How much bigger has the Earth become since the Late Heavy Bombardment?

Factual Questions
1

The Earth is partially warmed by radioactive decay and fission (q.v. Oklo reactor, and probably at the Earth’s core). The elements that result from that decay and fission are less dense than the original elements. So the density decrease means an increase in volume. Right?

So, how much has the Earth expanded since the Solar System quietened down?

2

Radioactive decay (and fission) will give you lighter nuclei, but I do not think that relates to density in any straightforward way. Osmium (atomic number 76) is denser than uranium (atomic number 92), for instance. Density of an element has more to do with how the electrons are arranged, and thus how closely the atoms can pack, than with atomic weight or number. Of course, deep in the Earth, everything will be at intense pressure, increasing as you go down, and you will not be dealing with pure elements but compounds and complex mixtures. I doubt if it is possible to say what effect the radioactive processes will have upon volume.

Of course, the heat generated by radioactivity will tend to increase volume, but you have to offset that against the heat the planet radiates into space, and also take into account the heat received from the Sun…and other stuff. It is going to be horrendously complicated and any effect on volume due to the change in elemental composition due to radioactivity is likely to be swamped by other effects.

3

Also, the Earth’s rotation has slowed down over geological time; I’d think that the centrifugal force of that spin would have an effect on volume as well. Plus there’s still a slow increase in the mass & volume of the planet from infalling matter.

4

A lot of radioactive byproducts are hydrogen and helium, both of which can escape out of the atmosphere in their elemental forms. If hydrogen does stick around, it’ll do so in a molecule so that it’s not adding much volume.

I think a bigger increase in volume would be due to daily light bombardments of new material that the Earth captures.

5

I remember from a trip to the Science Museum in London that many tons of material falls to earth every day. Because I couldn’t remember the figure I did an Internet search and it seems that every website gives a different answer. As far as I can work out, it’s between 40 and 100 tons per day depending who you believe…

6

The earth’s rotation counteracts about 0.03% of the effect of gravity at the surface of the earth at the equator; the effect is less, in a linear way, as you move toward the axis of rotation.