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

Abstract

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed seventeen Ryugu samples measuring 1-8 mm. CO 2 -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 Ca, Al-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed by aqueous alteration reactions at low temperature, high pH, and water/rock ratios < 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 Ryugu’s parent body formed ~ 2 million years after the beginning of Solar System formation.

Chondrule-like objects and CAIs in asteroid Ryugu: earlier generations of chondrules

“…Mineralogy and chemistry of the chondrule-like objects in the Ryugu samples Chondrule-like objects found in the Ryugu samples have rounded-to-spherical shapes with diameters of 10 -20 µm (Figs. 1a-c; see also Nakamura et al 23 ), which are as small as chondrule-like Wild2 particles 11 . Although remote sensing observations found mm-sized inclusions similar to chondrules on the surface of asteroid Ryugu 28 , sizes of the chondrule-like objects that we found are much smaller.…”

“…Occurrence of chondrule-like objects and CAIs in the Ryugu samples Chondrules and CAIs with sizes of ~ 100 µm -1 cm, which are typical for chondrites 1 , are not observed from the synchrotron radiation X-ray computed tomography of the bulk Ryugu samples with a resolution of 0.85 µm/voxel at BL20XU in SPring-8 (a synchrotron facility in Hyogo, Japan) 23 . A small number of chondrule-like objects and CAIs are found by elemental mapping using FE-EPMA and FE-SEM observation of 42 polished sections from the 13 Ryugu samples (52.6 mm 2 in total).…”

“…A small number of chondrule-like objects and CAIs are found by elemental mapping using FE-EPMA and FE-SEM observation of 42 polished sections from the 13 Ryugu samples (52.6 mm 2 in total). The chondrule-like objects and CAIs analyzed for oxygen isotopes occur along with isolated olivine, pyroxene and spinel grains in the polished sections of C0040-02 and C0076-10 and in less-altered clasts (clast 1 and 2) in the polished section C0002-P5 23 . Fractions of the surface areas of all chondrule-like objects and CAIs observed in the Ryugu polished sections including those reported in Nakamura et al 23 are estimated as ~ 15 ppm and 20…”

“…The Hayabusa2 spacecraft returned samples of ~ 5.4 g from C-type asteroid (162173) Ryugu 22 . The "stone" team, which is one of the six initial analysis teams, received 16 stone samples from the ISAS curation facility and conducted analyses for elucidation of early evolution of asteroid Ryugu 23 . The Ryugu samples mineralogically and chemically resemble CI chondrites [23][24][25][26] .…”

“…The "stone" team, which is one of the six initial analysis teams, received 16 stone samples from the ISAS curation facility and conducted analyses for elucidation of early evolution of asteroid Ryugu 23 . The Ryugu samples mineralogically and chemically resemble CI chondrites [23][24][25][26] . Remote sensing observations by the Hayabusa2 spacecraft suggested that asteroid Ryugu formed by reaccumulation of rubble ejected by impact from a larger asteroid 27 .…”