Jim Hurley asked:
"I thought that most of the matter in the solar system had undergone
countless collisions, breakups and reformations in the history of the
solar system."
"I was recollecting an old article from Scientific American that
modelled the early solar system from many small-bodied collisions.
I was thinking of the age far before the major planets were formed.
Of course, my recollection could be faulty. And I haven't kept up with
the advances in planetary sciences of the last few years."
It was a multi-stepped process, still not fully clear, but now thought to encompass:
Dust from the collapse of the solar nebula condensed into chondrules.
Chondrules condenses into debris.
Debris condensed into planetesimals that collided and ultimately condensed into the planets (Yes, there could have been false starts of planetesimals that were formed and torned apart, but eventually a larger body is formed and becomes a gravitational magnet for drawing in matter and growing larger).
Protoplanetary Earth differentiated relatively quickly into core/mantle/crust where intially the crust is a magma ocean (from the heat of core formation) while bombardment continues.
Heavy bombardment of Earth continued until 3.8Ga. -- but Earth was still Earth.
"This radioactive decay comes from the source that produced the dust
that formed the solar system, right? Neutron-rich heavy elements from
an exploding supernova. I would have thought the half-life of this
matter wouldn't have lasted long enough to heat up accreted bodies
in the later solar system.
Don't forget very short-lived, lighter isotopes (aluminum and iodine) that are no longer around but were important sources of heat for planetary differentiation.
Don't forget heat generated by core formaiton (loss of gravitational potential energy).
Don't forget accretionary heating from early heavy bombardment.
Hope this helps,
Steve Excell
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