Thermal Diffusivity, Thermal Conductivity and Thermal Inertia of Individual Lunar Regolith Grains: Case Study of Sample 70161 from Apollo 17

The regolith on the shallow lunar surface was formed through micrometeorite impacts over time. Investigating the thermophysical properties of the regolith provides valuable insights into the thermal history of the Moon as recorded by these surface materials and offers critical data for future lunar exploration. In several studies, the thermophysical properties of the regolith layer and rocks have been examined, but few studies have focused on individual regolith particles because of their limited size and irregular shapes, which are generally believed to have formed following intense activities, such as micrometeorite impacts. In this study, the local thermal diffusivity of individual particles from Apollo 17 sample 70161 was measured via the lock-in thermography (LIT) technique, and subsequently, the distribution of in-plane thermal diffusivity was provided. The particle was confirmed to be a typical breccia using X-ray tomography (XCT) assisted by X-ray diffraction (XRD). The local average thermal diffusivity values ranged from 2.9 m²·s⁻¹ to 3.6 × 10⁻⁷ m²·s⁻¹ and showed an anisotropic distribution. In addition, we calculated the representative thermal conductivity and thermal inertia of the particles via the specific heat and density, which are 0.738 ± 0.088 W.m⁻¹·K⁻¹ (300 K) and (1.231 ± 0.086) × 10³ J·m⁻²·s^−1/2·K⁻¹ (300 K), respectively. The specific heat was also obtained by differential scanning calorimetry (DSC) of fine samples from 70161. The density was calculated from the measured weight, and the volume was determined via XCT. On the one hand, our experimental results are in good agreement with previously reported measurements of Apollo lunar rocks (in terms of average values). On the other hand, our measurements also reveal an anisotropic distribution of thermal diffusivity within localized regions of the particle. This anisotropy is attributed to factors such as cracks and defects, which locally weaken heat conduction.