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Ever since the Apollo missions brought lunar soil back from the Moon, planetary scientists have been puzzling over how it came to contain nitrogen. Now, it seems the nitrogen originally came from Earth.

Previously, scientists thought the most likely explanation was that the solar wind deposited the nitrogen on the Moon. But the amount of lunar nitrogen and the ratios of its isotopes do not match the values in the solar wind.

So Frank Podosek of Washington University in St Louis, Missouri, US, and colleagues came up with an alternative explanation. “If you turn off Earth's magnetic field, the wind from the Sun can blow nitrogen from Earth's outer atmosphere onto the moon,” he says.

He thinks this is what happened early in the Earth's history, before its magnetic field became active. Once it became active due the convection of molten iron in the core, the magnetic field shielded the atmosphere from the solar wind - by deflecting its charged particles - and no more nitrogen was blown away.

The idea could be tested by studying lunar meteorites - rocks that have been blasted off the moon in asteroid impacts and then fallen to Earth.

Surface material from the dark side of the moon would not have been exposed to the Earth’s wind and so should contain less nitrogen. “We don't know where on the Moon these meteorites came from, but we guess that half come from the front and half from the back face,” he says. “If we test them all, and about half have a different composition of nitrogen, then we'll know that our theory is right.”
Mixed sources

Marc Chaussidon, at the French national research agency CNRS, says: “This is a very clever theory and might well be true, although it can’t be the whole story.”

Chaussidon has investigated the composition of nitrogen in the solar wind. He points out that some lunar soil samples still have levels of certain nitrogen isotopes that are higher than recorded in the terrestrial atmosphere. “The nitrogen probably comes from a combination of three places: the solar wind, meteorite impacts on the Moon, and from the Earth – as suggested here,” he says.

Geologists could potentially use the nitrogen composition of lunar samples to work out when the Earth’s magnetic field started up, says Podosek. If lunar samples older than a certain age have significantly more nitrogen than the younger ones, it could help estimates as to when the Earth’s magnetic field turned on.

Marc Chaussidon welcomes the idea. “We really know nothing about when the Earth’s magnetic field appeared,” he says. “This could be a very effective way to find out more.”

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