14,811 research outputs found
Chemical differentiation of a convecting planetary interior: Consequences for a one-plate planet such as Venus
Chemically depleted mantle forming a buoyant, refractory layer at the top of the mantle can have important implications for the evolution of the interior and surface. On Venus, the large apparent depths of compensation for surface topographic features might be explained if surface topography were supported by variations in the thickness of a 100-200 km thick chemically buoyant mantle layer or by partial melting in the mantle at the base of such a layer. Long volcanic flows seen on the surface may be explained by deep melting that generates low-viscosity MgO-rich magmas. The presence of a shallow refractory mantle layer may also explain the lack of volcanism associated with rifting. As the depleted layer thickens and cools, it becomes denser than the convecting interior and the portion of it that is hot enough to flow can mix with the convecting mantle. Time dependence of the thickness of a depleted layer may create episodic resurfacing events as needed to explain the observed distribution of impact craters on the venusian surface. We consider a planetary structure consisting of a crust, depleted mantle layer, and a thermally and chemically well-mixed convecting mantle. The thermal evolution of the convecting spherical planetary interior is calculated using energy conservation: the time rate of change of thermal energy in the interior is equated to the difference in the rate of radioactive heat production and the rate of heat transfer across the thermal boundary layer. Heat transfer across the thermal boundary layer is parameterized using a standard Nusselt number-Rayleigh number relationship. The radioactive heat production decreases with time corresponding to decay times for the U, Th, and K. The planetary interior cools by the advection of hot mantle at temperature T interior into the thermal boundary layer where it cools conductively. The crust and depleted mantle layers do not convect in our model so that a linear conductive equilibrium temperature distribution is assumed. The rate of melt production is calculated as the product of the volume flux of mantle into the thermal boundary layer and the degree of melting that this mantle undergoes. The volume flux of mantle into the thermal boundary layer is simply the heat flux divided by amount of heat lost in cooling mantle to the average temperature in the thermal boundary layer. The degree of melting is calculated as the temperature difference above the solidus, divided by the latent heat of melting. A maximum degree of melting is prescribed corresponding to the maximum amount of basaltic melt that the mantle can initially generate. As the crust thickens, the pressure at the base of the crust becomes high enough and the temperature remains low enough for basalt to transform to dense eclogite
Chemical differentiation on one-plate planets: Predictions and geologic observations for Venus
Recent studies have examined the partial melting of planetary interiors on one-plate planets and the implications for the formation and evolution of basaltic crust and the complementary residual mantle layer. In contrast to the Earth, where the crust and residual layer move laterally and are returned to the interior following subduction, one-plate planets such as Venus are characterized by vertical accretion of the crust and residual layer. The residual mantle layer is depleted and compositionally buoyant, being less dense than undepleted mantle due to its reduced Fe/Mg and dense Al-bearing minerals; its melting temperature is also increased. As the crust and depleted mantle layer grow vertically during the thermal evolution of the planet, several stages develop. As a step in the investigation and testing of these theoretical treatments of crustal development on Venus, we investigate the predictions deriving from two of these stages (a stable thick crust and depleted layer, and a thick unstable depleted layer) and compare these to geologic and geophysical observations, speculating on how these might be interpreted in the context of the vertical crustal accretion models. In each case, we conclude with an outline of further tests and observations of these models
Fluctuations of topological disclination lines in nematics: renormalization of the string model
The fluctuation eigenmode problem of the nematic topological disclination
line with strength is solved for the complete nematic tensor order
parameter. The line tension concept of a defect line is assessed, the line
tension is properly defined. Exact relaxation rates and thermal amplitudes of
the fluctuations are determined. It is shown that within the simple string
model of the defect line the amplitude of its thermal fluctuations is
significantly underestimated due to the neglect of higher radial modes. The
extent of universality of the results concerning other systems possessing line
defects is discussed.Comment: 6 pages, 3 figure
Multi-order interference is generally nonzero
It is demonstrated that the third-order interference, as obtained from
explicit solutions of Maxwell's equations for realistic models of three-slit
devices, including an idealized version of the three-slit device used in a
recent three-slit experiment with light (U. Sinha et al., Science 329, 418
(2010)), is generally nonzero. The hypothesis that the third-order interference
should be zero is shown to be fatally flawed because it requires dropping the
one-to-one correspondence between the symbols in the mathematical theory and
the different experimental configurations.Comment: Replaced Figs. 4,5 and caption of Fig.
Overturn of magma ocean ilmenite cumulate layer: Implications for lunar magmatic evolution and formation of a lunar core
We explore a model for the chemical evolution of the lunar interior that explains the origin and evolution of lunar magmatism and possibly the existence of a lunar core. A magma ocean formed during accretion differentiates into the anorthositic crust and chemically stratified cumulate mantle. The cumulative mantle is gravitationally unstable with dense ilmenite cumulate layers overlying olivine-orthopyroxene cumulates with Fe/Mg that decreases with depth. The dense ilmenite layer sinks to the center of the moon forming the core. The remainder of the gravitationally unstable cumulate pile also overturns. Any remaining primitive lunar mantle rises to its level of neutral buoyancy in the cumulate pile. Perhaps melting of primitive lunar mantle due to this decompression results in early lunar Mg-rich magmatism. Because of its high concentration of incompatible heat producing elements, the ilmenite core heats the overlying orthopyroxene-bearing cumulates. As a conductively thickening thermal boundary layer becomes unstable, the resulting mantle plumes rise, decompress, and partially melt to generate the mare basalts. This model explains both the timing and chemical characteristics of lunar magmatism
Crescimento e incremento de Sequoia sempervirens (D. Don) Endl., São Joaquim, SC.
O presente trabalho teve como objetivo estudar a forma de crescimento e calcular o incremento médio e corrente anual das variáveis dendrométricas de Sequoia sempervirens, de mudas originárias dos EUA e cultivadas no campo experimental da EPAGRI em São Joaquim, SC. Para isso, ajustou-se e selecionou-se um entre modelos lineares e não lineares que se mostraram eficientes, permitindo identificar as formas do crescimento de acordo com a idade. Os resultados obtidos demonstram que aos 18 anos o incremento médio anual em altura foi de 0,55 m/árvore/ano e o incremento médio em volume foi de 0,76 m3/árvore/ano, indicando um ótimo potencial de crescimento. Para volume e diâmetro a equação selecionada foi de Schumacker com R2aj. de 0,9558 e Syx de 0,7 cm para diâmetro, e para volume R2aj. de 0,9421 e Syx de 0,8694 m3, para a variável altura o modelo selecionado foi de Chapman-Richards com R2aj. de 0,98 e Syx de 0,5364 m. Os resultados demonstraram que a espécie teve um incremento médio em diâmetro de 2,5 cm na região, semelhante aos plantios comerciais de pinus, com ponto de rotação técnica em volume aos 70 anos, podendo ser cultivada comercialmente
Collective Modes of Tri-Nuclear Molecules
A geometrical model for tri-nuclear molecules is presented. An analytical
solution is obtained provided the nuclei, which are taken to be prolately
deformed, are connected in line to each other. Furthermore, the tri-nuclear
molecule is composed of two heavy and one light cluster, the later sandwiched
between the two heavy clusters. A basis is constructed in which Hamiltonians of
more general configurations can be diagonalized. In the calculation of the
interaction between the clusters higher multipole deformations are taken into
account, including the hexadecupole one. A repulsive nuclear core is introduced
in the potential in order to insure a quasi-stable configuration of the system.
The model is applied to three nuclear molecules, namely Sr + Be +
Ba, Mo + Be + Te and Ru + Be +
Sn.Comment: 24 pages, 9 figure
Spatiotemporal rheochaos in nematic hydrodynamics
Motivated by the observation of rheochaos in sheared wormlike micelles
[Bandyopadhyay et al., Phys. Rev. Lett, 84 2022, (2000); Europhys. Lett. 56,
447 (2001); Pramana 53, 223 (1999)] we study the coupled nonlinear partial
differential equations for the hydrodynamic velocity and order parameter fields
in a sheared nematogenic fluid. In a suitable parameter range, we find
irregular, dynamic shear-banding and establish by decisive numerical tests that
the chaos we observe in the model is spatiotemporal in nature.Comment: Slight changes in text, references and Fig. 5 inset; 6 eps figures
(figs 2,3,4 at lower resolution to reduce file size; full files available on
request); accepted for publication in Phys Rev Let
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