The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes

Hayabusa2 at the asteroid Ryugu Asteroids fall to Earth in the form of meteorites, but these provide little information about their origins. The Japanese mission Hayabusa2 is designed to collect samples directly from the surface of an asteroid and return them to Earth for laboratory analysis. Three papers in this issue describe the Hayabusa2 team's study of the near-Earth carbonaceous asteroid 162173 Ryugu, at which the spacecraft arrived in June 2018 (see the Perspective by Wurm). Watanabe et al. measured the asteroid's mass, shape, and density, showing that it is a “rubble pile” of loose rocks, formed into a spinning-top shape during a prior period of rapid spin. They also identified suitable landing sites for sample collection. Kitazato et al. used near-infrared spectroscopy to find ubiquitous hydrated minerals on the surface and compared Ryugu with known types of carbonaceous meteorite. Sugita et al. describe Ryugu's geological features and surface colors and combined results from all three papers to constrain the asteroid's formation process. Ryugu probably formed by reaccumulation of rubble ejected by impact from a larger asteroid. These results provide necessary context to understand the samples collected by Hayabusa2, which are expected to arrive on Earth in December 2020. Science, this issue p. 268, p. 272, p. eaaw0422; see also p. 230 The near-Earth asteroid Ryugu probably formed by reaccumulation of rubble from an impact on a larger parent asteroid. INTRODUCTION The asteroid 162173 Ryugu is the target of the Japanese Hayabusa2 mission, which is designed to collect samples from Ryugu’s surface and return them to Earth. We seek to understand Ryugu’s formation from a parent body, both to better explain the origin of near-Earth asteroids and to provide context for analyzing the samples. Theoretical calculations indicate that Ryugu-size asteroids are likely produced through catastrophic disruption of a parent body, formed in the early Solar System, whose fragments then reaccumulated. Ryugu later migrated from the main asteroid belt to its current near-Earth orbit. RATIONALE Hayabusa2 rendezvoused with the asteroid in June 2018. Detailed global observations of Ryugu were conducted with Hayabusa2’s remote-sensing instruments, including the optical navigation cameras (ONCs), laser altimeter [light detection and ranging (LIDAR) altimeter], and a thermal infrared camera (TIR). We examined the asteroid’s surface colors, geomorphological features, and thermal properties to constrain models of its formation. RESULTS Geologic features on Ryugu include a circum-equatorial ridge, an underlying east-west dichotomy, high boulder abundance, impact craters, and large-scale color uniformity. We estimate that the impact craters penetrating the top 10 meters of Ryugu’s surface have existed for 107 to 108 years, indicating that the last major resurfacing likely occurred while Ryugu was still located in the main asteroid belt. In contrast, the low number density of small craters (~10 m in diameter) suggests a very young resurfacing age (≲106 years) for the top 1-meter layer. Multicolor optical observations revealed that Ryugu possesses the average spectrum of a Cb-type asteroid and lacks a ubiquitous 0.7-µm absorption band. These spectral observations and a principal components analysis suggest that Ryugu originates from the Eulalia or Polana asteroid family in the inner main belt, possibly via more than one generation of parent bodies. Ryugu’s geometric albedo at 0.55 µm is 4.5 ± 0.2%, among the lowest in the Solar System. Moderately dehydrated carbonaceous chondrites and interplanetary dust particles (IDPs) are the only meteoritic samples with similarly low albedos. The high boulder abundance and the spectral properties of the boulders are consistent with dehydrated surface materials, which might be analogous to thermally metamorphosed meteorites. The spectra of Ryugu’s surfaces occupy a small area in the dehydration track of our principal component space, suggesting that a large volume of Ryugu’s original parent body experienced similar degrees of partial dehydration. Such uniformity is more consistent with internal heating on the parent body than heating due to multiple impacts. Nevertheless, it is possible that global partial dehydration could result from impacts if the parent body sustained many impacts before its catastrophic disruption. Geochemical analyses of thermally metamorphosed meteorites are consistent with short-term heating; thus, this scenario cannot be readily discarded. A third possibility is that Ryugu is covered with materials that experienced only incipient aqueous alteration, possibly similar to some IDPs. If so, the spectral trend observed in Ryugu’s boulders may be a progression of aqueous alteration. CONCLUSION Multiple scenarios remain viable, but the Hayabusa2 remote-sensing data are most consistent with parent-body partial dehydration due to internal heating. This scenario suggests that asteroids formed from materials that condensed at ≤150 K (the H2O condensation temperature under typical solar nebula conditions) must have either formed sufficiently early to contain high concentrations of radiogenic species, such as 26Al, or formed near the Sun, where they experienced other heating mechanisms. The degree of internal heating would constrain the location and/or timing of the snow line (the dividing line between H2O condensation and evaporation) in the early Solar System. Hayabusa2’s shadow on the surface of asteroid Ryugu. The shadow of the solar panels spans 6 m. The bright halo is due to the opposition effect, which enhances the reflectance at small solar phase angles. 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 multicolor 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.