Chondrule-like objects and Ca-Al-rich inclusions in Ryugu may potentially be the oldest Solar System materials

Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are (16)O-rich and -poor with Δ(17)O (=δ(17)O – 0.52 × δ(18)O) values of ~ –23‰ and ~ –3‰, resembling what has been proposed as early generations of chondrules. The (16)O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into (16)O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are (16)O-rich with Δ(17)O of ~ –23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body.