Sound velocities of Anorthite at high pressures and temperatures: Implications for estimating porosity in upper lunar crust

The porosity of the lunar crust serves as the key to deciphering crucial geological processes such as the Moon's impact history, volcanic activity, space weathering, and the formation and evolution of its internal structure. In this study, we conducted ultrasonic interferometry experiments on polycrystalline anorthite samples under high pressures and temperatures (up to 5.7 GPa and 873 K) using synchrotron radiation technology. We obtained the relationships between the compressional wave velocity (V_P) and shear wave velocity (V_S) of anorthite with varying pressure and temperature. The elastic properties of anorthite were fitted as follows: $K_{S0}=94.71\left(7\right)\mathrm{GPa},K_S^{\prime }=1.60\left(5\right),\partial K_S/\partial T=-0.004\left(7\right)\mathrm{GPa}/K,G_0=46.37\left(3\right)\mathrm{GPa},G^{\prime }=0.87\left(1\right),\partial G/\partial T=-0.007\left(1\right)\mathrm{GPa}/K$. By integrating these results with the lunar velocity model derived from the Apollo missions, we constructed a porosity model for the upper lunar crust, using anorthite as a representative mineral. The results indicate that the thickness of the lunar regolith (weathered surface layer) is approximately 30 km. Within the top 1 km of the lunar surface, porosity ranges from 30 % to 90 %. At the depth of 1–30 km, porosity ranges from 0 % to 50 %. At the depth of 30–50 km, porosity is less than 10 %. This study provides constraints on the porosity of the lunar surface and offers scientific guidance for the safety and design of future lunar exploration missions.