Constitution of the Moon: 4. Composition of the mantle from seismic data

Abstract

The main goal of this paper is to estimate the chemical composition of the lunar upper mantle as well as the bulk composition of the silicate portion of the Moon based on a method of thermodynamic modeling (phase equilibrium calculations in the CaOFeOMgOAl 2O 3SiO 2 system) and geophysical observations including the seismic data, moment of inertia and mass of the Moon. It has been found that the upper mantle is chemically uniform and may be composed of pyroxenite containing 0.5–2 mol.% of free silica. The calculated lunar bulk silicate composition (mantle + crust) as well as the bulk composition of the entire lunar mantle generated by the geophysical data suggest that the concentrations of FeO, SiO 2 and refractory elements (Ca, Al) are significantly higher than those in the Earth's upper mantle. The Fe Si atomic ratio is equal to 0.18 for the silicate portion of the Moon and 0.22 for the Moon as a whole (crust + mantle + core); the latter value is the lowest known Fe Si ratio of any object in the solar system. Phase changes in any model considered in the dry CFMAS system are not able to explain the nature of the 270 km and 500 km discontinuities; it is concluded that the lunar mantle is chemically stratified. Thermodynamic modeling and geophysical observations allow some constraints on the internal density distribution and suggest the presence of a lunar core: 480 km in radius for the FeS-core and 310 km for the Fe-core. As compared with the Moon, an allowed range of the Fe-core and FeS-core radius for Io is estimated to be 300–680 km and 450–1050 km, respectively. Without specifying a mechanism for the origin of the Moon, we may conclude that the Earth and its satellite formed from compositionally different materials. Composition of the Moon remains unusual in comparison with the Earth and chondrites.