|MadSci Network: Astronomy|
The classical explanation is that the heat for melting comes from several sources. The first is the release of gravitational potential energy. As those planetismals are slowly attracted toward the accreting planet, they accelerate and that potential is converted to kinetic energy. When the accelerating planetismal finally collides with the growing proto-planet, that energy is converted again, largely to heat. This is true, even today, of meteorite collisions with Earth. In addition, the early earth had a much higher concentration of radioactive elements than the present day Earth, many with short half-lives. As these elements decayed they heated Earth from within. In concert, these mechanisms plus further gravitiational compaction of the ball of stuff, released enough heat to cause partial melting to begin. Metallic iron or alloys of iron-nickel-sulfur are among the first things to melt out of the proto-planet. As they melt, they form droplets that can begin to migrate under the influence of gravity. Because the iron melt is denser than the silicate residual, as the droplets coalesce and get big enough, they migrate downward, which releases more potential energy, first as kinetic energy and ultimately as heat, which accelerates the melting. As shown in this web site, it would take about a half-billion years for the temperature to rise sufficiently by these processes alone http://www.indiana.edu/~geol105/images/gaia_chapter_3/earth_differentiation.htm Some geologists think now, however, that it may have happened much earlier in earth's history. The moon is now generally accepted to have formed from a collision between a young Earth and a Mars-sized proto-planet, which completely melted both bodies (and in fact turned a good part of them to vapor, which then condensed back into liquid as the bodies cooled and separated). See http://www.spike.com/video/naked-science-birth/2685304. This may have occurred very early in Earth's history (perhaps only 60 million years after the birth of the solar system, instead of 600 million in the classical model. http://news.nationalgeographic.com/news/2007/12/071219-moon-collision.html Either way, as a result of radioactive decay and collisions, there is plenty of heat available to melt the earth and allow differentiation of the iron-nickel core from the silicate mantle and crust. Dave Smith
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