This got me thinking about the earth's heat flux to the surface. It comes from primordial heat and radioisotope decay of thorium and uranium. Wild that geothermal energy is really just nuclear decay energy, just like the mars RTGs. But disappointing that it ain't all that much at just 47 TW, compared to the sun's 200,000 TW on the Earth's surface. And of those 47 TW, only 14 TW are on land. Not even close to enough to power mankind and our 20-40TW of primary energy today, and hopefully much much more in the future.