The thermal conditions of Venus

Models of Venus' thermal evolution are examined. The following subject areas are covered: (1) modified approximation of parameterized convection; (2) description of the model; (3) numerical results and asymptotic solution of the MAPC equations; (4) magnetism and the thermal regime of the cores of Earth and Venus; and (5) the thermal regime of the Venusian crust

FeO Content of Earth’s Liquid Core

The standard model of Earth’s core evolution has the bulk composition set at formation, with slow cooling beneath a solid mantle providing power for geomagnetic field generation. However, controversy surrounding the incorporation of oxygen, a critical light element, and the rapid cooling rates needed to maintain the early dynamo have called this model into question. The predicted cooling rates imply early core temperatures that far exceed estimates of the lower mantle solidus, suggesting that early core evolution was governed by interaction with a molten lower mantle. Here we develop ab initio techniques to compute the chemical potentials of arbitrary solutes in solution and use them to calculate oxygen partitioning between liquid Fe-O metal and silicate melts at the pressure-temperature (P−T) conditions expected for the early core-mantle system. Our distribution coefficients are compatible with those obtained by extrapolating experimental data at lower P−T values and reveal that oxygen strongly partitions into metal at core conditions via an exothermic reaction. Our results suggest that the bulk of Earth’s core was undersaturated in oxygen compared to the FeO content of the magma ocean during the latter stages of its formation, implying the early creation of a stably stratified oxygen-enriched layer below the core-mantle boundary (CMB). FeO partitioning is accompanied by heat release due to the exothermic reaction. If the reaction occurred at the CMB, this heat sink could have significantly reduced the heat flow driving the core convection and magnetic field generation

Numerical simulations of coarsening of eutectoid structures: Implications for grain size in the lower mantle

Since it has been realized that the viscosity of the lower mantle depends on the grain size, several studies have been conducted to constrain grain size in the lower mantle. One of the problems is that the initial structure generated by spinel-perovskite phase transformation is characterized by lamellae of alternating perovskite and magnesioüstite. It is not clear how long it takes for such structures to reach the asymptotic regime of conventional Ostwald ripening and whether this can happen on typical time scales of laboratory experiments. We performed numerical simulations of coarsening of lamellar structures using Monte Carlo Potts model. We find that at high temperatures, an isotropic lamellar structure degenerates into nearly equiaxed grains with the diameter which is 2 to 3 times larger that the original lamellar spacing. The duration of this process is comparable with that expected for conventional Ostwald

Viscosity of mafic magmas at high pressures

International audienceWhile it is accepted that silica-rich melts behave anomalously with a decrease of their viscosity at increased pressures (P), the viscosity of silica-poor melts is much less constrained. However, modeling of mantle melts dynamics throughout Earth's history, including the magma ocean era, requires precise knowledge of the viscous properties of silica-poor magmas. We extend here our previous measurements on fayalite melt to natural end-members pyroxenite melts (MgSiO 3 and CaSiO 3) using in situ X-ray radiography up to 8 GPa. For all compositions, viscosity decreases with P, rapidly below 5 GPa and slowly above. The magnitude of the viscosity decrease is larger for pyroxene melts than for fayalite melt and larger for the Ca end-member within pyroxene melts. The anomalous viscosity decrease appears to be a universal behavior for magmas up to 13 GPa, while the P dependence of viscosity beyond this remains to be measured. These results imply that mantle melts are very pervasive at depth