Yukio YAMAMOTO

<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.

Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth's atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid's boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m-2 s-0.5 K-1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.

<jats:p><jats:italic>Context.</jats:italic> Starting from late June 2018, the JAXA asteroid sample return mission Hayabusa2 acquired a large quantity of resolved images and spectra of the surface of the asteroid (162173) Ryugu.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> By studying the visible and near-infrared spectral behavior across the surface of Ryugu using a statistical analysis, we aim to distinguish spectral homogeneous groups and to detect the small heterogeneities. This allows us to better constrain the surface composition variations.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> In order to isolate and interpret the difference in the asteroid surface spectral behavior, we applied the <jats:italic>G</jats:italic>-mode multivariate statistical analysis to a set of pixels containing information of (i) the visible ONC-T spectrophotometry, and (ii) the near-infrared NIRS3 spectra thereby obtaining automatic statistical clustering at different confidence levels.</jats:p> <jats:p><jats:italic>Results.</jats:italic> The analysis of both ONC-T and NIRS3 data allows us to highlight small spectral variations on the Ryugu surface. At a 3<jats:italic>σ</jats:italic> confidence level, only two groups are evident, while going down to 2<jats:italic>σ</jats:italic> more groups are obtained with differences in spectral slope and band depth.</jats:p> <jats:p><jats:italic>Conclusions.</jats:italic> The identified groups have been associated with main morphological surface features. The spectral slope variations that characterize the small groups obtained by ONC-T data analysis, are interpreted as a consequence of space weathering with the presence of more or less fresh material and/or the different grain sizes of the regolith. The variations found analyzing the NIRS3 data are attributed to slightly different contents of hydrated material and different regolith sizes. The distribution on the Ryugu surface of the groups obtained by the analysis of the two instruments indicates a clear spectral dichotomy both between the east and west, and the north and south hemispheres. Small sized regolith grains associated to the redder spectra seem concentrated in the southwestern part of the body.</jats:p>

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>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.