Earth – still warm from the womb

Earth’s age has been derived in turn from creation myths and holy genealogies, classical thermodynamics (the study of steam engines, and how long it takes for your cup of tea to go cold), and measurements of the decay of radioactive elements such as uranium, thorium and potassium.

Once the holy books had been refiled as creative fiction, science had a first go at tackling the problem. In 1865, the Scottish physicist William Thomson (later Lord Kelvin) used thermodynamics to calculate Earth’s age as being between 20 million and 400 million years.

Thomson’s slightly rough estimate didn’t account for heat produced by radioactive decay, then unknown to science, or heat convection within Earth’s geological mantle. The naturally conservative Scot erred on the lower side of his broad calculated range, but even 400 million years was too short an age for biologists to accept, given what was known even then about evolution by natural selection.

Today, armed with radiometric dating of meteorites, terrestrial rocks and lunar samples, we can say with confidence that Earth is 4.54 billion years old, plus or minus one percent. Still, debate continues about the flux of heat radiating into space. The question is how much of this is due to continuing radioactive decay, and how much is residual cooling of the planet since its birth: the exhausting of the primordial heat supply.

The latest contribution comes from a team of scientists known as the KamLAND Collaboration. In a recent issue of Nature Geoscience, Azusa Gando and others present measurements of neutrino particles from opposite sides of the planet. Neutrinos are electrically neutral particles with virtually no mass which pass through Earth almost totally unaffected by obstructions in their path.

Gando and her colleagues found that radioactive decay of uranium-238 and thorium-232 contributes some 20 terawatts to the outgoing heat flux. This is but half of the total outgoing flux, which implies that the remaining heat is an afterglow of Earth’s formation.

Further reading: Gando et al., “Partial radiogenic heat model for Earth revealed by geoneutrino measurements”, Nature Geoscience 4, 647 (2011)