The Hayabusa2 spacecraft arrived at the near-Earth carbonaceous asteroid 162173 Ryugu in 2018. We present Hayabusa2 observations of Ryugu’s shape, mass, and geomorphology. Ryugu has an oblate ‘spinning top’ shape with a prominent circular equatorial ridge. Its bulk density, 1.19 ± 0.02 g cm–3, indicates a high porosity (>50%) interior. Large surface boulders suggest a rubble-pile structure. Surface slope analysis shows Ryugu’s shape may have been produced if it once spun at twice the current rate. Coupled with the observed global material homogeneity, this suggests that Ryugu was reshaped by centrifugally induced deformation during a period of rapid rotation. From these remote-sensing investigations, we identify a suitable sample collection site on the equatorial ridge.

The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced from a parent body that contained water ice and organic molecules. The Hayabusa2 spacecraft has obtained global multi-color images of Ryugu. Geomorphological features present include a circum-equatorial ridge, east/west dichotomy, high boulder abundances across the entire surface, and impact craters. Age estimates from the craters indicate a resurfacing age of ≲106 years for the top 1-meter layer. Ryugu is among the darkest known bodies in the Solar System. The high abundance and spectral properties of boulders are consistent with moderately dehydrated materials, analogous to thermally metamorphosed meteorites found on Earth. The general uniformity in color across Ryugu’s surface supports partial dehydration due to internal heating of the asteroid’s parent body.

The near-Earth asteroid 162173 Ryugu, the target of Hayabusa2 sample return mission, is thought to be a primitive carbonaceous object. We report reflectance spectra of Ryugu’s surface acquired with the Near Infrared Spectrometer (NIRS3) on Hayabusa2, to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 μm was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally- and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, consistent with Ryugu being a compositionally homogeneous rubble-pile object, generated from impact fragments of an undifferentiated aqueously altered parent body.

[1] We developed a new numerical model which can simulate the thermal history and metal-silicate separation of a growing Mars. In this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating, crater excavation, and isostatic rebound. A metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature. Sinking of the metal blobs is traced assuming Stokes' velocity. Their coalescence during sinking is treated by a Monte Carlo approach. A series of simulations is carried out assuming that Mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process. Our numerical results show that (1) no global magma ocean is formed during accretion, (2) metal-silicate separation takes place without global scale melting, and (3) instead of a metallic core, a metal-rich layer is formed at the late stage of accretion.

C-type asteroids are among the most pristine objects in the solar system, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith covered surface with low thermal conductivity and particle sizes in the centimeter range. This includes observations of C-type asteroid (162173) Ryugu, for which average grainsizes of 3-30 mm have been derived 1,2,3. However, upon arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover