Å®ÉúСÊÓƵ

All of the radioactive elements that made up the early Earth started out in the hot ash of ancient supernova explosions. This means we’ve been working through their half-lives for at least 5 billion years since our planet was born. How much hotter was Earth’s interior then? What would the heat from nuclear reactions mean for tectonic activity and the evolution of life in our world’s feverish youth?

• An earlier answer to this question said Earth’s internal heat is generated by the radioactive decay of uranium, thorium and potassium. There was, however, no mention of the gravitational heating caused by the pull of the moon in its elliptical orbit around Earth. That pull must have been considerable in the distant past, when the moon was much closer in to our planet. The heating effect is particularly evident today on Jupiter’s satellite Io.

In Earth’s case, has anyone calculated how much heating is due to radioactive decay and how much to the moon’s gravitational pull, both now and in the past?

Neil Macnaughtan, Edinburgh, UK