First Author

The composition of our planet has long fascinated Bernard Wood. Currently based at Macquarie University in New South Wales, Australia, Wood has spent the past ten years trying to determine the chemical processes that occurred as the Earth's core separated from the mantle.

This work had led him to most of the reactions that were responsible for the composition of the planet's core. But there was one that had him stumped — how and when did sulphur get added to the mix?

The paper on page 1345 of this issue reveals how Wood solved the riddle. He reasoned that the impact on Earth of the body that resulted in the Moon's formation could also have caused the introduction of sulphur to the core. To help him clarify this, he hooked up with Alex Halliday, a geochemist at the University of Oxford, UK. Together they generated a fresh estimate for Earth's cooling age — when cooled from a molten state to how it is now.

Your paper refers back to work published in Nature in 1895 by Lord Kelvin. How do your results square with Kelvin's original estimate of cooling and the age of Earth?

Our ‘late‘ sulphide addition suggests that the cooling time for Earth after the great impact was 30 million years, which is close to Kelvin's estimate of 24 million years. But, as one of the reviewers pointed out, modern numerical models indicate that Earth would have cooled much more quickly than that, becoming mostly crystalline after a few thousand years. So we think that our estimate reflects the time taken to separate the sulphide from the rest of Earth, rather than being a cooling age.

How do you think Kelvin would react to your findings?

Kelvin had broad interests and so would be intrigued by the findings. But he would recognize them as not being any vindication of his view that Earth was only about 24 million years old. He probably realized by the time of his death that his calculated ‘cooling age’ of Earth had ignored the most important heat source — that provided by radioactive decay of potassium, uranium and thorium.

You'd never worked with Alex Halliday before. How did he come on board?

I knew that a late sulphide addition to the core would take a lot of lead and so affect the apparent age of the core. I called Alex, told him the story and got him interested in figuring out the isotopic consequences of the model.

What's next for you?

I don't know exactly, but I am looking at other ways of testing and constraining our hypothesis of late sulphide addition.