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Density Constraints on the Composition of Venus K. A. Goettel

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Density Constraints on the Composition of Venus K. A. Goettel DENSITY CONSTRAINTS ON THE COMPOSITION OF VENUS Kenneth A. Goettel, Dept. of Earth and Planetary Sciences, and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130 Inferences about compositional differences between Venus and Earth have generally been drawn principally from the observed difference in mean density between the two planets. The mean density of Venus, 5.245 g/cm3, which is well determined by the observed mass and mean radius, is nearly 5% less than the mean density of Earth, 5.517 g/cm3. However, most of this density difference is attributable to greater self-compression in Earth, due to its greater mass. Pressures in both planets are sufficiently high that uncertain- ties in equations of state introduce significant uncertainties in densities of model planets ; therefore, density differences between Venus and Earth can be examined most sensitively by considering differences between the observed density of Venus and the density of an Earth-1 i ke Venus. Ri ngwood and Anderson (1977) estimated that an Earth-1 i ke Venus would have a mean density of 5.34 g/cm3. Their model for an Earth-1 i ke Venus postulates a planet with the observed mass of Venus, with the same composition and structure (i.e., ratios of core/mantle/crust) as Earth, and with internal temperatures equal to temperatures in Earth at corresponding pressures. This model , however, overestimates the density of an Earth-1 i ke Venus; the smal ler mass of Venus implies that the depth at which a given pressure is reached is deeper in Venus than in Earth and thus temperatures at corresponding pressures should be higher in Venus than in Earth. With the same assumptions as Ringwood and Anderson, except that temperatures at corresponding depths are equal in Venus and Earth, the density of an Earth-like Venus is ~5.32g/cm3. In addition, the higher surface temperature of Venus lowers the density of Venus by ~0.2%(Ri ngwood and Anderson, 1977). Therefore, an Earth-1 i ke Venus with a hot surface would have a mean density of %5.31 g/cm3, which is about 1.2% higher than actually observed (see Table 1 ) . Viable models for the composition and internal constitution of Venus must account for a 1.2% density difference between Venus and Earth. The four main hypotheses which have been proposed are listed in Table 2. Three of the models appear to be capable of accounting for most or all of the observed density difference. Iron fractionation can, at least in principle, produce a large density change, because of the large density difference between iron and silicates. About 5% less metallic iron in Venus than in Earth (with si 1 icate compositions equal ) would account for the 1.2% density difference. Aerodynamic fractionation between iron and silicates may have occurred in the solar nebula (Weidenschi 11 i ng , 1977) ; the extent to which this process affected the compositions of Venus and Earth is unknown. Differences in oxidation state between Venus and Earth could account for the density difference (Ringwood and Anderson, 1977). In this model, the core of Venus would be smaller than Earth's core, with more iron present as FeO in mantle silicates ($13 wt.% FeO in Venus vs. -8-9 wt.% in the terrestrial mantle). Differences in basalt/eclogite ratio between Venus and Earth due to the high surface temperature of Venus could account for much of the density difference (Anderson, 1980), if it is assumed that the Earth's upper mantle contains very 1 arge amounts of eclogi te (,4nderson, 1979). The equi 1 i brium condensation model was deemed by Ringwood and Anderson (1977) to be capable of accounting for only a 0.4% density difference between Venus and Earth. However, more detailed consideration of the imp1 ications of this model with respect to phase changes and the thermal state of Venus suggests that this model is also viable (see Table 3). O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System COMPOSITION OF VENUS Goettel , K.A. The end-member equi 1i brium condensation model for Venus (Lewis, 1972) postulates that Venus contains virtually no S or FeO. According to Ringwood and Anderson (1977), the absence of S and FeO lowers the density of Venus by only 0.3% relative to Earth (0.4% if changes in the depth of phase boundaries are included). However, they have underestimated the effect of phase changes and ignored the thermal consequences of the composition postulated by the equilibrium condensation model. The melting temperatures of both the core and mantle of Venus will be substantially higher than for Earth, because of the postulated absence of S and FeO. Assuming that internal temperatures in Venus are regulated by convection (e.g., Tozer, 1972), then temperatures in Venus will be substantially higher than in Earth. Using a mean thermal expansion coefficient of 2 x 10-5/oC, temperatures in Venus 200-4000C higher than in Earth lower the density of Venus by 0.4-0.82. Ringwood and Anderson (1977) also underestimated the effect of phase changes on the density of Venus. The FeO-free composition postulated by the equi 1i brium condensation model increases the pressures for the 01 ivine/spinel and spinel/perovskite phase boundaries; each of these changes lowers the density of Venus by %0.1%. Higher temperatures in Venus will further depress the olivine/spinel boundary (but not the spinel/perovskite boundary which probably has dP/dT QO) , and lower dens'ity by another ~0.1%. Even if Earth has only one-quarter as much eclogite as postulated by Anderson (1979), then suppression of the basal t/eclogi te transformation on Venus (Anderson, 1980) will lower the density of Venus predicted by the equilibrium condensation model by 0.2-0.4%. Thus, changes in the depth of phase transformations could account for a 0.5-0.7% density change. Overall, the equilibrium condensation model can account for a density difference of 1.2-1.8% and thus is also a viable model for the composition of Venus. The principal conclusion which arises from these calculations is that none of the models discussed can be excluded solely on the basis of the observed density difference between Venus and Earth. These models postulate rather disparate co~positionsand constitutions for Venus: the FeO content of the mantle varies from q;O to $13 wt.%, the size of the core varies from ~23to ~32wt.% of Venus, and the model moment of inertia factor of Venus varies markedly. These compositional differences of course also imply major differences in the petrologic and thermal evolution of Venus. Other constraints exist (e.g., composition of the atmosphere, meagre data on the composition of surface materials, etc.) ; however, appl ication of these data to the bulk composition of Venus is tenuous at best. Therefore, evaluation of these models for Venus must be based principally on the plausibility of their constituent assumptions. Perhaps the most 1i kely model is an inter- mediatelhybrid model: Venus with substantially less S and FeO than Earth, with less eclogite than Earth, with perhaps a little less total iron, and with somewhat hotter internal temperatures. Rigorous determination of the composition and constitution of Venus probably requires seismic data to obtain directly density vs. depth data, the size of the core, etc. References Anderson, D. L . (1979), Geophys. Res. Lett. 6, 433-436. Anderson, D .L . (1980), Geophys. ELett.7,--- 101-102. Lewis, J.S. ( 1972), Earth Planet. Sci. Lett. 15, 286-290. Ringwood , A. E. and A-on, D.L. (1977),aKs 30, 243-253. Tozer, D.C. ( 1972) , Ph s . Earth. Planet. 1mrs6,- 182-1 97 Weidenschilli ng, S. J .*7m. --Not. Roy. Astron. --Soc. 180, O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System COMPOSITION OF VENUS Goettel, K.A. TABLE 1 Density Difference Between Venus and Earth mean density (g/cm3) Venus (observed) 5.245 Venus (~arth-1ike)a 5.34 (1.8% denser than observed) b Venus (Earth-1 ike) 5.32 (1.4% denser than observed) Venus (~arth-1ike)c 5.31 (1 .2% denser than observed) a~ingwood and Anderson (1977) : Venus with same composition, structure, and T(P) as Earth. bpresent estimate: as "a", except Venus with same T(Z) as Earth L present estimate: as "b", including effect of high surface T TABLE 2 Explanations for the Density Difference between Venus and Earth model density effect reference iron fractionation several % many authors higher oxidation state 1.9% Ringwood and Anderson, 1977 basal t/eclogi te ratio 0.8-1.5% Anderson, 1980 equilibrium condensation 0.4%~ Lewis , 1972 aaccordi ng to Ringwood and Anderson, 1977 TABLE 3 Equi 1i brium Condensation Model for Venus change in Venus vs. Earth density effect reference no S -1 .l% Ringwood and Anderson, 1977 no FeO +O .8% Ringwood and Anderson, 1977 deeper phase boundaries -0.5 - -0.7% present estimate higher interior temperatures -0.4 - -0.8% present estimate TOTAL -1.2 - -1.8% O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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