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In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center, while less dense materials rise to the surface. Such a process tends to create a core and mantle. Sometimes a chemically distinct crust forms on top of the mantle. The process of planetary differentiation has occurred on planets, dwarf planets, the asteroid 4 Vesta, and natural satellites (such as the Moon). ==Heating== When the Sun ignited in the solar nebula, hydrogen, helium and other volatile materials were evaporated in the region around it. The solar wind and radiation pressure forced these low-density materials away from the Sun. Rocks, and the elements comprising them, were stripped of their early atmospheres, but themselves remained, to accumulate into protoplanets. Protoplanets had higher concentrations of radioactive elements early in their history, the quantity of which has reduced over time due to radioactive decay. Heating due to radioactivity, impacts, and gravitational pressure melted parts of protoplanets as they grew toward being planets. In melted zones, it was possible for denser materials to sink towards the center, while lighter materials rose to the surface. The compositions of some meteorites (achondrites) show that differentiation also took place in some asteroids (e.g. Vesta), that are parental bodies for meteoroids. The short-lived radioactive isotope 26Al was probably the main source of heat.〔de Pater, I., and Lissauer, J.J. 2001. Planetary Sciences, Cambridge Univ. Press.〕〔Prialnik D., Merk R., 2008. Growth and evolution of small porous icy bodies with an adaptive-grid thermal evolution code. I. Application to Kuiper Belt objects and Enceladus. Icarus 197: 211–220.〕 When protoplanets accrete more material, the energy of impact causes local heating. In addition to this temporary heating, the gravitational force in a sufficiently large body creates pressures and temperatures which are sufficient to melt some of the materials. This allows chemical reactions and density differences to mix and separate materials, and soft materials to spread out over the surface. On Earth, a large piece of molten iron is sufficiently denser than continental-crust material to force its way down through the crust to the mantle. In the outer Solar System a similar process may take place but with lighter materials: they may be hydrocarbons such as methane, water as liquid or ice, or frozen carbon dioxide. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Planetary differentiation」の詳細全文を読む スポンサード リンク
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