The Moon has no real atmosphere, but Earth has, in a sense, been generously trying to share its own for billions of years. A new study has found that our planet's magnetic field could be what's funnelling particles from Earth's atmosphere onto the lunar surface.
A surprising abundance of volatile elements has been detected in the lunar regolith – the fine, rocky dust covering the Moon's surface – ever since the Apollo astronauts brought back samples.
Solar wind is one possible source of volatiles, but that alone couldn't account for these levels, especially the nitrogen. Tiny meteorites smashing into the Moon could also be altering the lunar surface.
Related: Sound of Earth's Flipping Magnetic Field Haunts Again From 780,000 Years Ago
Earth's atmosphere has been proposed as a potential source as well, but it was assumed that this would have been possible only before our planet's magnetic field developed; once it did, the field would have trapped most atmospheric particles.
The new study, conducted by astrophysicists at the University of Rochester, investigated that assumption.
The team simulated two scenarios to see which fit the data best: an 'early Earth' model with no magnetic field and a stronger solar wind, or a 'modern Earth' model with a strong magnetic field and a weaker solar wind.
Somewhat surprisingly, the modern Earth scenario fit the bill better. The solar wind knocks charged particles out of the atmosphere, sending them careening along the planet's magnetic field lines.
Earth's magnetosphere isn't, as the name suggests, a perfect sphere. Instead, it's shaped more like the tail of a comet thanks to the constant pressure of the solar wind. And when the Moon passes through that tail, particles are deposited on the lunar surface.
Previous studies have suggested a similar mechanism could be delivering oxygen to the Moon, creating water and even rust.
The new study suggests that this process has been ongoing for billions of years, giving these volatile particles plenty of time to build up in the lunar regolith.
And since Earth's atmosphere has changed drastically in that period, a valuable time capsule of historical data could be preserved on the Moon's surface.
The research was published in the journal Nature Communications Earth & Environment.