Subsequent assessments of Earth’s surface heat flux require more heat than compositions based on the CI class can produce. The latest value for Qtot of 44 TW (Pollack et al., 1993) is 2.3 times QR provided by various CI models (compiled by Lodders and Fegley, 1998). To explain this difference, additional heat sources and processes have been proposed. Consensus does not exist, and the hypotheses fall into several classes:
(i) delayed secular cooling, wherein the surface flux includes stored internal emissions from an earlier age that exceed the current amount generated (see discussions in Van den Berg and Yuen, 2002; Van den Berg et al., 2002), or (ii) high K content in the core (e.g. Breuer and Spohn, 1993), or (iii) remnants of primordal heat (QP) (e.g. Anderson, 1988a,b) delivered early in Earth’s history from impacts of accretion or decay of short-lived isotopes. Other possible heat releasing processes, such as crystallization of the inner core, contribute insignificantly (e.g. Stacey, 1969).
Geological observations, inferred mantle overturn rates, estimated mantle cooling rates, and recent geodynamic models independently suggest that neither delayed secular cooling nor primordal heat are currently significant sources, necessitating that current heat production predominately originates in radioactive decay and is quasi-steady-state. Models of Earth’s bulk composition based on CI chondritic meteorites provide an unrealistically low radioactive power of ~20 T
W, whereas enstatite chondrites are sufficiently radioactive to supply the observed heat flux, contain enough iron metal to account for Earth’s huge core, and have the same oxygen isotopic ratios as the bulk Earth. We devise a method to obtain K/U/Th ratios for the Earth and other planetary bodies from their power, including secular delays, and use this to constrain Earth’s cooling rate.