巽 瑛理

The carbonaceous asteroid Ryugu has been explored by the Hayabusa2 spacecraft to elucidate the actual nature of hydrous asteroids. Laboratory analyses revealed that the samples from Ryugu are comparable to unheated CI carbonaceous chondrites; however, reflectance spectra of Ryugu samples and CIs do not coincide. Here, we demonstrate that Ryugu sample spectra are reproduced by heating Orgueil CI chondrite at 300°C under reducing conditions, which caused dehydration of terrestrial weathering products and reduction of iron in phyllosilicates. Terrestrial weathering of CIs accounts for the spectral differences between Ryugu sample and CIs, which is more severe than space weathering that likely explains those between asteroid Ryugu and the collected samples. Previous assignments of CI chondrite parent bodies, i.e., chemically most primitive objects in the solar system, are based on the spectra of CI chondrites. This study indicates that actual spectra of CI parent bodies are much darker and flatter at ultraviolet to visible wavelengths than the spectra of CI chondrites.

Abstract Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu’s surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu’s global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa’s spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids.

The zodiacal light (ZL) is sunlight scattered by interplanetary dust (IPD) in the optical wavelengths. The spatial distribution of IPD in the Solar system may hold an important key to understanding the evolution of the Solar system and material transportation within it. The IPD number density can be expressed as n(r)∼r^{−α}, and the result of α∼1.3 was obtained by the previous observations from the interplanetary space by Helios 1/2 and Pioneer 10/11 in the 1970s and 1980s. However, no direct measurements of α based on the ZL observation from the interplanetary space outside the Earth's orbit have been conducted since then. Here we introduce the initial result of the ZL radial profile at optical wavelengths observed at 0.76-1.06 au by ONC-T with Hayabusa2# mission in 2021-2022. The obtained ZL brightness is well reproduced by the model brightness, but there is a small excess of the observed ZL brightness over the model brightness at around 0.9 au. The obtained radial power-law index is α=1.30±0.08, which is consistent with the previous results based on the ZL observations. The dominant uncertainty source in α arises from the uncertainty in the Diffuse Galactic Light estimation.

In the framework of the Visible NEAs Observations Survey (ViNOS) that uses several telescopes at the Canary Islands observatories since 2018, we observed two super fast rotator NEAs, 2021 NY₁ and 2022 AB. We obtained photometry and spectrophotometry of both targets and visible spectroscopy of 2022 AB. Light curves of 2021 NY₁ obtained in 4 different nights between Sept. 30 and Oct. 16, 2021 return a rotation period P = 13.3449 ± 0.0013 minutes and a light curve amplitude A = 1.00 mag. We found that 2021 NY₁ is a very elongated super fast rotator with an axis ratio a/b ≥ 3.6. We also report colours (g - r) = 0.664 ± 0.013, (r - i) = 0.186 ± 0.013, and (i - z_s) = -0.117 ± 0.012 mag. These are compatible with an S-type asteroid. The light curves of 2022 AB obtained on Jan. 5 and Jan. 8, 2021 show a rotation period P = 3.0304 ± 0.0008 minutes, with amplitudes A = 0.52 and A = 0.54 mag. 2022 AB is also an elongated object with axis ratio a/b ≥ 1.6. The obtained colours are (g - r) = 0.400 ± 0.017, (r - i) = 0.133 ± 0.017, and (i - z_s) = 0.093 ± 0.016. These colours are similar to those of the X-types, but with an unusually high (g - r) value. Spectra obtained on Jan. 12 and Jan. 14, 2022, are consistent with the reported colours. The spectral upturn over the 0.4 - 0.6 $\mu \mathrm{m}$ region of 2022 AB does not fit with any known asteroid taxonomical class or meteorite spectrum, confirming its unusual surface properties....

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37 degrees +/- 10 degrees C, about 5.2(-0.7)(+0.8) million (statistical) or 5.2(-2.1)(+1.6) million (systematic) years after the formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above similar to 100 degrees C. The samples have a chemical composition that more closely resembles that of the Sun's photosphere than other natural samples do.

Context. Observational and instrumental difficulties observing small bodies below 0.5 μm make this wavelength range poorly studied compared with the visible and near-infrared. Furthermore, the suitability of many commonly used solar analogues, essential in the computation of asteroid reflectances, is usually assessed only in visible wavelengths, while some of these objects show spectra that are quite different from the spectrum of the Sun at wavelengths below 0.55 μm. Stars HD 28099 (Hyades 64) and HD 186427 (16 Cyg B) are two well-studied solar analogues that instead present spectra that are also very similar to the spectrum of the Sun in the wavelength region between 0.36 and 0.55 μm. <BR /> Aims: We aim to assess the suitability in the near-ultraviolet (NUV) region of the solar analogues selected by the team responsible for the asteroid reflectance included in Gaia Data Release 3 (DR3) and to suggest a correction (in the form of multiplicative factors) to be applied to the Gaia DR3 asteroid reflectance spectra to account for the differences with respect to the solar analogue Hyades 64. <BR /> Methods: To compute the multiplicative factors, we calculated the ratio between the solar analogues used by Gaia DR3 and Hyades 64, and then we averaged and binned this ratio in the same way as the asteroid spectra in Gaia DR3. We also compared both the original and corrected Gaia asteroid spectra with observational data from the Eight Color Asteroid Survey (ECAS), one UV spectrum obtained with the Hubble Space Telescope (HST) and a set of blue-visible spectra obtained with the 3.6 m Telescopio Nazionale Galileo (TNG). By means of this comparison, we quantified the goodness of the obtained correction. <BR /> Results: We find that the solar analogues selected for Gaia DR3 to compute the reflectance spectra of the asteroids of this data release have a systematically redder spectral slope at wavelengths shorter than 0.55 μm than Hyades 64. We find that no correction is needed in the red photometer (RP, between 0.7 and 1 μm), but a correction should be applied at wavelengths below 0.55 μm, that is in the blue photometer (BP). After applying the correction, we find a better agreement between Gaia DR3 spectra, ECAS, HST, and our set of ground-based observations with the TNG. <BR /> Conclusions: Correcting the near-UV part of the asteroid reflectance spectra is very important for proper comparisons with laboratory spectra (minerals, meteorite samples, etc.) or to analyse quantitatively the UV absorption (which is particularly important to study hydration in primitive asteroids). The spectral behaviour at wavelengths below 0.5 μm of the selected solar analogues should be fully studied and taken into account for Gaia DR4....

Context. After landing on C-type asteroid Ryugu, MASCOT imaged brightly colored, submillimeter-sized inclusions in a small rock. Hayabusa2 successfully returned a sample of small particles from the surface of Ryugu, but none of these appear to harbor such inclusions. The samples are considered representative of Ryugu. Aims. To understand the apparent discrepancy between MASCOT observations and Ryugu samples, we assess whether the MASCOT landing site, and the rock by implication, is perhaps atypical for Ryugu. Methods. We analyzed observations of the MASCOT landing area acquired by three instruments on board Hayabusa2: a camera (ONC), a near-infrared spectrometer (NIRS3), and a thermal infrared imager. We compared the landing area properties thus retrieved with those of the average Ryugu surface. Results. We selected several areas and landforms in the landing area for analysis: a small crater, a collection of smooth rocks, and the landing site itself. The crater is relatively blue and the rocks are relatively red. The spectral and thermophysical properties of the landing site are very close to those of the average Ryugu surface. The spectral properties of the MASCOT rock are probably close to average, but its thermal inertia may be somewhat higher. Conclusions. The MASCOT rock can also be considered representative of Ryugu. Some of the submillimeter-sized particles in the returned samples stand out because of their atypical spectral properties. Such particles may be present as inclusions in the MASCOT rock.

<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.

ONC (Optical Navigation Camera; 光学航法カメラ) は探査機はやぶさ２の目であり， リュウグウを訪れた際には科学的にも工学的にも広報的にも幅広く活⽤された．ONCは広域撮像⽤のONC-W1，ONC-W2，望遠カメラ且つ7色のバンドパスフィルターをもつONC-Tで構成されている．ONC-Tは科学観測において特に重要で，フィルターを活⽤し⼩惑星表⾯の色の変化を記載することや解像度の高い画像から詳細な地形の観測を目的としている．試料採取地点の選定にも，粒径や風化作⽤の推定といった核となる情報を得て貢献してきた．本稿では，今後のサンプル分析を見据えて，主にONCチームのONC-Tを⽤いた分光観測における活動とその結果として得られた“仮説”を振り返りたいと思う．