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.

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Spin-polarized band structure of the three-dimensional quantum spin Hall insulator Bi 1−x Sb x ͑x = 0.12-0.13͒ was fully elucidated by spin-polarized angle-resolved photoemission spectroscopy using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. Between the two timereversal-invariant points, ⌫ and M , of the ͑111͒ surface Brillouin zone, a spin-up band ͑⌺ 1 Ј band͒ was found to cross the Fermi energy only once, providing unambiguous evidence for the strong topological insulator phase. The observed spin-polarized band dispersions determine the "mirror chirality" to be −1, which agrees with the theoretical prediction based on first-principles calculations.The spin Hall effect, which makes it possible to produce spin currents without magnet, has recently attracted a lot of attention for its potential impact on future spintronics. 1,2 The spin Hall effect has also stimulated physicists not only to understand the intriguing phenomena, but also to extend the theoretical frameworks to the "quantum spin Hall" ͑QSH͒ effect, 3-6 which is realized in a topologically nontrivial electronic state, as in the case of the quantum Hall effect. The theories of QSH effect have been constructed for two and three dimensions, and experimentalists have already attempted to obtain evidence for those topologically nontrivial states of matter. The QSH phase in two dimensions is gapped in the bulk but is gapless for edge modes. Unlike the spin itself, the spin current is time-reversal invariant. Hence, these modes carry spin currents without breaking time-reversal symmetry. Furthermore, the edge modes are robust against disorder or modest changes in boundary conditions, such as surface roughness or nonmagnetic impurities. The Z 2 topological number , which represents the number of Kramers pairs in the edge states, characterizes this topological protection 7,8 in two dimensions and distinguishes the topological insulator ͑ =1͒ from an ordinary insulator ͑ =0͒. This two-dimensional ͑2D͒ QSH phase has been theoretically proposed to be realized in bismuth bilayers 5 and in CdTe/HgTe/CdTe quantum wells. 9 In the latter case, the edge states were indeed observed in recent transport experiments. 10