What is Earth's internal heat budget?
[[File:Earth heat flow.jpg|thumb|upright=1.5|Global map of the flux of heat, in mW/m<sup>2</sup>, from Earth's interior to the surface.<ref name=Davies /> The largest values of heat flux coincide with [[mid-ocean ridge]]s, and the smallest values of heat flux occur in stable continental interiors.]] '''Earth's internal heat budget''' is about heat that comes from the [[Earth]] itself, not heat from the [[Sun]]. The flow of heat from Earth's interior to the surface is estimated at 47±2 [[terawatt]]s (TW).<ref name=Davies>Davies J.H. & D.R. 2010. ''Earth's surface heat flux''. ''Solid Earth'', '''1'''(1), 5–24p.</ref> It comes from two main sources in roughly equal amounts. There is the ''radiogenic heat'' produced by the [[radioactive decay]] of isotopes in the [[Mantle (geology)|mantle]] and [[Earth's crust|crust]]. Secondly, there is the ''primordial heat'' left over from the [[Earth#Formation|formation of Earth]].<ref name=Turcotte>Donald L. Turcotte & Gerald Schubert 2002. ''Geodynamics''. Cambridge University Press. ISBN 978-0-521-66624-4</ref> As many people know, the deeper down you go, the hotter the Earth gets. [[Mine]] shafts are a good example of this. == Radiogenic heat == [[File:Evolution of Earth's radiogenic heat-with total.svg|thumb|upright=1|The evolution of Earth's [[radiogenic]] heat flow over time]] This is still being produced. but its amount reduces very slowly as time passes. Remember, these radioactive elements are produced in [[Supernova|supernovae]], and are quite different from the Sun, which is mostly hydrogen. The planets are completely different from the Sun, historically and in their make-up. == Primordial heat == Primordial heat is the heat lost by the Earth as it continues to cool from its original formation, in contrast to its still actively-produced radiogenic heat. The Earth core's heat flow—heat leaving the core and flowing into the overlying mantle is thought to be due to primordial heat. It is estimated as 5–15&nbsp;TW.<ref name=Lay>Lay T; Hernlund J., & Buffett, B.A