Main silicate-rich zone within a The term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More between the Outermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More and metallic In the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More. The mantle accounts for 82% of Earth’s volume and is composed of The most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes Transfer of heat energy by moving material. Temperatures increases with depth in planetary objects. Deep hot less-dense material physically rises and cools, releasing heat and becoming denser. The now cooler denser material sinks back into deeper regions, where it will be reheated and rise again. Convection is an important mechanism Click on Term to Read More currents in the semi-liquid mantle; rock rises and then slowly sinks again as it cools, driving the tectonic plates. The crust-mantle boundary, which occurs at an average depth of ~32 km, is called the Mohorovicic discontinuity The core-mantle boundary (CMB), the Gutenburg discontinuity, lies at an average depth of 2,900 km.The mantle is subdivided into upper and lower mantle, based upon changes in seismic Propagating disturbance which transmits energy from one point to another without physically transporting the oscillating quantity. A wave is characterized by wavelength and frequency. velocity. The uppermost non-convecting mantle comprises the lower part of the Rigid outer layer of a planet. The base of the lithopshere is defined by the temperature at which the brittle/ductile transition occurs in the mantle. Click on Term to Read More. Beneath the lithosphere at ~72–250 km depth, is the low-velocity zone; also called the asthenosphere. A rapid increase of seismic velocities occurs at a depth of ~670 km; this corresponds to a mantle phase change. Seismic velocities in the upper mantle are overall lower than those of the lower mantle, indicating that the lower mantle is much denser than the upper mantle. Other smaller changes in velocity correspond to other phase changes.
Some or all content above used with permission from J. H. Wittke.