When planets are forming, does the densest material sink the core and the less dense material "float" on top? What I'm asking is that are the more dense materials more likely to appear in the core rather than the surface. I'm also wondering, can a planet have an iron surface and still have rock beneath the surface in its crust and core?
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$\begingroup$ See this question which explains the source of less dense objects rising (and conversely more dense objects sinking). $\endgroup$– zephyrMar 30, 2017 at 13:17
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$\begingroup$ In a world-building sense, a planet could be covered with oceans, and those oceans, rich in Iron, like the young Earth, then, much of the water evaporates and you're left with an Iron dust over the planet. Mars is like that a bit. Solid Iron, probably not possible, but an Iron rich coating is possible. $\endgroup$– userLTKMar 30, 2017 at 19:22
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2 Answers
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Essentially, a planet can be considered as a self-gravitating fluid. As was already known to Archimedes, under gravity low-density fluids are buoyant and rise, while high-density fluids sink. So, with some local exceptions, planets are always well structured from the densest material in the core to lower-density material further up.
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Apparently you don't fully appreciate the results of gravity. Try to model a fluid sphere (for our purposes here, a collection of rock, liquid, and gas is semifluidic) such that somehow there's less gravitational pressure near the center than near the outermost shell. Can't happen. Next, think about trying to make chunks of lead, or granite, float on top of a pond (of liquid water). Won't happen.
Your last question is kind of pointless - there are "rocks" with densities both greater and less than the density of pure iron.
answered Mar 30, 2017 at 12:41
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$\begingroup$ You explained by you didn't answer the question. $\endgroup$ Mar 31, 2017 at 21:05