Measurement of Microscopic Thermal Diffusivity Distribution for Ryugu Sample by Infrared Lock-in Periodic Heating Method

Abstract

The thermophysical properties of small Solar System bodies are essential to be determined, on which the thermal evolution of small bodies largely depends. The carbonaceous asteroid Ryugu is one of the small undifferentiated bodies formed in the early Solar System. Hayabusa2 explored the asteroid Ryugu and returned the surface samples in 2020 for detailed on-ground investigation, including measurements of thermal properties. Because the available sample amount was limited, this study developed a novel method to measure the thermal diffusivity of small and irregularly shaped samples of about 1 mm in diameter by combining lock-in thermography and periodic heating methods on the microscale. This method enables us to measure the thermal diffusivity of both flat-plate and granular shape samples by selecting the suitable detecting direction of the temperature response. Especially, when the sample has a flat-plate shape, the anisotropic distribution of the in-plane thermal diffusivity can be evaluated. This method was applied to six Ryugu samples, and the detailed anisotropic distribution of the thermal diffusivity was obtained. The measurement results showed that the samples show local thermal anisotropy caused by cracks and voids. The average thermal diffusivity among all samples was (2.8 to 5.8) × 10⁻⁷ m²·s. Based on the density and specific heat of the samples obtained independently, the thermal effusivity was estimated to be 791 J·(s^1/2·m²·K) to 1253 J·(s^1/2·m²·K), which is defined as the resistance of surface temperature to the change of thermal input. The determined thermal effusivity, often called thermal inertia in planetary science, is larger than the observed value of 225 ± 45 J· (s^1/2·m²·K) of the asteroid Ryugu's surface, obtained from the diurnal temperature change of the rotating asteroid by a thermal infrared camera onboard Hayabuas2. This difference is likely to be attributed to the difference in the analytical scale between the sample and the surface boulders compared with the thermal diffusion length. Consequently, it was found that the present result is more representative of the thermal diffusivity and thermal inertia of local part of individual Ryugu particles.

Appendix

Figures 11, 12, 13, 14, 15, 16 and 17 show the thermal diffusivity distribution and experimental plots of phase lag vs radial distance with a linear approximation line used in thermal diffusivity analysis at the angles of 0° and 90° as representative angles. When analyzing the slope of the phase lag with respect to the radial distance, the range of phase lag used in the analysis in each direction was fixed to be constant to eliminate arbitrariness in the selection of the analysis region and to maintain the same quality of the scattering of the results. The upper and lower limits of the phase lag range used in the analysis are indicated by the red plots in figures (b, c). The slope is calculated by a linear approximation for the phase lag in this range. The linear approximation line of the A0026 sample shown in Fig. 15 does not seem to fit to the whole experimental plots, this is due to the following. Figure 15b and c show that the change of the phase lag decreases around the distance of 0.2 to 0.4 mm, and this is possibly due to the effect of the reflected temperature wave at the sample edge (0°.) or on the back surface on the sample (90°). The fitting analysis was, therefore, performed for the phase lag plots only in the region closer to the heating point.

Fig. 11

(a) thermal diffusivity distribution of the C0002-plate 3 sample and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°

Fig. 12

(a) thermal diffusivity distribution of the C0002-plate 4 sample measured under vacuum pressure and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°

Fig. 13

(a) thermal diffusivity distribution of the C0002-plate 4 sample measured under atmospheric pressure and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°

Fig. 14

(a) thermal diffusivity distribution of the C0025 sample and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°

Fig. 15

(a) thermal diffusivity distribution of the A0026 sample and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°

Fig. 16

(a) thermal diffusivity distribution of the C0033 sample and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°

Fig. 17

(a) thermal diffusivity distribution of the C0002-plate 3 sample and experimental plots of phase lag vs distance and a linear approximation line used to obtain the thermal diffusivity at the angle of (b) 0° and (c) 90°