Dept. of Solar System Sciences
基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 太陽系科学研究系 特任助教
- 連絡先
- sakatani.naoya
jaxa.jp - 研究者番号
- 70795187
- J-GLOBAL ID
- 201901019739774118
- researchmap会員ID
- B000365669
研究分野
1経歴
4-
2022年4月 - 現在
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2020年4月 - 2022年3月
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2018年4月 - 2020年3月
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2016年8月 - 2018年3月
主要な論文
80-
INTERNATIONAL JOURNAL OF THERMOPHYSICS 43(6) 2022年6月The thermal conductivity of planetary soils, or regolith, is essential for understanding the present global thermal state of bodies. The thermal conductivity of lunar soils is important with respect to lunar crustal heat flow. Although in situ measurements were performed by the Apollo 15 and 17 missions, laboratory measurements of the returned lunar samples have not reproduced the estimated subsurface values. Since the amount of extraterrestrial soil samples is limited, a small apparatus is needed to measure their thermal conductivity. In this study, we developed an apparatus enabling the measurement of the thermal conductivity of a small amount of soil (< 10 g) via the line heat source method as a function of compressional pressure under vacuum conditions. The thermal conductivity of glass beads and lunar regolith simulant derived by the new apparatus is higher than that obtained from the larger line heat source, and then, reliable apparatus. To evaluate the experimental results, we performed numerical simulation of the temperature evolution during the line heat source measurement, and found that the thermal conductivity derived from the simulation data is higher than the input thermal conductivity. This is consistent with the experimental results and is caused by the heat loss through a line heat source with a limited length. The difference depends on the contact conductance between the sample and the line heat source, and the calibration factors for each sample are determined.
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Nature Astronomy 5(8) 766-774 2021年8月24日Planetesimals—the initial stage of the planetary formation process—are considered to be initially very porous aggregates of dusts1,2, and subsequent thermal and compaction processes reduce their porosity3. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. 4–6), higher than meteorites but lower than cometary nuclei7, which are considered to be remnants of the original planetesimals8. Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (<20 m in diameter), boulders with reflectance (~0.015) lower than the Ryugu average6 and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment9 is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction3. Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations10,11.
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ICARUS 309 13-24 2018年7月Many air-less planetary bodies, including the Moon, asteroids, and comets, are covered by regolith. The thermal conductivity of the regolith is an essential parameter controlling the surface temperature variation. A thermal conductivity model applicable to natural soils as well as planetary surface regolith is required to analyze infrared remote sensing data. In this study, we investigated the temperature and compressional stress dependence of the thermal conductivity of the lunar regolith simulant JSC-1A, and the temperature dependence of sieved JSC-1A samples under vacuum conditions. We confirmed that a series of the experimental data for JSC-1A are fitted well by our analytical model of the thermal conductivity (Sakatani et al., 2017). Comparison with the calibration data of the sieved samples with those for original JSC-1A indicates that the thermal conductivity of natural samples with a wide grain size distribution can be modeled as mono-sized grains with a volumetric median size. The calibrated model can be used to estimate the volumetric median grain size from infrared remote sensing data. Our experiments and the calibrated model indicates that uncompressed JSC-1A has similar thermal conductivity to lunar top surface materials, but the lunar subsurface thermal conductivity cannot be explained only by the effects of the density and self-weighted compressional stress. We infer that the nature of the lunar subsurface regolith grains is much different from JSC-1A and lunar top-surface regolith, and/or the lunar subsurface regolith is over-consolidated and the compressional stress higher than the hydrostatic pressure is stored in the lunar regolith layer. (C) 2018 Elsevier Inc. All rights reserved.
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AIP Advances 7(1) 2017年1月1日The thermal conductivity of powdered media is characteristically very low in vacuum, and is effectively dependent on many parameters of their constituent particles and packing structure. Understanding of the heat transfer mechanism within powder layers in vacuum and theoretical modeling of their thermal conductivity are of great importance for several scientific and engineering problems. In this paper, we report the results of systematic thermal conductivity measurements of powdered media of varied particle size, porosity, and temperature under vacuum using glass beads as a model material. Based on the obtained experimental data, we investigated the heat transfer mechanism in powdered media in detail, and constructed a new theoretical thermal conductivity model for the vacuum condition. This model enables an absolute thermal conductivity to be calculated for a powder with the input of a set of powder parameters including particle size, porosity, temperature, and compressional stress or gravity, and vice versa. Our model is expected to be a competent tool for several scientific and engineering fields of study related to powders, such as the thermal infrared observation of air-less planetary bodies, thermal evolution of planetesimals, and performance of thermal insulators and heat storage powders.
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ICARUS 267 1-11 2016年3月Thermal conductivity of powdered materials under vacuum conditions is a valuable physical parameter in the context of planetary sciences. We report results of thermal conductivity measurements of 90-106 mu m and 710-1000 mu m glass beads, and lunar regolith simulant using two different experimental setups for varying the compressional stress and the temperature, respectively. We found the thermal conductivity increase with the compressional stress, for example, from 0.003 to 0.008 W m(-1) K-1 for the glass beads of 90-106 mu m in diameter at the compressional stress less than 20 kPa. This increase of the thermal conductivity is attributed the areal enlargement of the contacts between particles due to their elastic deformation. The thermal conductivity increased also with temperature, which primarily represented enhancement of the radiative heat conduction between particles. Reduction of the estimated radiative conductivity from the effective thermal conductivity obtained in the first experiment yields the relation between the solid conductivity (conductive contribution through inter-particle contacts) and the compressional stress. We found that the solid conductivity is proportional to approximately 1/3 power of the compressional stress for the glass beads samples, while the regolith simulant showed a weaker exponent than that of the glass beads. We developed a) semi-empirical expression of the thermal conductivity of the lunar regolith using our data on the lunar regolith simulant. This model enabled us to estimate a vertical distribution of the lunar subsurface thermal conductivity. Our model provides an examination for the density and compressional stress relationships to thermal conductivity observed in the in-situ measurements in Apollo 15 and 17 Heat Flow Experiments. (C) 2015 Elsevier Inc. All rights reserved.
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ICARUS 221(2) 1180-1182 2012年11月We measured the thermal conductivity of glass beads as a simple model material in order to investigate the compressional stress dependence of the thermal conductivity of powder materials. The measurements were conducted in a vertically elongated cylindrical sample container under vacuum conditions. Our results suggest that the compressional stress is one of the essential factors to understand the thermal conductivity structure in the regolith layer. (C) 2012 Elsevier Inc. All rights reserved.
MISC
21-
Icarus 400 2023年8月This paper summarizes the evidence for the optical effects of space weathering, as well as the properties of the surface that control optical changes, on asteroid (101955) Bennu. First, we set the stage by briefly reviewing what was known about space weathering of low-albedo materials from telescopic surveys, laboratory simulations, and sample return analysis. We then look at the evidence for the nature of space weathering on Bennu from recent spacecraft imaging and spectroscopy observations, including the visible to near-infrared and thermal infrared wavelengths, followed by other measurements such as normal albedo measurements from LIDAR scans. We synthesize these different lines of evidence in an effort to describe a general model of space weathering processes and resulting color effects on dark C-complex asteroids, with hypotheses that can be tested by analyzing samples returned by the mission. A working hypothesis that synthesizes findings thus far is that the optical effects of maturation in the space environment depend on the level of hydration of the silicate/phyllosilicate substrate. Subsequent variations in color depend on surface processes and exposure age. On strongly hydrated Bennu, in color imaging data, very young craters are darker and redder than their surroundings (more positive spectral slope in the wavelength range 0.4–0.7μm) as a result of their smaller particle sizes and/or fresh exposures of organics by impacts. Solar wind, dehydration, or migration of fines may cause intermediate-age surfaces to appear bluer than the very young craters. Exposed surfaces evolve toward Bennu's moderately blue global average spectral slope. However, in spectroscopic and LIDAR data, the equator, the oldest surface on Bennu, is darker and redder (wavelength range 0.55–2.0μm) than average and has shallower absorption bands, possibly due to dehydration and/or nanophase and/or microphase opaque production. Bennu is a rubble pile with an active surface, making age relationships, which are critical for determining space weathering signals, difficult to locate and quantify. Hence, the full story ultimately awaits analyses of the Bennu samples that will soon be delivered to Earth.
共同研究・競争的資金等の研究課題
3-
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日本学術振興会 科学研究費助成事業 特別研究員奨励費 2014年4月 - 2016年3月
