The mystery of where the moon's water came from may soon be solved. Evidence from NASA's LCROSS mission suggests much of it was delivered by comets rather than forming on the surface through an interaction with the solar wind.

Volatiles, including hydrocarbons known to be present in comets, have been detected in lunar material kicked up by NASA's LCROSS mission (Image: T.A.Rector/I.P.Dell'Antonio

These compounds should have been mostly lost to space billions of years ago, when the moon coalesced from the debris of an impact between the Earth and a Mars-sized object. Water formed through an interaction with the solar wind would therefore be relatively pure – and free of volatiles.

But comets, which are thought to have been responsible for many of the moon's impact scars, are "dirty iceballs" known to contain volatiles such as methane. "If you can nail down the source of the water [on the moon], that could tell us a lot about the cometary history of the moon for the last couple of billion years," says Larry Taylor of the University of Tennessee.

High concentrations

The second line of evidence pointing to comets comes from the amount of water detected. The solar wind is expected to form water in minute amounts, amounting to concentrations of no more than 1 per cent in the lunar soil.

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