RESEARCH ◥ Multicolor optical observations revealed RESEARCH ARTICLE SUMMARY that Ryugu possesses the average spectrum of a Cb-type asteroid and lacks a ubiquitous 0.7-µm absorption band. These spectral obser- ASTEROIDS vations and a principal components analysis suggest that Ryugu originates from the Eulalia or Polana asteroid family in the inner main Thegeomorphology,color,and belt,possiblyviamorethanonegenerationof parent bodies. thermal properties of Ryugu: Ryugu’s geometric albedo at 0.55 µm is 4.5 ± 0.2%, among the lowest in the Solar Implications for parent-body processes System. Moderately dehydrated carbonaceous chondrites and interplanetary dust particles S. Sugita*, R. Honda, T. Morota, S. Kameda, H. Sawada, E. Tatsumi, M. Yamada, ◥ (IDPs) are the only me- ON OUR WEBSITE C. Honda, Y. Yokota, T. Kouyama, N. Sakatani, K. Ogawa, H. Suzuki, T. Okada, teoritic samples with sim- N. Namiki, S. Tanaka, Y. Iijima, K. Yoshioka, M. Hayakawa, Y. Cho, M. Matsuoka, Read the full article ilarly low albedos. The N. Hirata, N. Hirata, H. Miyamoto, D. Domingue, M. Hirabayashi, T. Nakamura, at http://dx.doi. high boulder abundance T. Hiroi, T. Michikami, P. Michel, R.-L. Ballouz, O. S. Barnouin, C. M. Ernst, org/10.1126/ and the spectral proper- science.aaw0422 ties of the boulders are S. E. Schröder, H. Kikuchi, R. Hemmi, G. Komatsu, T. Fukuhara, M. Taguchi, T. Arai, .................................................. H. Senshu, H. Demura, Y. Ogawa, Y. Shimaki, T. Sekiguchi, T. G. Müller, consistent with dehydrated Downloaded from A. Hagermann, T. Mizuno, H. Noda, K. Matsumoto, R. Yamada, Y. Ishihara, H. Ikeda, surface materials, which might be analogous H. Araki, K. Yamamoto, S. Abe, F. Yoshida, A. Higuchi, S. Sasaki, S. Oshigami, to thermally metamorphosed meteorites. ’ S. Tsuruta, K. Asari, S. Tazawa, M. Shizugami, J. Kimura, T. Otsubo, H. Yabuta, The spectra of Ryugu s surfaces occupy a S. Hasegawa, M. Ishiguro, S. Tachibana, E. Palmer, R. Gaskell, L. Le Corre, small area in the dehydration track of our R. Jaumann, K. Otto, N. Schmitz, P. A. Abell, M. A. Barucci, M. E. Zolensky, F. Vilas, principal component space, suggesting that a ’ F. Thuillet, C. Sugimoto, N. Takaki, Y. Suzuki, H. Kamiyoshihara, M. Okada, K. Nagata, largevolumeofRyugusoriginalparentbody M. Fujimoto, M. Yoshikawa, Y. Yamamoto, K. Shirai, R. Noguchi, N. Ogawa, F. Terui, experienced similar degrees of partial dehy- http://science.sciencemag.org/ S. Kikuchi, T. Yamaguchi, Y. Oki, Y. Takao, H. Takeuchi, G. Ono, Y. Mimasu, dration. Such uniformity is more consistent K. Yoshikawa, T. Takahashi, Y. Takei, A. Fujii, C. Hirose, S. Nakazawa, S. Hosoda, with internal heating on the parent body than O. Mori, T. Shimada, S. Soldini, T. Iwata, M. Abe, H. Yano, R. Tsukizaki, M. Ozaki, heating due to multiple impacts. Nevertheless, K. Nishiyama, T. Saiki, S. Watanabe, Y. Tsuda it is possible that global partial dehydration could result from impacts if the parent body sustained many impacts before its catastrophic INTRODUCTION: The asteroid 162173 Ryugu disruption. Geochemical analyses of thermally is the target of the Japanese Hayabusa2 metamorphosed meteorites are consistent with mission, which is designed to collect samples short-term heating; thus, this scenario cannot from Ryugu’s surface and return them to be readily discarded. on April 25, 2019 Earth. We seek to understand Ryugu’sfor- A third possibility is that Ryugu is covered mation from a parent body, both to better with materials that experienced only incipient explain the origin of near-Earth asteroids and aqueous alteration, possibly similar to some to provide context for analyzing the samples. IDPs. If so, the spectral trend observed in Theoretical calculations indicate that Ryugu- Ryugu’s boulders may be a progression of size asteroids are likely produced through cata- aqueous alteration. strophic disruption of a parent body, formed in the early Solar System, whose fragments then CONCLUSION: Multiple scenarios remain vi- reaccumulated. Ryugu later migrated from the able, but the Hayabusa2 remote-sensing data main asteroid belt to its current near-Earth orbit. are most consistent with parent-body partial dehydration due to internal heating. This RATIONALE: Hayabusa2 rendezvoused with scenario suggests that asteroids formed from theasteroidinJune2018.Detailedglobal ≤ Hayabusa2’s shadow on the surface of materials that condensed at 150 K (the H2O observations of Ryugu were conducted with asteroid Ryugu. The shadow of the solar condensation temperature under typical solar Hayabusa2’s remote-sensing instruments, in- panels spans 6 m. The bright halo is due to nebula conditions) musthaveeitherformed cluding the optical navigation cameras (ONCs), the opposition effect, which enhances the sufficiently early to contain high concentrations laser altimeter [light detection and ranging 26 reflectance at small solar phase angles. of radiogenic species, such as Al, or formed (LIDAR) altimeter], and a thermal infrared near the Sun, where they experienced other camera (TIR). We examined the asteroid’s We estimate that the impact craters pene- heating mechanisms. The degree of internal surface colors, geomorphological features, and trating the top 10 meters of Ryugu’s surface heating would constrain the location and/or thermal properties to constrain models of its have existed for 107 to 108 years, indicating that timing of the snow line (the dividing line be- formation. the last major resurfacing likely occurred while tween H2O condensation and evaporation) in Ryugu was still located in the main asteroid the early Solar System. ▪ RESULTS: Geologic features on Ryugu include belt. In contrast, the low number density of a circum-equatorial ridge, an underlying east- small craters (~10 m in diameter) suggests a The list of author affiliations is available in the full article online. ð≲ 6 Þ *Corresponding author: Email: [email protected] west dichotomy, high boulder abundance, im- very young resurfacing age 10 years for the Cite this article as S. Sugita et al., Science 364, eaaw0422 pact craters, and large-scale color uniformity. top 1-meter layer. (2019). DOI: 10.1126/science.aaw0422 Sugita et al., Science 364, 252 (2019) 19 April 2019 1of1 RESEARCH ◥ its surface. Detailed global observations of Ryugu RESEARCH ARTICLE were conducted with Hayabusa2’s remote-sensing instruments, including the optical navigation cam- eras (ONCs), one of which is the nadir-viewing ASTEROIDS telescopic camera (ONC-T), with seven narrow- band filters [0.40 mm (ul), 0.48 mm (b), 0.55 mm (v), 0.59 mm (Na), 0.70 mm (w), 0.86 mm (x), and Thegeomorphology,color,and 0.95 mm (p)] (3–5); a laser altimeter [light de- tection and ranging (LIDAR) altimeter] (6); and a thermal infrared camera (TIR) (7), sensitive to thermal properties of Ryugu: wavelengths from 8 to 12 mm(8). Global images were obtained from the home Implications for parent-body processes position located 20 km above the asteroid (9), from which we constructed a 0.55-mm map of S. Sugita1,2*, R. Honda3, T. Morota4, S. Kameda5, H. Sawada6, E. Tatsumi1, Ryugu (Fig. 1A). Major geomorphologic fea- M. Yamada2, C. Honda7, Y. Yokota6,3, T. Kouyama8, N. Sakatani6, K. Ogawa9, tures visible in this map include impact craters, H. Suzuki10, T. Okada6,1, N. Namiki11,12, S. Tanaka6,12, Y. Iijima6†, K. Yoshioka1, boulders, troughs, and an equatorial ridge (Fig. M. Hayakawa6, Y. Cho1, M. Matsuoka6, N. Hirata7, N. Hirata9, H. Miyamoto1, 1B and table S3). D. Domingue13, M. Hirabayashi14, T. Nakamura15, T. Hiroi16, T. Michikami17, Impact craters P. Michel18, R.-L. Ballouz6,19, O. S. Barnouin20, C. M. Ernst20, S. E. Schröder21, H. Kikuchi1, R. Hemmi1, G. Komatsu22,2, T. Fukuhara5, M. Taguchi5, T. Arai23, Impact crater morphologies, including rim and Downloaded from H. Senshu2, H. Demura7, Y. Ogawa7, Y. Shimaki6, T. Sekiguchi24, T. G. Müller25, floor characteristics, provide indicators of surface A. Hagermann26, T. Mizuno6, H. Noda11, K. Matsumoto11,12, R. Yamada7, Y. Ishihara6‡, age and mechanical properties. Approximately 30 circular depressions ≥20 m in diameter have been H. Ikeda27, H. Araki11, K. Yamamoto11, S. Abe28, F. Yoshida2, A. Higuchi11, S. Sasaki29, identified on Ryugu, many (at least half) with S. Oshigami11, S. Tsuruta11, K. Asari11, S. Tazawa11, M. Shizugami11, J. Kimura29, raised rims (Fig. 1A). Several craters also exhibit T. Otsubo30, H. Yabuta31, S. Hasegawa6, M. Ishiguro32, S. Tachibana1, E. Palmer13, bowl-like shapes (Fig. 2, A and B), whereas others R. Gaskell13, L. Le Corre13, R. Jaumann21, K. Otto21, N. Schmitz21, P. A. Abell33, 34 33 13 18 1 1 have shallow floors (fig. S10). The bowl-shaped http://science.sciencemag.org/ M. A. Barucci , M. E. Zolensky , F. Vilas , F. Thuillet , C. Sugimoto , N. Takaki , depressions are classified on the basis of rim mor- 1 1 1 8 6 6,12 Y. Suzuki , H. Kamiyoshihara , M. Okada , K. Nagata , M. Fujimoto , M. Yoshikawa , phology and shape, providing confidence levels 6,12 6 6 6 6 6 Y. Yamamoto , K. Shirai , R. Noguchi , N. Ogawa , F. Terui , S. Kikuchi , (CLs) to their identification as impact craters. CL1 6 1 1 6 27 6 T. Yamaguchi §, Y. Oki , Y. Takao , H. Takeuchi , G. Ono , Y. Mimasu , features are circular with a clearly identifiable 27 6 6,27 6 27 6 K. Yoshikawa , T. Takahashi , Y. Takei , A. Fujii , C. Hirose , S. Nakazawa , rim, CL2 depressions are circular but exhibit no S. Hosoda6, O. Mori6, T. Shimada6, S. Soldini6, T. Iwata6,12, M. Abe6,12, H. Yano6,12, rim, CL3 depressions are quasi-circular, and CL4 R. Tsukizaki6, M. Ozaki6,12, K. Nishiyama6, T. Saiki6, S. Watanabe4,6, Y. Tsuda6,12 features are circular patterns of boulders with no clear topography. CL1 and CL2 depressions are The near-Earth carbonaceous asteroid 162173 Ryugu is thought to have been produced most likely impact craters. The group of CL3 and from a parent body that contained water ice and organic molecules. The Hayabusa2 CL4 features may include a few craters.