Why wouldn't friction be the source of heat? The very center of the earth has a balanced gravity pull outwards in every direction, creating a gradient that should condense the heavier atoms around the mantle. The internal core is spinning at a different rate that the crust creating a velocity gradient between the core, crust, and mantle. Regardless if there was no nuclear energy in the core, the friction alone should be substantial. And we know solid particles make up the core. Ancient Earth had some rather nasty experiences with meteors. That solid core settles to the center, but basic common sense would suggest massive meteor strikes would bounce that baby around like a rattle, fracturing the crust and mantle from the inside out not much different than how a baseball to the head can cause the brain to strike the opposite side of the head and create a skull fracture. The core being off center for any length of time has to ratchet up the internal friction.