A Warm and Hospitable Planet
Editorial Revision 2026
The Blue Planet
Radioactivity has helped give Earth its character as a blue and hospitable planet by preventing it from cooling too rapidly. Together with heat from the Sun, it has enabled the mild temperatures that favored the development of life as we know it on its surface.
© CEA
Our planet is warm. Radioactivity contributes to the mild temperatures we experience on its surface. Like an almost eternal hot-water bottle, it has slowed Earth’s cooling and thus helped sustain life. About half of Earth’s internal heat comes from radioactive decay occurring within rocks, in the Earth’s crust and deeper within its core. The other half of the heat output comes mainly from the very slow cooling of the Earth since its formation, a process known as secular cooling.
At the end of the 19th century, the British physicist Lord Kelvin calculated from the heat flux emerging from the ground that the Earth could not be older than 100 million years. However, Lord Kelvin was unaware of the existence of radioactive decay…
A Permanent Source of Heat
Deep within, the Earth functions like a gigantic thermal power plant driven by movements that dissipate through convection the heat released by the natural radioactivity of deep rocks. At the very center lies the solid inner core, surrounded by the liquid outer core. Moving toward the surface, the next layer is the mantle. The uppermost part of the mantle and the continental and oceanic crusts together form the lithosphere. The lithosphere is a puzzle made up of twelve plates that slowly drift under the influence of deep currents, deforming continents and oceans along their boundaries. It is at these boundaries that the most visible manifestations of Earth’s activity occur: volcanic eruptions and earthquakes.
© CNRS
The thermal energy emerging from the interior of our planet is estimated at 46 trillion watts (46 TW or terawatts), of which 2 TW are released by volcanic eruptions and earthquakes. Apart from earthquakes and volcanic eruptions, most of this energy is released continuously: this is known as the geothermal flux.
Part of this geothermal flux—between 15 and 25 TW—results from a small but constant release of heat produced by the decay of long-lived radioactive atoms present in rocks. It is small because it amounts to only about 0.1 watt/m² or 8 nW per tonne, requiring enormous quantities of rock to power a light bulb; it is nearly constant because this heat output has decreased by only half since the Earth formed.
This figure includes the contribution of the decay products of uranium-238, including radium, which are in radioactive equilibrium and whose predominantly alpha decays produce a similar amount of heat. Taking the entire decay chain into account increases the heat production of uranium-238 alone by a factor of ten. The same applies to uranium-235 and thorium-232.
In addition to these modest heat releases, the concentrations of radioactive elements in rocks are low. However, the quantities involved are enormous on the scale of the Earth. The amounts of uranium and thorium present in the Earth’s crust and mantle are estimated at 50,000 and 160,000 billion tonnes respectively. According to this estimate, uranium alone would produce the electrical output equivalent to 4,620 nuclear power plants of 1 gigawatt (1,000 MW) each.
Only a small fraction of this heat escapes because of the Earth’s immense size. Radioactivity from uranium-235, uranium-238, thorium-232, and potassium-40 has decreased since the Earth’s early history because shorter-lived radioactive elements have long since disappeared. The heat production rate, on the order of a few billionths of a watt (nW) per tonne for uranium and thorium, is lower for potassium because only a tiny fraction of natural potassium consists of radioactive potassium-40 (0.0117%), even though potassium itself is more abundant in the Earth’s crust.
Geothermal energy is one of the renewable energy sources frequently discussed today. Can this vast source of energy present within the Earth, part of which originates from radioactivity, be exploited? In general, this energy is too dispersed to be efficiently recovered: a geothermal flux of 80 milliwatts per square meter would require an area of 1,000 m² to power an 80-watt light bulb. However, efforts are made to use heat stored in deep groundwater for district heating, as in the Paris Basin, or in volcanic regions to generate electricity from steam, as in Iceland.
Interested readers may consult the excellent article, The Earth’s Heat and Geothermal Energy, by Pierre Thomas, ENS Lyon – Laboratory of Geology of Lyon, 2014.