature origin, such as melted silicate-rich particles, all being smaller than 100 μm. Electron microscope observations showed that the samples are breccias, assemblies of numerous small rock fragments with different compositions, mineralogies, and histories. The most common mineralogy contains Mg-rich hydrous silicates, MgCa and MgFe carbonates, hydroxyapatite, Fe sulfides, and Fe oxides. The mineralogy of this major lithology is consistent with classification as a CI chondrite. It also indicates widespread aqueous alteration (reactions with liquid water) on the parent body. Some Ryugu fragments have a different mineralogy, containing anhydrous silicates (olivine and pyroxene), amorphous silicates, Ca carbonate, phosphides, Fe–Ni sulfide, Fe oxide, and poorly crystalline phyllosilicates. Some small objects (<~30 μm) that formed at high temperatures were also found. This mineralogy suggests that these fragments experienced less aqueous alteration. We measured mechanical and thermal properties from the Ryugu samples. We found that they are similar, but not identical, to hydrated CI chondrites. Numerical simulations of the thermal history and impact disruption processes of the Ryugu parent asteroid were performed by incorporating the physical and mineralogical properties and appropriate water/rock ratios. CONCLUSION We propose that Ryugu’s parent asteroid formed ~1.8 million to 2.9 million years after the beginning of Solar System formation, in the outer Solar System, where water and CO2 were present as ice. It acquired a water ice/rock mass ratio in the range of 0.2 to 0.9. In this region, material formed at low temperatures is dominant, whereas material of high temperature origin is rare. In the interior of the parent asteroid, radioactive heating caused the water ice to melt at ~3 million years; water-rock reactions then gradually changed the initial anhydrous mineralogy to a largely hydrous mineralogy. At shallow depths, the original material was less altered, at a low water/rock ratio of <0.2. At ~5 million years, all material in the parent asteroid experienced its peak temperature, and aqueous alteration continued. An impact occurred ~1 billion years ago, disrupting the parent asteroid. Some fragments, originating away from the impact point, then reassembled to form Ryugu. Proposed model of Ryugu’s formation history. (1) A parent body asteroid forms in the outer Solar System, containing abundant ice. (2 and 3) Radioactive heating causes the ice to melt, which modifies the mineralogy through aqueous alteration reactions. (4 and 5) An impact then disrupts the parent body but does not cause widespread heating. (6) Ryugu formed from reaccumulation of ejected material that originated away from the impact point. All times were measured from the start of Solar System formation. Colors indicate estimated temperatures from our thermal interior and impact models.