1. There's far less of such heavier elements in the Universe as a whole. Iron is the end-result of all stellar fusion. Heavier elements form only in massive astronomical events --- novas of huge stars, or in neutron-star collisions, black-hole formation, and the like. So most of the heavy stuff is iron or adjacent elements.
2. Most of the heavy stuff most likely is in the inner core, though what the fluid-dynamics of this during and following planet formation are ... is tenuously known at best. Most likely arrived at through modeling and some very limited remote sensing (seismic, neutrino flux), density estimates, and gravimetr (noting differences in Earth's gravitational field).
3. Most of the crustal prevalence of trans-iron elements in the Earth's crust is likely a result of late bombardment. The Giant Impact Hypothesis thought to have formed the Moon may variously account for greater prevalence (more material deposited to the crust from deeper within the Earth) or less (re-liqufication of the crust and mantle leading to more heavy material sinking to greater depths).
4. From an earlier HN thread, @perihelions states that evidence suggests that there's not a sufficient concentration of fissible elements to support sustained nuclear chain reactions within Earth's core, though radioactive decay does contribute about half the source of geothermal energy within the Earth (the other half being latent heat from gravitational energy of formation). See: <https://news.ycombinator.com/item?id=31281424>
Whichever way, generally, yes, heavier elements increase in prevalence with depth.
From Wikipedia:
The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).
2. Most of the heavy stuff most likely is in the inner core, though what the fluid-dynamics of this during and following planet formation are ... is tenuously known at best. Most likely arrived at through modeling and some very limited remote sensing (seismic, neutrino flux), density estimates, and gravimetr (noting differences in Earth's gravitational field).
3. Most of the crustal prevalence of trans-iron elements in the Earth's crust is likely a result of late bombardment. The Giant Impact Hypothesis thought to have formed the Moon may variously account for greater prevalence (more material deposited to the crust from deeper within the Earth) or less (re-liqufication of the crust and mantle leading to more heavy material sinking to greater depths).
4. From an earlier HN thread, @perihelions states that evidence suggests that there's not a sufficient concentration of fissible elements to support sustained nuclear chain reactions within Earth's core, though radioactive decay does contribute about half the source of geothermal energy within the Earth (the other half being latent heat from gravitational energy of formation). See: <https://news.ycombinator.com/item?id=31281424>
Whichever way, generally, yes, heavier elements increase in prevalence with depth.
From Wikipedia:
The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).
<https://en.wikipedia.org/wiki/Internal_structure_of_Earth#Co...>