17:15 〜 18:45
[SIT14-P02] Origin of ancient magnetic field of the Moon decuced from electrical conductivity of ilmenite
キーワード：月、磁場、電気伝導度、マグマオーシャン、イルメナイト
Planetary bodies produce magnetic fields through convection of molten metals in a planetary core. The constant flow of electrically conductive material can produce a magnetic field known as a core dynamo, which protects the surface from the sun’s most dangerous radiation. Rocks returned to Earth during NASA’s Apollo program from 1968 to 1972 have provided that some rocks have formed in the presence of a strong magnetic field. However, it wasn’t clear how a Moon-sized body could have generated a magnetic field. The Moon lost a magnetic field, and models of its core suggest that it was probably too small and lacked the convective force to have ever produced a continuously strong magnetic field. In the case of the early Moon, the mantle surrounding the core wasn't much cooler than the core itself. Scheinberg et al. (2018) proposed the early lunar dynamo was driven by convection in a basal magma ocean formed from the final stages of an early lunar magma ocean; this material is expected to be dense, radioactive, and metalliferous. This model assumed that basal magma had sufficiently electrically conducting.
Very early in its history, the Moon is thought to have been covered by an ocean of molten rock. As the vast magma ocean began to cool and solidify, minerals like olivine and pyroxene that were denser than the liquid magma sank to the bottom, while less dense minerals like anorthosite floated to form the crust. The remaining liquid magma was rich in titanium as well as heat-producing elements like thorium, uranium and potassium, so it took a bit longer to solidify. When this titanium layer finally crystallized just beneath the crust, it was denser than the earlier-solidifying minerals below it. Over time, the titanium formations sank through the less-dense mantle rock underneath, a process known as gravitational overturn. When this dense titanium-rich layer settled at the core-mantle boundary, it can melt and convect, producing a dynamo until it solidifies. Electrical conductivity is one of key physical parameters to control dynamo of the planetary interior.
In this study, we measured the electrical conductivity of Ti-rich Lunar basalt and ilmenite, which may exist at the bottom of the Moon, at 5 GPa. The measured electrical conductivity of lunar basalt was about 100 S/m even under melting conditions, but the electrical conductivity of ilmenite was about 10^4 S/m. According to Scheinberg et al. (2018), a lunar basal magma ocean requires electrical conductivity on the order of 10^4 S/m to generate a dynamo. This study suggests that if a layer of molten ilmenite existed at the core-mantle boundary early in the moon's history, it could have sustained the magnetic field until this layer solidifies and no longer flows
Very early in its history, the Moon is thought to have been covered by an ocean of molten rock. As the vast magma ocean began to cool and solidify, minerals like olivine and pyroxene that were denser than the liquid magma sank to the bottom, while less dense minerals like anorthosite floated to form the crust. The remaining liquid magma was rich in titanium as well as heat-producing elements like thorium, uranium and potassium, so it took a bit longer to solidify. When this titanium layer finally crystallized just beneath the crust, it was denser than the earlier-solidifying minerals below it. Over time, the titanium formations sank through the less-dense mantle rock underneath, a process known as gravitational overturn. When this dense titanium-rich layer settled at the core-mantle boundary, it can melt and convect, producing a dynamo until it solidifies. Electrical conductivity is one of key physical parameters to control dynamo of the planetary interior.
In this study, we measured the electrical conductivity of Ti-rich Lunar basalt and ilmenite, which may exist at the bottom of the Moon, at 5 GPa. The measured electrical conductivity of lunar basalt was about 100 S/m even under melting conditions, but the electrical conductivity of ilmenite was about 10^4 S/m. According to Scheinberg et al. (2018), a lunar basal magma ocean requires electrical conductivity on the order of 10^4 S/m to generate a dynamo. This study suggests that if a layer of molten ilmenite existed at the core-mantle boundary early in the moon's history, it could have sustained the magnetic field until this layer solidifies and no longer flows