Abstract: Thermal stress of the moon due to cooling process is non-negligible in lunar evolution. We calculate the accumulation of thermal stress with 3-D viscoelastic model, and explore the influence of viscosity parameters on thermal stress through comparative experiments. Numerical results suggest that the thermal stress of lithosphere is utterly distinct from deep mantle. The lithosphere is under tangential compression that concentrates at the bottom of the crust because of unevenly distributed cooling rate and elastic strength; on the other hand, the accumulation and relaxation of thermal stress in deep mantle is balanced due to low viscosity, and the thermal stress is in a “hydrostatic” state, which is mainly controlled by the elastic surface. Under the assumption that viscosity of lunar lithosphere is greater than 10²⁸ Pa·s, the tangential compressive stress in lithosphere accumulates to several hundreds of MPa in the present day, while the tensile stress in deep mantle reaches up to 100 MPa. Consequently, part of the shallow moonquake events can be explained by thermal stress. However, the focal mechanism of deep moonquakes is still unclear. We speculate that the tensile thermal stress in deep mantle helps to develop pore structures, and the melting layer provides pore fluid with high pressure, which reduces the fracture strength of mantle medium.

Key words: viscoelastic medium, finite element method, thermal stress, moonquake