Heating
When the Sun ignited in the solar nebula, hydrogen, helium and other volatile materials were evaporated in the area near the Sun. The solar wind and radiation pressure forced such low-density material away from the Sun. Rocks, and the elements comprising them, were stripped of their early atmospheres, but themselves remained, to accumulate in 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 show that differentiation also took place in some asteroids.
When protoplanets accrete more material, the energy at 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.
Read more about this topic: Planetary Differentiation
Famous quotes containing the word heating:
“If the factory people outside the colleges live under the discipline of narrow means, the people inside live under almost every other kind of discipline except that of narrow means—from the fruity austerities of learning, through the iron rations of English gentlemanhood, down to the modest disadvantages of occupying cold stone buildings without central heating and having to cross two or three quadrangles to take a bath.”
—Margaret Halsey (b. 1910)