吉川 真

<title>Abstract</title>Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.

<title>Abstract</title> C-type asteroids are considered to be primitive small Solar-System bodies enriched in water and organics, providing clues for understanding the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing and on-asteroid measurements with Hayabusa2, but further studies are expected by direct analyses of returned samples. Here we describe the bulk sample mainly consisting of rugged and smooth particles of millimeter to submillimeter size, preserving physical and chemical properties as they were on the asteroid. The particle size distribution is found steeper than that of surface boulders11. Estimated grain densities of the samples have a peak around 1350 kg m-3, which is lower than that of meteorites suggests a high micro-porosity down to millimeter-scale, as estimated at centimeter-scale by thermal measurements. The extremely dark optical to near-infrared reflectance and the spectral profile with weak absorptions at 2.7 and 3.4 microns implying carbonaceous composition with indigenous aqueous alteration, respectively, match the global average of Ryugu, confirming the sample’s representativeness. Together with the absence of chondrule and Ca-Al-rich inclusion of larger than sub-mm, these features indicate Ryugu is most similar to CI chondrites but with darker, more porous and fragile characteristics.

&amp;lt;p&amp;gt;JAXA&amp;amp;#8217;s Hayabusa2 is a sample-return mission was launched on Dec. 3, 2014 for bringing back first samples from a C-complex asteroid [1,2]. It arrived at asteroid Ryugu on June 27, 2018 and left for Earth on Nov. 13, 2019 after conducting global remote-sensing observations, two touchdown sampling operations, rover deployments, and an artificial impact experiment. We review our science results and update the mission status of Hayabusa2 in this presentation.&amp;amp;#160;&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;The global observations revealed that Ryugu has a top-shaped body with very low density (1.19&amp;amp;#177;0.02 g/cc) [3], spatially uniform Cb-type spectra without strong Fe-rich serpentine absorption at 0.7-um [4], and a weak but significant OH absorption at 2.7 um [5]. Based on these observations, we proposed that Ryugu materials may have experienced aqueous alteration and subsequent thermal metamorphism due to radiogenic heating [4]. However, other scenarios, such as impact-induced thermal metamorphism and extremely primitive carbonaceous materials before extensive alteration, were also considered because there were many new properties of Ryugu whose origins are unclear. Also, numerical calculations show that impact heating can raise the temperatures high enough to dehydrate serpentine at typical collision speed in the asteroid main belt [6]. &amp;amp;#160;&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;Further analysis using high-resolution data obtained at low-altitude descents for both rehearsal and actual touchdown operations as well as the artificial impact experiment by small carryon impactor (SCI) and landers observations the Ryugu surface on allowed us to find out what caused the properties of Ryugu. For example, subtle but distinct latitudinal variation of spectral slope in optical wavelengths found in the initial observations [4] turned out be caused by solar heating or space weathering during orbital excursion toward the Sun and subsequent erosion of the equatorial ridge owing to slowdown in Ryugu&amp;amp;#8217;s spin rate [7]. The SCI impact created a very large (~17 m in crest diameter) crater consistent with gravity-controlled scaling showing that Ryugu surface has very low intra-boulder cohesion and the Ryugu surface is very young and well mixed [8].&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;Furthermore, the MASCOT lander also showed that typical boulders on Ryugu is not covered with a layer of fine regolith [9] and yet possess very low thermal inertia (282+93/-35 MKS) consistent with highly porous structure [10]. This value is consistent with the global values or Ryugu [4, 11], suggesting that the vast majority of boulders on Ryugu are very porous. However, thermal infrared imager (TIR) also found that Ryugu has a number of &amp;amp;#8220;dense boulders&amp;amp;#8221; with high thermal inertia (&amp;gt;600 MKS) consistent with typical carbonaceous chondrites, showing that Ryugu&amp;amp;#8217;s parent body must have had a large enough gravity and pressure to compress the constituent materials [11]. This observation supports that Ryugu originated from a large parent body, such as proto-Polana and proto-Eulalia, which are estimated to be ~100 km in diameter.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;Some of the dense boulders were also covered by multi-band images of optical navigation camera (ONC-T) and turned out to have C-type spectra with albedos much higher than the Ryugu average [12]. These spectra and albedos are similar to carbonaceous chondrites heated at low temperatures. Although the total mass of these high-albedo boulders on Ryugu is estimated to be very small (&amp;lt; 1%), the spectral and albedo varieties are much greater than the bulk Ryugu surface and approximately follow the dehydration track of carbonaceous chondrites [12]. These spectral match supports that Ryugu materials experienced aqueous alteration and subsequent thermal metamorphism. The dominance of a high-temperature component and scarcity of lower temperature components are consistent with radiogenic heating in a relatively large parent body because large bodies would have only thin low-temperature thermal skin and large volume of high-temperature interior.&amp;amp;#160;&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;If radiogenic heating is really responsible for Ryugu&amp;amp;#8217;s moderate dehydration, this may place a very important constraint on the timing of the formation of Ryugu&amp;amp;#8217;s parent body. Because the radiogenic heat source for most meteorite parent bodies are likely extinct species, such as 26Al, the peak temperature is chiefly controlled by the timing of accretion [13]. Thus, high metamorphism temperatures (several hundred degrees in Celsius) of Ryugu&amp;amp;#8217;s bulk materials inferred from spectral comparison with laboratory heated CM and CI meteorites [4, 12] require Ryugu&amp;amp;#8217;s parent body formed early in the Solar System. Because Ryugu&amp;amp;#8217;s parent body contained substantial amount of water at the time of formation, it must have been formed outside the snowline. Thus, the birth place of Ryugu&amp;amp;#8217;s parent body would be a high-accretion-rate location outside the snowline.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;Recent high-precision measurements of stable isotopes of meteorites have found that there is a major dichotomy between carbonaceous chondrites (CCs) and some iron meteorites, which formed outside Jupiter&amp;amp;#8217;s orbit, and non-carbonaceous meteorites (NCs), which formed inside Jupiter&amp;amp;#8217;s orbit [e.g., 14]. If Ryugu belongs to CCs, then Ryugu materials could be form near Jupiter, where accretion could occur early. Thus, measurements of stable isotopes of elements, such as Cr, Ti and Mo, of Ryugu samples to be returned to Earth by the end of 2020 would be highly valuable for constraining the original locations of Polana or Eulalia, among the largest C-complex asteroids in the inner main belt.&amp;amp;#160;&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Acknowledgements:&amp;lt;/strong&amp;gt; This study was supported by JSPS Core-to-Core program &amp;amp;#8220;International Network of Planetary Sciences&amp;amp;#8221;, CNES, and Univ. Co?te d&amp;amp;#8217;Azur.&amp;amp;#160;&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;References:&amp;lt;/strong&amp;gt;&amp;amp;#160; [1] Watanabe et al., SSR, 208, 3-16, 2017. [2] Tsuda et at., Acta Astronaut. 91, 356-363, 2013. [3] Watanabe et al., Science, 364, 268-272, 2019. [4] Sugita et al., Science, 364, eaaw0422, 2019. [5] Kitazato et al., Science, 364, 272-275, 2019. [6] Michel et al., Nature Comm., 11, 5184, 2020. [7] Morota et al., Science, 368, 654-659, 2020. [8] Akarawa et al. Science, 368, 67-671, 2020. [9] Jaumann et al. Science, 365, 817-820, 2019.&amp;amp;#160; [10] Grott et al., Nature Astron. 3, 971-976, 2019.&amp;amp;#160; [11] Okada et al., Nature, 579, 518-522, 2020. [12] Sugimoto et al. 51st LPSC, #1770, 2020.&amp;amp;#160; [13] Grimm and McSween, Science, 259, 653-655, 1993.&amp;amp;#160; [14] Kruijer et al., PNAS, 114, 6712-6716, 2017.&amp;amp;#160;&amp;lt;/p&amp;gt;