چگونگی تشکیل اقیانوس های بازالتی ماه: همرفتی ناشی از برخورد شهاب سنگی و تولید مذاب در جبه ماه

We present impact-induced convection models in the Moon that explain the formation of lunar mare basalt, and the long duration of the basaltic flow. A large projectile that produces an Imbrium-size impact basin can induce three stage of melting at different depths. The first occurs near the surface, by strong heating of the projectile and target that results in melting and vaporization of the major part of the projectile and near-surface part of the target. This rapid melting does not create mare basalt. The second melting arises from the sudden reduction of the pressure in the upper mantle due to excavation of the basin. The ascending upper mantle material in the process of isostatic adjustment in response to the excavation partially melts by decompression within less than a few Myr after the impact. The possible related volcanism cannot explain basaltic flow on the Moon that lasted about ٨٠٠ Myr. The local impact-induced perturbations, however, trigger global-scale convection, and a large plume from lower mantle ascends directly beneath the impact basin. This convection causes the third stage of melting in the mantle and produces well over ١٠٧ km٣ of mare basalt that give rise to substantial mare flooding. We investigate the effects of an impact on the thermal evolution of the Moon and melt production in the mantle, using convection calculations in an axi-symmetric cylindrical coordinate system. The models have temperature-dependent viscosity, and time dependent heat sources arising from decay of radioactive materials. The mantle is allowed to melt as it crosses the solidus temperature and partial melting is calculated. We consider two different models, permeable and impermeable. Five different viscosity models are examined, where the ratio between the viscosity at the surface and at the bottom of the computation domain is ١٠٠, ٥٠٠, ١٠٠٠, ١٥٠٠ and ٢٠٠٠, respectively. The permeable model with viscosity contrast of ١٠٠٠ can explain the observed amount of basaltic flow as well as the duration of the volcanic eruption.