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Thermal properties of C-type asteroid Ryugu have been investigated through remote sensing using the Thermal Infrared Imager (TIR), on the surface using the radiometer MARA on MASCOT lander, and the analysis of return sample. The global average and the local distribution of thermal inertia were mapped by TIR observations, with the lower thermal inertia than that of typical chabonaceous chondrite meteorites. surface boulders and their surroudings have almost the same thermal inertia of 200 to 400 J m-2 kg-1 s-0.5 (tiu, hereafter), indicating that most of boulders are relatively porous and not completely consolidated and the surroundings are covered with boulders and rocks (not sandy regolith) [Okada et al., 2020), which was confirmed during the descent operations for sampling. Boulders have a variety of thermal inertia, with more than 80 % of boulders with 200 to 400 tiu, while some portions have very low or very high thermal inertias. They are identified as Hot Spots and Cold Spors, because they are exceptionally hot or cold compared with their surroundings (Sakatani et al., 2021). The surface experiment by MARA indicated the simiar thermal inertia for a single boulder (Grott et al., 2019), so that the rough boulders should be the representative material on the asdteroid. The thermal properties of return sample do not seem to be the same, although they are not always representative regaring the physical properties since fragile sample might have been broken during the impact sampling process, during the severe shock and shaking in the return capsule when entry to the Earth surface. The return sample dseem to be more consolidated and flatter surface feature, instead of fragile and porous features. The return sample are more like CI chondrite meteorites with darker, more porous and fragile characteristics (Tada et al., 2021). The difference of thermal inertia between la arger scale (> 1mm) and a small scale (

Thermal Infrared Imager TIRI onboard Hera is now being developed for the investigation of the binary asteroid Didymos and Dimorphos in the European Space Agency Hera mission. This instrument uses a heritage from the Thermal Infrared Imager TIR on Hayabusa2, which observed the C-type asteroid 162173 Ryugu. In the Hera mission, its updated version of instrument is based on Lynred PICO 1024 Gen2 bolometer array (1024 x 768 effective pixels) with almost 4 times higher spatial resolution than TIR. The wavelength range shows 8-14 µm for the thermal average images, as well as 7.8, 8.6, 9.6, 10.6, 13.0 deg for multi-bands). Its FOV covers 13.3 x 10.0 deg with the spatial resolution of 0.013 deg/pixel. TIRI is now in the test process to check its full functions and performances as well. The calibration for TIRI is to check its focus, to conduct radiometric and geometrical corrections, identification of materials (like terrestrial rocks and meteorites). A set of apparatus were prepared by the TIRI PI team using the collimator (Aperture: 200mm dia., temperature range of -20 to 150℃, Focal position is 60 deg or higher), the flat cavity blackbody ( 175 x 175 mm, -20 to 125 ℃ for fact change of temperature), and a cold plate plus the cryocooler for the target sample (room temperature to -123 deg). There are some rocks and meteorites to be set as the multiband test samples at the target material, With these information and the presentation, details of the TIRI preparation and development are shown. The test results for TIRI will be presented with some data set and physical properties using the sample.

The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU$$_3$$ 3 ) 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 $$0.5^\circ$$ 0 . 5 ∘ by $$0.5^\circ$$ 0 . 5 ∘ maps. The estimated thermal inertia, for instance, was approximately 300$$\sim$$ ∼ 350 Jm$$^{-2}$$ - 2 s$$^{-0.5}$$ - 0.5 K$$^{-1}$$ - 1 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.

We present a numerical method for simulating a disk-resolved thermal image of an asteroid with small-scale roughness. In our method, we carry out numerical thermal evolution model of a small but rough area taking into account its latitude, shadowing effect, and re-absorption of the thermal radiation by neighbor. By visualization of the resulting temperature distribution for an observation direction, we obtain the thermal flux from the area as a function of the observation direction. Then thermal image of an asteroid with random topography is constructed. The resulting daytime temperature evolution profile is different from the well-known parabolic shape due to the surface roughness, implying that the daytime temperature evolution profile is a diagnostic to evaluate the surface roughness. Although this model is inapplicable to a morphologically complex asteroid such as Itokawa, the target body of Hayabusa2, Ryugu is generally convex and suitable for application of our model. Furthermore, the study presents predictions of the location shift of Ryugu trajectory after one orbital rotation due to the thermal moment caused by the rebound force from thermally emitted photons known as the Yarkovsky effect. This model is thus verifiable by precise calculation of the ephemeris of Ryugu.

著者人数: 66名 (所属. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS): 岩田, 隆浩; 安部, 正真; 大竹, 真紀子; 仲内, 悠佑; 松岡, 萌; L. Riu; 中藤, 亜衣子; 横田, 康弘; 坂谷, 尚哉; 山本, 幸生; 岡田, 達明; 澤田, 弘崇, 早川, 雅彦; 白井, 慶; 嶌生, 有理; 尾川, 順子; 照井, 冬人; 竹井, 洋; 佐伯, 孝尚; 中澤, 暁; 田中, 智; 吉川, 真; 渡邊, 誠一郎; 津田, 雄一), Accepted: 2019-03-07, 資料番号: SA1180385000

Thermal imaging, or thermography, has revealed the surface physical state of the C-type near-Earth asteroid 162173 Ryugu (Okada et al., 2020). The asteroid is the target body of JAXA Hayabsua2 asteroid sample return mission, and it has been characterized through remote sensing and surface experiments, and will be deeply and accurately investigated by analysis of returned sample. Thermal observations are among such multi-scale observations, providing a new insight into understanding planetary evolution process. Thermal infrared imager TIR (Okada et al., 2017; 2020) was used to take one-rotation global thermal images of Ryugu at every 6° step, from the home position (20 km altitude) or from the Mid-Altitude (5 km altitude). There were two big surprises contrary to the predictions before arrival at Ryugu: i) flat diurnal temperature profiles compared to the case of non-rough surface, and ii) non-cold spots identified for most of boulders. The flat diurnal temperature profiles and its maximum temperature in a day indicate that Ryugu must have very rough surfaces made of highly porous materials, derived from the thermal inertia of 300 ± 100 J K-1s-0.5m-2 (hereafter, tiu). Non-cold boulders indicate that boulders are less consolidated or compacted than typical carbonaceous chondrite meteorites, and shows the same thermophysical properties as the surroundings. TIR was also used to take close-up thermal images during the descent operations, and to have proven that the surface of asteroid is covered with fragments of porous rocks, larger than several centimeters in diameter. The typical size of fragments larger than thermal skin depth (~35 mm) results in similar thermal properties between the boulders and their surroundings. We also consider the surface roughness effect (Shimaki et al., 2020) to obtain the maps of thermal inertia ( 225 ± 45 tiu) and the roughness (0.41 ± 0.05) at the same time, corresponding to very rough surfaces made of highly-porous materials. This thermal inertia is basically consistent with the value (282 +93/-35 tiu) by in situ measurement using a thermal radiometer MARA on MASCOT lander (Grott et al., 2019). Furthermore, in the close-up thermal images, there were found boulders colder by 20 °C or more, indicating the thermal inertia of typical carbonaceous chondrite meteorites. Considering these results, we proposed a formation scenario of Ryugu: fluffy cosmic dusts gathered to form porous planetesimals, and then much larger sized but still porous bodies. A low degree of consolidation and alteration has occurred at most of the body, while a higher degree of consolidation or alteration proceeded at the deep interior. Huge meteoritic impacts destroyed and fragmented the bodies, and part of those fragments were re-accreted to form the next generation, rubble-pile bodies (asteroids). Boulders found on Ryugu might have originated from the deep interior of parent bodies, so that most of them are very porous and less consolidated but some of them are relatively dense materials similar to carbonaceous chondrites, which might have originated from the interior. Due to YORP effect, the rotation rate decreased to current one, and the current shape of a spinning top-shape were formed. Analysis of returned sample will make progress in our knowledge of the planetary formation process.

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.

Accepted: 2017-03-15, 資料番号: SA1170013000

著者人数: 37名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 岡田, 達明; 坂谷, 尚哉; 田中, 智; 矢野, 創), Number of authors: 37 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Okada, Tatsuaki; Sakatani, Naoya; Tanaka, Satoshi; Yano, Hajime）, Accepted: 2019-06-04, 資料番号: SA1190164000

International audience; 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 bodies1

著者人数: 120名(所属. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS): 澤田, 弘崇; 横田, 康弘; 坂谷, 尚哉; 岡田, 達明; 田中, 智; 飯島, 祐一; 早川, 雅彦; 松岡, 萌; R.-L. Ballouz; 嶌生, 有理; 水野, 貴秀; 石原, 吉明; 長谷川, 直; 藤本, 正樹; 吉川, 真; 山本, 幸生; 白井, 慶; R. Noguchi; 尾川, 順子; 照井, 冬人; 菊地, 翔太; 山口, 智宏; 竹内, 央, 三桝, 裕也; 高橋, 忠輝; 竹井, 洋; 藤井, 淳; 中澤, 暁; 細田, 聡史; 森, 治; 嶋田, 貴信; Soldini, Stefania; 岩田, 隆浩; 安部, 正真; 矢野, 創; 月崎, 竜童; 尾崎, 正伸; 西山, 和孝; 佐伯, 孝尚; 渡邊, 誠一郎; 津田, 雄一), Accepted: 2019-03-12, 資料番号: SA1180387000

TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m−2 s−0.5 K−1, which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.

The X-ray spectrometer (XRS) on the SELENE (SELenological and ENgineering Explorer) spacecraft, XRS, will observe fluorescent X-rays from the lunar surface. The energy of the fluorescent X-ray depends on the elements of which the lunar soil consists, therefore we can determine elemental composition of the upper most lunar surface. The XRS consists of three components: XRF-A, SOL-B, and SOL-C. XRF-A is the main sensor to observe X-rays from the lunar surface. SOL-B is direct monitor of Solar X-ray using Si-PIN photodiode. SOL-C is another Solar X-ray monitor but observes the X-rays from the standard sample attached on the base plate. This enables us to analyze by a comparative method similar to typical laboratory XRF methods. XRF-A and SOL-C adopt charge coupled device as an X-ray detector which depletion layer is deep enough to detect X-rays. The X-ray spectra were obtained by the flight model of XRS components, and all components has been worked well to analyze fluorescent X-rays. Currently, development of the hardware and software of the XRS has been finished and we are preparing for system integration test for the launch.

To establish the preparation method of colloidal suspensions with low viscosity and long term stability against sedimentation, the effects of polymer and surfactant are examined for silica suspensions. The suspensions are flocculated by bridging mechanism, in which one polymer chain is simultaneously adsorbed onto two particles. Since the adsorption affinity of polymer for the silica surfaces is governed by a balance between polymer and surfactant concentrations, two bridging conformations, that is, the reversible bridging and irreversible bridging are induced. In suspensions flocculated by reversible bridging, the interparticle bonds are constantly forming, breaking, and re-forming in a quiescent state and the suspensions behaves as fluids even at very low shear rates. The sedimentation stability is evaluated through the time dependence of intensities of backscattering lights from the suspensions. The fluctuation of backscattering intensity is detected for suspensions flocculated by reversible bridging. The local motion and rearrangement of particles may be possible in the sediment. The reversible bridging is very attractive as a new technique of controlling the viscosity and sedimentation stability of flocculated suspensions.

著者人数: 13名, Accepted: 2017-8-18, 資料番号: SA1170090000

Chemically treated Si(111) surfaces in aqueous HF (1.5%), NH4F (40%), and HBF4 (42%) solutions at 20 °C have been studied using spectroellipsometry (SE), ex situ atomic force microscopy (AFM), and contact‐angle measurement techniques. The SE data clearly indicate that the solutions cause the removal of the native SiO2 film with etch rates of ∼72 (HF), ∼13 (NH4F), and ∼8 A/min (HBF4), respectively. The SE data also indicate that when the native oxide film is completely etch removed, the resulting surface is slightly roughened. The roughened surface thicknesses are estimated to be ∼3 A, in reasonable agreement with the AFM rms values. The measured contact angles suggest that the HF‐ and NH4F‐treated surfaces are hydrophobic, while the HBF4‐treated surface is, if anything, hydrophilic.

The decay kinetics of the 7.6 eV optical absorption band of hydroxy-free silica glasses during heating in oxygen gas flow was determined. It was found that annihilation of oxygen-deficient centers, ODC(I)s, agrees with the tarnishing reaction model. The absorption cross-section of that center, σ, was determined to be 7.8 ± 0.5 × 10 −17 cm 2 . This value is close to 7.5 × 10 −17 cm 2 which was previously determined spectroscopically by hydrogen-gas treatment of oxygen-deficient silica glass.

Accepted: 2006-04-20, 資料番号: SA1000462000

X-ray fluorescence spectrometry of asteroid 25143 Itokawa was performed by the x-ray spectrometer onboard Hayabusa during the first touchdown on 19 November 2005. We selected those data observed during relatively enhanced solar activity and determined average elemental mass ratios of Mg/Si = 0.78 ± 0.09 and Al/Si = 0.07 ± 0.03. Our preliminary results suggest that Itokawa has a composition consistent with that of ordinary chondrites, but primitive achondrites cannot be ruled out. Among ordinary chondrites, LL- or L-chondrites appear to be more likely than H-chondrites. No substantial regional difference was found on the asteroid surface, indicating its homogeneity in composition.

著者人数：11名, 資料番号: SA1000968000

Accepted: 2007-12-02, 資料番号: SA1000246000

The x-ray fluorescence spectrometer (XRS) on board Japanese lunar polar orbiter SELENE (Kaguya) will provide global distribution of major elemental composition on the lunar surface in energy range of characteristic K-α x-ray line emission for Mg, Al, Si, Ca, Ti, and Fe. These measurements will contribute to research of lunar origin and its evolution. The XRS shows a good energy resolution within 200 eV at 5.9 keV relying on charge coupled device (CCD) as photon energy dispersive detector. Total collective area of 100 cm2 for main detector facing the lunar surface is composed of 16 CCD chips. Instrumentation of the XRS and its performance evaluated in laboratory are presented.

This thesis reports results on the major elemental composition ratios of asteroid 25143 Itokawa through remote X-ray fluorescence spectroscopy using the X-ray fluorescence spectrometer (XRS) onboard the Japanese asteroid spacecraft Hayabusa. Much more detailed and plausible results are provided than in the previous preliminarystudy (Okada et al. Science 312, 1338-1341, 2006), utilizing newly released spacecraft's position and attitude data. With these results, a relationship between asteroids and meteorites is discussed as well as the origin and evolution ofthe asteroid. Hayabusa successfully carried out in situ observations of 25143 Itokawa, an S (IV)-class near-Earth asteroid. The primary scientific goal of the mission is to unveil an asteroid-meteorite connection and consequently apply the evidence of meteorites to the solar system evolution. A fundamental question remains unsolved for decades whether asteroids are truly parent bodies of meteorites or not. There has been a "paradox" in asteroid science that reflectance spectra of S-class asteroids, most abundantly discovered in the solar system, have features more reddened, darkened, and shallower in absorption band depth than those of ordinary chondrite meteorites, most frequently found on earth. On the other hand, these spectral features rather resemble those of less frequently found iron-rich stony iron meteorites. Some explanations were proposed that S-class asteroids are substantially enriched in iron or they are intrinsically like ordinary chondrites but only the uppermost surface has been altered by some processes like space weathering. The XRS observed X-ray fluorescence emission from the uppermost surface materials of Itokawa, excited by solar X-rays, in order to determine major elemental composition of surface of the asteroid. This is the second in situ asteroid X-ray fiuorescence observation in history next to the Near-Earth Asteroid Rendezvous Shoemaker mission (Trombka et al., 2000; Nittler et al., 2001), and the first one with the advanced instruments based on charge-coupled device (CCD). In this study, composition of major elements such as Mg, Al, Si, S, Ca, and Fe was estimated and the relation of the asteroid to some possible candidates of meteorite-analogues was investigated. For that purpose, analyses of the XRS data have been carefully carried out. The failure of two reaction wheels of the spacecraft caused severe disturbance of the attitude control and significantly reduced duration to view the asteroid by the XRS. Many observational data include cosmic X-ray backgrounds and solar wind particles other than instrumental noises. Effective energy resolution was also degraded due to frequent thermal changes of the XRS. Thus, XRF spectra were extracted by subtraction of all these effects. Only a few percents of useful data out of three month observation were finally extracted for further detailed analysis under appropriate conditions. Considering the instrumental characteristics, intensities of XRF line spectra were determined by fitting with the response function of each CCD chip that has been evaluated in the laboratory. Temporal fluctuation of incident solar X-rays as well as regional variation of surface geomorphology of Itokawa were also the important points considered. Geometric corrections were carried out using the shape model developed by Aizu University (Demura et al., 2006) and the updated spacecraft's position and attitude data relative to the asteroid. A key result of analysis indicates that major elemental composition of the surface materials of Itokawa resembles that of ordinary chondrite meteorite-analogues. In particular, the results show that (1) elemental mass ratio of Fe/Si is somewhat larger than that of the average composition of ordinary chondrites, but its corrected result with mineral mixing effect indicates the ratio is considered almost equal to it, (2) elemental mass ratio of Mg/Si is globally homogeneous on the surface of Itokawa, neglecting the occurrence of substantial chemical differentiation, (3) in spite of large uncertainties, elemental mass ratios of best-fit Al/Si and S/Si may slightly vary in local areas, and the corrected results with mineral mixing effect suggest that elemental mass ratio of S/Si is less than, or almost the same as that of average composition of ordinary chondrites, indicating slight processes underwent on the surface. Asteroid 25143 Itokawa, an S (IV)-class of the taxonomy by Gaffeyet al. (1993), is attempted to be classified as meteorite-analogues referring to the elemental mass ratios of Mg/Si, Al/Si, S/Si, Ca/Si and Fe/Si. The results indicate that iron and stony iron meteorites such as IAB, pallasites, and mesosiderites are ruled out due to its Fe/Si. This fact strongly supports the idea that the reddened and darkened features as well as shallower absorption band depth found in visible and near infrared reflectance spectra is not by substantial amount of iron but by the alteration due to uppermost surface processes such as space weathering. Additionally, these elemental composition ratios suggest that most probable candidates for Itokawa are ordinary chondrite meteorites, especially L- or LL-chondrites. But other types of meteorites including EL-chondrites, or primitive achondrites such as Brachinites, Acapulcoites or Winonaites still cannot be ruled out when Mg/Si vs. Fe/Si are compared. These results imply that Itokawa is like a primitive meteorite. Regional distribution of S/Si found on local areas implies that some surface alteration might occur on Itokawa. However, sulfur does not seem globally depleted and has regional distribution. Only the surface material on Itokawa might be altered but the inner materials are expected to remain fresh and primordial. Resurfacing on Itokawa due to seismic destruction by impacts and/or tidal destruction by planet encounters (Miyamoto, et al., 2007) could cause such surface heterogeneity. Low average density and global compositional homogeneity of Itokawa implies that the asteroid is a rubble-pile (Fujiwara et al., 2006) and assembled by compositiomally homogeneous rocks originated from the parent bodies.