The chemical composition of the cores of the terrestrial planets and the moon

Using models of the quasi-chemical theory of solutions, the activity coefficients of silicon are calculated in the melts Fe-Si, Ni-Si, and Fe-Ni-Si. The calculated free energies of solution of liquid nickel and silicon in liquid iron in the interval 0 to 1400 kbar and 1500 to 4000 K, shows that Fe-Ni-Si alloy is stable under the conditions of the outer core of the earth and the cores of the terrestrial planets. The oxidation-reduction conditions are studied, and the fugacity of oxygen in the mantles of the planets and at the core-mantle boundary are calculated. The mechanism of reduction of silicon is analyzed over a broad interval of p and T. The interaction between the matter of the core and mantle is studied, resulting in the extraction of silicon from the mantle and its solution in the material of the core. It is concluded that silicon can enter into the composition of the outer core of the earth and Venus, but probably does not enter into the composition of the cores of Mercury, Mars, and the moon, if in fact the latter possesses one

PhysEth1107004KronrodLO.fm

Abstract-We model the internal structure of the Moon, initially homogeneous and later differentiated due to partial melting. The chemical composition and the internal structure of the Moon are retrieved by the Monte Carlo inversion of the gravity (the mass and the moment of inertia), seismic (compressional and shear velocities), and petrological (balance equations) data. For the computation of phase equilibrium relations and physical properties, we have used a method of minimization of the Gibbs free energy combined with a Mie Gr@uneisen equation of state within the CaO FeO MgO Al 2 O 3 -SiO 2 system. The lunar models with a different degree of constraints on the solution are considered. For all models, the geophysically and geochemically permissible ranges of seismic velocities and concentrations in three mantle zones and the sizes of Fe 10%S core are estimated. The lunar mantle is chemically stratified; different mantle zones, where orthopyroxene is the dominant phase, have different concentrations of FeO, Al 2 O 3 , and CaO. The silicate portion of the Moon (crust + mantle) may contain 3.5-5.5% Al 2 O 3 and 10.5-12.5% FeO. The chemical boundary between the middle and the lower mantle lies at a depth of 620-750 km. The lunar models with and without a chemical boundary at a depth of 250-300 km are both possible. The main parameters of the crust, the mantle, and the core of the Moon are estimated. At the depths of the lower mantle, the P and S velocities range from 7.88 to 8.10 km/s and from 4.40 to 4.55 km/s, respectively. The radius of a Fe 10%S core is 340 ± 30 km. In the present work, we suggest a new model of the constitution and internal structure of the Moon. This model is based on the hypothesis of the magma ocean; it involves modern mathematical processing of the P and S travel time data Keywords PETROLOGICAL AND GEOPHYSICAL CONSTRAINTS Models of the Magma Ocean The early differentiation of the Moon with the for mation of the continental feldspar crust, which has a thickness of about 50-60 km and ~25% Al 2 O 3 , as well as the age of the lunar rocks, have motivated the hypothesis of the Magma Ocean. The latter is com monly understood as the outer lunar shell, which had undergone partial melting By analyzing the thermoelastic stresses, Solomon [1986] showed that there is no tectonic evidence for a large scale expansion or compression of the Moon over the past four billion years (after a period of its intense bombardment). He estimated the lunar radius to have been changed by about a kilometer, which dis agrees with the concept of extensive melting. The set of petrological, geochemical, and geophysical data offers no reasons to believe that the Moon had ever been totally molten and formed a continuous magma ocean. This is also supported by the lunar asymmetry (the center of figure of the Moon is offset by 2 km from the center of mass). By analyzing the volumetric effects of differentiation of the Moon, The melting depth of 500-600 km well agrees with the experimental data on the crystallization of lunar basalts and green and picrite glass [Ringwood and Ess ene, 1970; Some information on the thickness of MO can be inferred from the geophysical data. The results of the Apollo mission infer that one or a few seismic bound aries exist in the mantle at a depth of 400-750 km In our works Seismic Data The seismic data are a kind of the Rosetta Stone for understanding the internal structure of the Moon. Processing the data obtained in the experiment that lasted for eight years (1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977) and included seis mic measurements at four landing sites of Apollo 12, 14, 15, and 16 missions revealed the seismic structure of the lunar interiors shown in The mathematical processing of travel times of P and S waves suggests a zonal structure of the lunar mantle