I was reading an article about super-Earths in the latest Scientific American, and there was one paragraph that I didn't quite follow:
ok, I understand that the greater pressures of a super-Earth would cause an iron core to solidify at temperatures where it would remain liquid on Earth.But at the pressures that exist in a large planet's core, iron can solidify even at temperatures as high as 10,000 Kelvins, according to recent theoretical calculations.These high temperatures are probably exceeded only when the planets are very young. But a little cooling would be sufficient for the cores of super-Earths to solidify.Thus, a typical super-Earth may have a completely solid iron core and no global magnetic field.
But what I don't get is how this translates into a completely solid iron core. Wouldn't the pressure on the core decrease further away from the center, getting to a spot on the Fe state diagram to provide for a liquid Fe layer?
Fe state diagrams are not easy to come by, for obvious reasons, but this paper takes a good stab at it (figure 2 on pg. 2): http://www.gps.caltech.edu/~sue/TJA_...eismo_2069.pdf
This all assumes, of course, that the core has not cooled so sufficiently that Fe is solid even at low pressures, rendering the discussion moot. Since Earth still has enough heat for a liquid layer after 4B+ years, I think a super-Earth would be at least as warm.
The only way I can envision the scenario described in the article is if the non-Fe mass is so great that it alone can generate the pressures needed to solidify even the outermost region of the iron core regardless of temperature. But, in that case, it seems like the remaining layers of the planet would be extremely iron-deficient as iron sank to the core due to earlier convective processes.