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

DOI：

10.1126/science.aaw0422

发表时间：

2019-03

期刊：

Science

影响因子：

56.9

通讯作者：

中科院分区：

综合性期刊1区

文献类型：

作者：

S. Sugita;R. Honda;T. Morota;S. Kameda;H. Sawada;E. Tatsumi;M. Yamada;C. Honda;Y. Yokota;T. Kouyama;N. Sakatani;K. Ogawa;H. Suzuki;T. Okada;N. Namiki;S. Tanaka;Y. Iijima;K. Yoshioka;M. Hayakawa;Y. Cho;M. Matsuoka;N. Hirata;N. Hirata;H. Miyamoto;D. Domingue;M. Hirabayashi;T. Nakamura;T. Hiroi;T. Michikami;P. Michel;R. Ballouz;O. Barnouin;C. Ernst;S. Schröder;H. Kikuchi;R. Hemmi;G. Komatsu;T. Fukuhara;M. Taguchi;T. Arai;H. Senshu;H. Demura;Y. Ogawa;Y. Shimaki;T. Sekiguchi;T. Müller;A. Hagermann;T. Mizuno;H. Noda;K. Matsumoto;R. Yamada;Y. Ishihara;H. Ikeda;H. Araki;K. Yamamoto;S. Abe;F. Yoshida;A. Higuchi;S. Sasaki;S. Oshigami;S. Tsuruta;K. Asari;S. Tazawa;M. Shizugami;J. Kimura;T. Otsubo;H. Yabuta;S. Hasegawa;M. Ishiguro;S. Tachibana;E. Palmer;R. Gaskell;L. Le Corre;R. Jaumann;K. Otto;N. Schmitz;P. Abell;M. Barucci;M. Zolensky;F. Vilas;F. Thuillet;C. Sugimoto;N. Takaki;Y. Suzuki;H. Kamiyoshihara;M. Okada;K. Nagata;M. Fujimoto;M. Yoshikawa;Y. Yamamoto;K. Shirai;R. Noguchi;N. Ogawa;F. Terui;S. Kikuchi;Toshiko Yamaguchi;Y. Oki;Y. Takao;H. Takeuchi;G. Ono;Y. Mimasu;K. Yoshikawa;T. Takahashi;Y. Takei;A. Fujii;C. Hirose;S. Nakazawa;S. Hosoda;O. Mori;T. Shimada;S. Soldini;T. Iwata;M. Abe;H. Yano;R. Tsukizaki;M. Ozaki;K. Nishiyama;T. Saiki;S. Watanabe;Y. Tsuda

小行星龙宫上的隼鸟二号小行星以陨石的形式坠落到地球上，但这些陨石提供的有关其起源的信息很少。日本隼鸟二号任务旨在直接从小行星表面收集样本并将其带回地球进行实验室分析。本期的三篇论文描述了隼鸟二号团队对近地碳质小行星 162173 Ryugu 的研究，该航天器于 2018 年 6 月抵达该行星（参见 Wurm 的观点）。渡边等人。测量了这颗小行星的质量、形状和密度，表明它是由松散岩石组成的“碎石堆”，在快速旋转的前期形成了陀螺形状。他们还确定了合适的样本收集着陆点。北里等人。使用近红外光谱来寻找表面普遍存在的水合矿物，并将 Ryugu 与已知类型的碳质陨石进行比较。杉田等人。描述 Ryugu 的地质特征和表面颜色，并结合所有三篇论文的结果来限制小行星的形成过程。龙宫可能是由一颗较大的小行星撞击喷出的碎石重新堆积而形成的。这些结果为理解隼鸟二号收集的样本提供了必要的背景，这些样本预计将于 2020 年 12 月抵达地球。 268，p。 272，p。 eaaw0422；另见 p. 230 近地小行星龙宫可能是由一颗较大的母小行星撞击时碎石重新堆积而成。简介小行星 162173 Ryugu 是日本隼鸟 2 号任务的目标，该任务旨在从龙宫表面收集样本并将其带回地球。我们试图了解龙宫的母体形成过程，以便更好地解释近地小行星的起源，并为分析样本提供背景。理论计算表明，龙宫大小的小行星很可能是由早期太阳系形成的母体发生灾难性破坏而产生的，其碎片随后重新聚集。龙宫后来从小行星主带迁移到目前的近地轨道。理由 Hayabusa2 于 2018 年 6 月与小行星会合。利用 Hayabusa2 的遥感仪器对龙宫进行了详细的全球观测，包括光学导航相机 (ONC)、激光高度计 [光探测和测距 (LIDAR) 高度计] 和热红外相机 (TIR)。我们检查了小行星的表面颜色、地貌特征和热特性，以约束其形成模型。结果龙宫的地质特征包括环赤道山脊、潜在的东西二分法、高巨石丰度、撞击坑和大范围的颜色均匀性。我们估计，穿透龙宫表面 10 米的撞击坑已经存在了 107 到 108 年，这表明最后一次重大表面修复可能发生在龙宫仍位于主小行星带时。相比之下，小陨石坑（直径约 10 m）的低数量密度表明顶部 1 米层的重铺年龄非常年轻（≲106 年）。多色光学观测表明，Ryugu 拥有 Cb 型小行星的平均光谱，但缺乏普遍存在的 0.7 µm 吸收带。这些光谱观测和主成分分析表明，龙宫起源于内部主带的尤拉利亚或波拉纳小行星家族，可能经过不止一代的母体。龙宫在 0.55 µm 处的几何反照率为 4.5 ± 0.2%，是太阳系中最低的之一。中度脱水的碳质球粒陨石和行星际尘埃颗粒（IDP）是唯一具有类似低反照率的陨石样本。高巨石丰度和巨石的光谱特性与脱水的表面材料一致，这可能类似于热变质陨石。龙宫表面的光谱在我们的主成分空间的脱水轨迹中占据了一小部分区域，这表明龙宫的原始母体的大部分经历了类似程度的部分脱水。这种均匀性与母体的内部加热比由于多次冲击而产生的加热更一致。然而，如果母体在灾难性破坏之前遭受了多次撞击，则撞击可能会导致全球部分脱水。热变质陨石的地球化学分析与短期加热一致；因此，这种情况不能轻易放弃。第三种可能性是龙宫覆盖的物质仅经历了初期的水蚀变，可能类似于一些国内流离失所者。如果是这样，在龙宫巨石中观察到的光谱趋势可能是水蚀变的过程。结论多种情况仍然可行，但隼鸟二号遥感数据与母体因内部加热而部分脱水最为一致。这种情况表明，由在 ≤150 K（典型太阳星云条件下的 H2O 凝结温度）凝结的材料形成的小行星必须要么形成得足够早，以包含高浓度的放射性物质，例如 26Al，要么在太阳附近形成，在那里它们经历了其他加热机制。内部加热程度会限制早期太阳系中雪线（水凝结和蒸发之间的分界线）的位置和/或时间。隼鸟二号在小行星龙宫表面的影子。太阳能电池板的阴影跨度为6 m。明亮的光晕是由于对立效应而产生的，它增强了小太阳相位角下的反射率。近地碳质小行星 162173 Ryugu 被认为是由含有水冰和有机分子的母体产生的。隼鸟二号航天器获得了龙宫的全球彩色图像。呈现的地貌特征包括环赤道山脊、东西二分法、整个表面的高巨石丰度以及撞击坑。陨石坑的年龄估计表明，顶部 1 米层的重铺年龄为≲106 年。龙宫是太阳系中已知最暗的天体之一。巨石的高丰度和光谱特性与适度脱水的材料一致，类似于地球上发现的热变质陨石。龙宫表面颜色的总体均匀性支持了由于小行星母体内部加热而导致的部分脱水。

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.

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

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