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AN ANALYSIS of moon rocks has answered two long-standing questions about the Earth’s companion: how old it is and what it’s made of.

Our satellite was born when a massive asteroid crashed into the early Earth, throwing a moon-sized blob of material into orbit. But this begs the question: how much of the moon is Earth matter and how much came from the asteroid?

Carsten Münker at the University of Münster in Germany and his colleagues have now worked this out (Science, vol 301, p 84). They measured the relative amounts of two elements – niobium and tantalum – in Earth rocks, bits of asteroid and moon samples.

By comparing the ratio of these elements in all three sources they deduced that between 35 and 65 per cent of the moon came from the asteroid. Since this ratio in the Earth’s crust has changed over time, they could also estimate that the collision happened 4.533 billion years ago.