Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 2—Three-Layer Mantle

Internally consistent models of the thermal state, chemical composition and mineralogy of the three-layer mantle of the Moon are constructed based on the joint inversion of gravity, seismic and petrological-geochemical data within the Na₂O-TiO₂-CaO-FeO-MgO-Al₂O₃-SiO₂ system. Geochemical constraints on the chemical composition and physical properties in three zones of the mantle are obtained in terms of the cold and hot models. Velocities of P-waves in the lower mantle (～8 km/s) are higher than in the upper mantle (～7.7 km/s). The behavior of velocities of S-waves is conservative, they are observed in the interval 4.40–4.45 km/s in all zones of the mantle. It was found that, independently of the temperature distribution, the most probable concentrations of FeO, ～11–14 wt % and MgO, 28–31 wt % and the values of the magnesian number MG# 80–83 are approximately the same in the upper and the lower mantles of the Moon, but drastically differ from those in the bulk composition of the silicate Earth (Bulk Silicate Earth, BSE, FeO 8%, MG# 89). On the contrary, the estimates of Al₂O₃ concentration in the three-layer mantle noticeably depend on the thermal state. The results of solution of the inverse problem indicate the trend towards the gradual increase in the Al₂O₃ content with depth, from the upper to the lower mantle to 4–7% with the higher content of garnet. For the cold models of the lower mantle of the Moon, the bulk content of Al₂O₃ is ～1 × BSE, and for the hot models it can be in the interval of 1.3 × BSE-1.7 × BSE. The abundance of SiO₂ depends, to a lesser degree, on the thermal state and is 50–55% in the upper and 45–50% in the lower mantle. The high pyroxene content of the upper mantle of the Moon is the geochemical consequence of the geophysical models used with the inversion into composition and temperature relations; orthopyroxene, instead of olivine, is the dominant mineral of the upper mantle. Concentrations of SiO₂ in the lower (undifferentiated) mantle showing the bulk composition of the silicate Moon (Bulk Silicate Moon, BSM), are consistent with the geochemical estimates of 45–48% of SiO₂ for the BSM and close to those for the Earth’s mantle (45–47%). The composition of the mantle middle zone remains discussible, since it might be partially overlapped with compositions of the over- and underlying envelopes. The results of the model suggest that the mantle of the Moon is stratified in chemical composition. For the considered thermal state models, the mantle of the Moon is enriched in FeO and depleted in MgO in relation to the primitive Earth mantle, which indicates considerable differences between compositions of the Earth and its satellite.

     

Thermal structure and thickness of the lithospheric mantle underlying the Siberian Craton from the kraton and kimberlit superlong seismic profiles

A self-consistent approach is proposed for the investigation of the thermal conditions, chemical composition, and internal structure of the upper mantle of the Earth. Using this approach, the thermal state of the lithospheric mantle beneath the Siberian Craton (SC) is reconstructed from P velocities, taking into account the phase transitions, anharmonicity, and the effects of anelasticity. The velocities of seismic waves are more sensitive to temperature than to the composition of the mantle rocks, which allows the velocity models to be effectively used for reconstruction of the thermal regime of the mantle. The temperature at depths 100–300 km is reconstructed by inversion of the Kraton and Kimberlit superlong seismic profiles for compositions of the garnet harzburgite, lherzolite, and intermediate composition of garnet peridotite. The averaged temperature in the normal continental mantle is reconstructed by inversion of the IASP91 reference model for depleted and fertile substance. One-dimensional models and two-dimensional thermal fields undergo a substantial fall in temperature (～300–600°C) beneath the Siberian Craton as compared to the temperatures of the continental mantle and paleotemperatures inferred from the thermobarometry of xenoliths. Temperature profiles of the Siberian Craton deduced from seismic data lie between the conductive geotherms of 32.5–40.0 mW/m² and below the P(H)-T values obtained for low- and high-temperature xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlite pipes. The thickness of the thermal lithosphere estimated from the intersection with the potential adiabat is 300–320 km, which is consistent with the data on heat flows and seismotomographic observations. This provides grounds for the assumption that the low-temperature anomalies (thermal roots of continents) penetrate down to a depth of 300 km. The analysis of the sensitivity of seismic velocity and density to the variations in temperature, pressure, and chemical and phase composition of petrological models shows that recognition of fine differences in chemical composition of the lithospheric rocks by seismic methods is impossible.     

Inversion of seismic and gravity data for the composition and core sizes of the Moon

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-Al₂O₃-SiO₂ 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, Al₂O₃, and CaO. The silicate portion of the Moon (crust + mantle) may contain 3.5–5.5% Al₂O₃ 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.     

Methods of Venus radiolocation map synthesis using strip images of VENERA-15 and VENERA-16 space stations

The methodology and the results of Venus northern hemisphere (approximately to 30°N) map synthesis are described. The synthesis was carried out in the Institute for Information Transmission Problems of the USSR Academy of Sciences using the data of strip survey of Venus surface received from VENERA-15 and VENERA-16 space stations. Interactive image processing system of the institute was used for this purpose. The problem of map synthesis was divided into four stages— pre-processing of about 300 strips images available, geometrical transformation of the strips and the synthesis of so-called sectors from 10–16 strips, joining all the 19 sectors into 34 map sheets and filling the polar area in the middle of the central sheet. The main mode of work in the course of map synthesis was the interaction of the operator with the system. This enabled to achieve high accuracy of strips referencing, indistinguishability of the borders between strips and sectors, the absence of false objects, and, as a result, high visual quality of the map.     

Physico-chemical models of the internal structure of partially differentiated Titan

We analyze models of the internal structure of Titan, a large icy satellite of the Saturn system. Calculations are carried out using information on the mass, mean density, moment of inertia, orbital parameters, and elastic properties of the satellite obtained by the Cassini–Huygens mission, as well as geochemical data on the composition of chondrite materials, equations of state of water and ices I, III, V, VI, and VII, and thermodynamic models for conductive heat transfer in the outer icy crust and of global convection in the interior zones of the satellite. The analysis of the models shows that models of partially differentiated Titan are most consistent; they include an outer water–ice shell, an intermediate ice–rock mantle, and an inner rock–iron core. It is shown that for the models of this type the maximum thickness of the water–ice shell is 460–470 km; it can be composed of an outer conductive crust of Ih ice 80–110 km thick and a subsurface water ocean 200–300 km deep. The maximum radius of the central rock–iron core of Titan can reach ~1300 km. The thickness of Titan’s ice–rock mantle does not exceed 2100 km at a density of 1.22–2.64 g/cm³. The model of partially differentiated Titan is feasible in the moment of inertia range of 0.312 < I/MR ² < ~0.350.     

Geochemical constraints on the model of the composition and thermal conditions of the Moon according to seismic data

A new method of reconstruction of the temperature profile in the lunar mantle from the velocities of seismic P- and S-waves for different models of chemical composition is developed. The procedure of the solution of an inverse problem is realized with the help of the minimization of the Gibbs free energy and the equations of state of a mantle substance, taking into account phase transformations, anharmonicity, and the effects of inelasticity. The geophysical and geochemical constraints on composition and temperature distribution in Moon’s mantle are established. The upper mantle can be composed of olivine pyroxenite, depleted by low-volatile oxides (∼2 wt % of CaO and Al₂O₃). On the contrary, the lower mantle must be enriched by low-volatile oxides (∼4–6 wt % of CaO and Al₂O₃). Its composition can be represented by a mineral association of the olivine + clinopyroxene + garnet or olivine + orthopyroxene + clinopyroxene + garnet type, which is close in composition to pyrolite. The temperature distribution at depths 50–1000 km are approximated by the equation: T(°C) = 351 + 1718[1–exp (−0.00082H)]. The constraints inferred make it possible to conclude that the published values of the velocities of P- and S-waves for the lunar mantle, obtained by processing the data of seismic experiments of the Apollo lunar mission are inconsistent with each other at depths below 300 km. Otherwise, the variations in the velocities of P- and S-waves disturb the symmetry between the petrological model (composition), the temperature profile, and the seismic profile.     

L- and LL-Chondritic Models of the Chemical Composition of Io

A geologically admissible range for the density of Io's mantle, its chemical and mineral compositions in the system, and the size of an Fe–FeS core are determined by mathematical simulation based on the fitting of the calculated mass and moment of inertia of Io to the experimental data. It is demonstrated that the bulk chemical composition of Io (crust + mantle + core) most closely resembles the composition of ordinary L and LL chondrites. The densities of mineral assemblages in the mantle and the iron concentration in the core, calculated on the basis of the L- and LL-chondritic models, meet the geophysical constraints on the mass and the moment of inertia of Io. The core radius is estimated to be 590–640 km for the Fe core (8–10% of Io's mass) and 820–890 km for the Fe–FeS eutectic core (13.5–17%).