11:00 AM - 11:15 AM
[PPS05-02] Effects of magma-generation and transport of heat and heat producing elements by migrating magma on the thermal history of the Moon.
Keywords:The Moon, Thermal history, Magma migration, Radial expansion/contraction, Numerical analysis
Previous geological and geophysical observations of the Moon indicate its radial expansion in its early history and radial contraction in the later history. The most optimal models that explain this history commonly start from a low initial temperature in the deep mantle. However, this cold origin model is at odds with the giant-impact hypothesis. In order to resolve this difficulty, we numerically calculated the thermal history of the spherically symmetric mantle of the Moon, taking account of thermal diffusion, magma-generation, decay of heat-producing elements (HPEs), and the transport of HPEs by migration of the generated magma.
The initial temperature in the deep mantle is high enough to make the temperature in the shallow mantle equal to the solidus temperature. We considered the various amount of the melt-content in the shallow mantle. The temperature is fixed at 270K on the surface boundary, while the core is modeled as a heat bath of uniform temperature. The initial temperature on the CMB is about 1900 K. To take account of the blanket effect of the lunar crust and regolith layer, the thermal conductivity in the crust is about half that of the mantle. We also assume that the mantle is depleted in HPEs relative to the crust by a factor of 8 or 16, as suggested from studies of the Th concentration in the putative primary magmas of mare basalts.
The calculated temperature first increases and then remains equal to the solidus for billions of years due to the contribution from the latent heat of melting, and then decreases due to decay of HPEs. The melt-content increases at first and then gradually diminishes; magma migrates upward and accumulates at the top of the partially molten region. The globally averaged temperature first decreases due to conductive cooling at the base of the mantle, then rises due to internal heating, and after that, gradually decreases owing to the cooling from the surface boundary. The radius first increases mostly by melting and then decreases due to cooling and solidification of the magma in the models that initial melt-content is 0 in the entire mantle.
The calculated temperature distribution at 4.4 Gyr is consistent with the thermal structure inferred from magnetotelluric observation of the Moon.The calculated history of radial change is consistent with that inferred from observations of the lunar gravity field and lobate scarps. Realistic modeling of generation and migration of magma as well as HPE-transport by migrating magma are crucial for understanding the thermal history of the Moon.
The initial temperature in the deep mantle is high enough to make the temperature in the shallow mantle equal to the solidus temperature. We considered the various amount of the melt-content in the shallow mantle. The temperature is fixed at 270K on the surface boundary, while the core is modeled as a heat bath of uniform temperature. The initial temperature on the CMB is about 1900 K. To take account of the blanket effect of the lunar crust and regolith layer, the thermal conductivity in the crust is about half that of the mantle. We also assume that the mantle is depleted in HPEs relative to the crust by a factor of 8 or 16, as suggested from studies of the Th concentration in the putative primary magmas of mare basalts.
The calculated temperature first increases and then remains equal to the solidus for billions of years due to the contribution from the latent heat of melting, and then decreases due to decay of HPEs. The melt-content increases at first and then gradually diminishes; magma migrates upward and accumulates at the top of the partially molten region. The globally averaged temperature first decreases due to conductive cooling at the base of the mantle, then rises due to internal heating, and after that, gradually decreases owing to the cooling from the surface boundary. The radius first increases mostly by melting and then decreases due to cooling and solidification of the magma in the models that initial melt-content is 0 in the entire mantle.
The calculated temperature distribution at 4.4 Gyr is consistent with the thermal structure inferred from magnetotelluric observation of the Moon.The calculated history of radial change is consistent with that inferred from observations of the lunar gravity field and lobate scarps. Realistic modeling of generation and migration of magma as well as HPE-transport by migrating magma are crucial for understanding the thermal history of the Moon.