山本 幸生

<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>The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU<inline-formula><alternatives><tex-math>$$_3$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow /> <mml:mn>3</mml:mn> </mml:msub> </mml:math></alternatives></inline-formula>) from June 2018 to November 2019. Our previous reports revealed that the surface of Ryugu was globally filled with porous materials and had high surface roughness. These results were derived from making the observed temperature maps of TIR using a projection method onto the shape model of Ryugu as geometric corrections. The pointing directions of TIR were calculated using an interpolation of data from the SPICE kernels (NASA/NAIF) during the periods when the optical navigation camera (ONC) and the light detection and ranging (LIDAR) observations were performed. However, the mapping accuracy of the observed TIR images was degraded when the ONC and LIDAR were not performed with TIR. Also, the orbital and attitudinal fluctuations of Hayabusa2 increased the error of the temperature maps. In this paper, to solve the temperature image mapping problems, we improved the correction method by fitting all of the observed TIR images with the surface coordinate addressed on the high-definition shape model of Ryugu (SFM 800k v20180804). This correction adjusted the pointing direction of TIR by rotating the TIR frame relative to the Hayabusa2 frame using a least squares fit. As a result, the temperature maps spatially spreading areas were converged within high-resolved <inline-formula><alternatives><tex-math>$$0.5^\circ$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msup> <mml:mn>5</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math></alternatives></inline-formula> by <inline-formula><alternatives><tex-math>$$0.5^\circ$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:msup> <mml:mn>5</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math></alternatives></inline-formula> maps. The estimated thermal inertia, for instance, was approximately 300<inline-formula><alternatives><tex-math>$$\sim$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>∼</mml:mo> </mml:math></alternatives></inline-formula>350 Jm<inline-formula><alternatives><tex-math>$$^{-2}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math></alternatives></inline-formula>s<inline-formula><alternatives><tex-math>$$^{-0.5}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msup> </mml:math></alternatives></inline-formula>K<inline-formula><alternatives><tex-math>$$^{-1}$$</tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mrow /> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math></alternatives></inline-formula> at the hot area of the Ejima Saxum. This estimation was succeeded in case that the surface topographic features were larger than the pixel scale of TIR. However, the thermal inertia estimation of smooth terrains, such as the Urashima crater, was difficult because of surface roughness effects, where roughness was probably much smaller than the pixel scale of TIR.

<title>Abstract</title> The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU3) from June 2018 to November 2019. In this study, we performed a geometric correction for TIR images by making a one-to-one correspondence between the observed areas and the surface coordinates derived from a shape model of Ryugu. The pointing direction, which is an alignment direction of TIR, was adjusted by rotating the TIR frame relative to the base of the Hayabusa2 frame using a least-squares fit. This geometric correction allows us to identify observed local areas within one pixel, which corresponds to 5 m error in a 5 km altitude observation. The corrected temperature images projected onto the shape model were constructed. Hot temperature regions were found at the base of Ejima Saxum and Otohime Saxum, for instance. A simulation result indicates that multiple radiations from the surrounding terrains generate hot regions. The estimated thermal inertia of the base of Ejima Saxum as characteristic shape area is approximately 300 Jm -2 s -0.5 K -1 within the error bars of the observed temperature profile. This estimation is succeeded by performing the geometric correction in case that the surface topographic features are greater than the spatial resolution of the pixel. However, thermal inertia estimations of smooth terrains, such as the floor of Urashima crater, were difficult probably because of surface roughness effects. Our results suggest the necessity to develop a hybrid thermophysical model that implements large- and small-scale surface roughness.

The near-Earth asteroid 162173 Ryugu, the target of the Hayabusa2 sample-return mission, is thought to be a primitive carbonaceous object. We report reflectance spectra of Ryugu's surface acquired with the Near-Infrared Spectrometer (NIRS3) on Hayabusa2, to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 micrometers was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, which is consistent with Ryugu being a compositionally homogeneous rubble-pile object generated from impact fragments of an undifferentiated aqueously altered parent body.

月周回衛星「かぐや」に搭載されたハイビジョンカメラ(HDTV)は600以上の映像を取得した。広報目的に加え科学的な価値を見出すため、他の観測機器と同様に科学データとしてアーカイブを行った。HDTV映像は標準で30fpsの60秒間の1800枚のフレームから構成され、2倍速、4倍速、8倍速で撮像可能である。データアーカイブでは動画をフレームに分割して静止画として扱い、撮像時刻、動作モード、衛星位置、撮像範囲等を含め、総計100万枚以上の静止画にメタデータの付与を行った。ファイルフォーマットは天文学分野標準のFITSを用い、ディレクトリ構造やメタデータ記述は惑星探査データの標準規格であるPlanetary Data System (PDS)を用いた。FITSとPDSは親和性が高く2つの基準を満たすことが容易である。そのためHDTVデータは天文学分野と惑星探査分野の両方のツールを利用することが可能なハイブリッド仕様である。本発表ではHDTVデータの長期保存への取り組みをその背景と共に報告するものである。

The existence of lightning discharges in the Venus atmosphere has been controversial for more than 30 years, with many positive and negative reports published. The lightning and airglow camera (LAC) onboard the Venus orbiter, Akatsuki, was designed to observe the light curve of possible flashes at a sufficiently high sampling rate to discriminate lightning from other sources and can thereby perform a more definitive search for optical emissions. Akatsuki arrived at Venus during December 2016, 5 years following its launch. The initial operations of LAC through November 2016 have included a progressive increase in the high voltage applied to the avalanche photodiode detector. LAC began lightning survey observations in December 2016. It was confirmed that the operational high voltage was achieved and that the triggering system functions correctly. LAC lightning search observations are planned to continue for several years.

<p> 金星探査機「あかつき」によって取得されたデータのアーカイブを 2017年 7月に公開しました．本稿ではデータアーカイブの概要と今後のリリース予定について紹介します．</p>

<p>「はやぶさ2」は，C 型小惑星リュウグウ（Ryugu）にランデブーし，母船からのリモートセンシング観測及び小型着陸機によるその場観測を行うとともに，最大3 回の表面物質サンプリングを行うこととなっている．サンプリング地点には，リュウグウそのものや母天体，さらには太陽系形成時の惑星集積過程と物質進化について，最大の情報を得られる場所を選定する必要があるが，選定のために必要となる情報はランデブー後取得されるリモートセンシング観測の結果を待たねばならない．そのため，限られた時間の中で小惑星の特徴を把握し，安全性と科学価値の評価（Landing Site Selection, LSS）を行う手順を確立しておくことは必須である．本稿では，はやぶさ2 プロジェクトが来年に迫ったLSS 本番に向けて実施したLSS 訓練について概説する．</p>

<p>We are planning to explore the caverns through the skylight holes on the Moon and Mars. The holes and their associated subsurface caverns are among the most important future exploration targets. The importance of the lunar and Martian holes and their associated caverns is categorized from two aspects: (1) fresh materials are easily observed and sampled there, and (2) the subsurface caverns provide a safe, quiet environment. The expectation of lunar and Martian hole and cavern exploration is increasing in Japan. We name the project as UZUME (Unprecedented Zipangu (Japan) Underworld of the Moon Exploration) whose name is after a Japanese mythology. The ultimate purpose of the UZUME project is to investigate how to expand human activity and survival in space and on extraterrestrial bodies. </p>

The status of the X-ray spectrometer XRS onboard SELENE (Kaguya) is reported. The XRS is to map major elemental composition of lunar surface but is subject to instrumental trouble and unexpectedly faint solar activity. However it observed X-rays from the onboard standard sample excited by solar X-rays and non-solar X-ray events, maybe by particle events.