10:45 〜 11:00
[PPS03-06] Implication of Ryugu’s physical property by comparison of thermal diffusivity with carbonaceous chondrites
キーワード：リュウグウ、小惑星、熱物性、熱拡散率
The thermal inertia and thermal diffusivity of materials composing asteroids are crucial for conducting thermal physics simulations, investigating the temperature histories and physical states experienced by celestial bodies during their evolutionary processes. Thermal inertia can be obtained by observing temperature changes in asteroids’ surface, and it can also be acquired by measuring the thermal diffusivity and specific heat of returned samples. However, the observed thermal inertia values are significantly smaller than those extrapolated from meteorite samples. On the other hand, the results from several Ryugu samples were close to the values of meteorite samples [Ishizaki et al., Int. J. Thermophys., 2023]. This study aims to elucidate the reasons behind this discrepancy and focuses on gaining a more detailed understanding of the thermal evolution process of asteroids. In this presentation, we employ the Lock-in Thermography (LIT) periodic heating method used in the Hayabusa2 initial analysis to measure carbonaceous chondrites with petrological classifications similar to Ryugu. By comparing the results with those from Ryugu, we aim to confirm the validity of the measurements obtained during the initial analysis and enhance our understanding of the thermal properties of Ryugu particles.
In this study, the thermal diffusivities of Ryugu and fourteen grains from seven types of carbonaceous chondrites were measured. Furthermore, the physical properties of astromaterials are not only influenced by their composition but also strongly affected by the mechanical conditions within the samples, such as the distribution of internal voids. Therefore, it is crucial to simultaneously analyze and evaluate the mechanical state as a correlated property when assessing these physical values. In this study, we utilized X-ray computed tomography to visualize the internal condition of the samples and calculate the bulk volume to determine bulk density. This information was then used to assess the relationship between bulk density and thermal diffusivity. Additionally, for several meteorite samples, we conducted further measurements of elastic wave velocities and explored their implications for thermal diffusivity with respect to the elastic property.
Consequently, the results show a positive correlation between thermal diffusivity and bulk density. Ryugu grains show a thermal diffusivity-bulk density relationship more similar to the Tagish Lake than to the Ivuna sample, which has a similar elemental composition to Ryugu. This result is in agreement with the elastic properties and suggests that in carbonaceous chondrites with petrological classifications 1 or 2, thermo-mechanical properties may be dominated by mechanical and structural features consisting of the matrix, inclusions, and voids inside the grain.
When comparing the thermal diffusivity and bulk density relationship of Ryugu with those measured for carbonaceous chondrites, it was found that Ryugu aligns with the trends observed in carbonaceous chondrites. In other words, it was revealed that Ryugu does not possess a uniquely low thermal diffusivity. Therefore, the thermal inertia of the asteroid Ryugu's surface obtained from observations by Hayabusa2 indicates a much smaller value than predicted, not due to the low thermal inertia of the materials of the surface boulders themselves, but rather attributed to thermal resistance arising from cracks developing within the boulders. Since such cracks are believed to occur due to thermal fatigue , it is expected that they concentrate in the surface layer of the boulders on a scale corresponding to skin depth of diurnal temperature changes. The fact that few crack-rich grains were found in the return sample, whose effective thermal inertia matches that of the asteroid's surface, suggests that the return sample may have been finely separated from the cracks that developed on the boulder surface.
In this study, the thermal diffusivities of Ryugu and fourteen grains from seven types of carbonaceous chondrites were measured. Furthermore, the physical properties of astromaterials are not only influenced by their composition but also strongly affected by the mechanical conditions within the samples, such as the distribution of internal voids. Therefore, it is crucial to simultaneously analyze and evaluate the mechanical state as a correlated property when assessing these physical values. In this study, we utilized X-ray computed tomography to visualize the internal condition of the samples and calculate the bulk volume to determine bulk density. This information was then used to assess the relationship between bulk density and thermal diffusivity. Additionally, for several meteorite samples, we conducted further measurements of elastic wave velocities and explored their implications for thermal diffusivity with respect to the elastic property.
Consequently, the results show a positive correlation between thermal diffusivity and bulk density. Ryugu grains show a thermal diffusivity-bulk density relationship more similar to the Tagish Lake than to the Ivuna sample, which has a similar elemental composition to Ryugu. This result is in agreement with the elastic properties and suggests that in carbonaceous chondrites with petrological classifications 1 or 2, thermo-mechanical properties may be dominated by mechanical and structural features consisting of the matrix, inclusions, and voids inside the grain.
When comparing the thermal diffusivity and bulk density relationship of Ryugu with those measured for carbonaceous chondrites, it was found that Ryugu aligns with the trends observed in carbonaceous chondrites. In other words, it was revealed that Ryugu does not possess a uniquely low thermal diffusivity. Therefore, the thermal inertia of the asteroid Ryugu's surface obtained from observations by Hayabusa2 indicates a much smaller value than predicted, not due to the low thermal inertia of the materials of the surface boulders themselves, but rather attributed to thermal resistance arising from cracks developing within the boulders. Since such cracks are believed to occur due to thermal fatigue , it is expected that they concentrate in the surface layer of the boulders on a scale corresponding to skin depth of diurnal temperature changes. The fact that few crack-rich grains were found in the return sample, whose effective thermal inertia matches that of the asteroid's surface, suggests that the return sample may have been finely separated from the cracks that developed on the boulder surface.