Radioactive heating of the EarthWatts/kg half life mantle Watts/kg of isotope (years) abundance of mantle Uranium-238 9.46e-5 4.47e9 30.8 e-9 2.91 e-12 Uranium-235 56.9 e-5 .70e9 .22e-9 .125e-12 Thorium-232 2.64e-5 14.0 e9 124 e-9 3.27 e-12 Potassium-40 2.92e-5 1.25e9 36.9 e-9 1.08 e-12 The Earth loses heat at a rate of .087 Watts/m^2. Global heat loss = 4.42e13 Watts. 80% of the Earth's heat is from radioactivity and 20% is from accretion. The radioactive heating rate 3 billion years ago is twice that of today. The Earth's core temperature is ~ 7000 K. Given the above data, estimate how long will it take for the Earth's radioactive power to decrease by 1/2.