Our planet doesn’t preserve information about its distant past very well, but close by, a hunk of orbiting rock might hold a record of the Earth’s atmosphere going back billions of years.
For five days every month, oxygen particles shower the moon during the brief period in which the planet’s magnetosphere shields it from harsh solar weather. Preserved on the moon’s surface, these particles may constitute a unique record of life on Earth.
The discovery was made by a team led by Kentaro Terada from Osaka University in Japan, which analysed data from the Kaguya lunar orbiter. The team found evidence of “biogenic terrestrial oxygen” preserved on the lunar surface.
Published in Nature Astronomy, the study suggests that transported oxygen ions get stuck in a nanometre-deep layer of soil – the “lunar regolith” – and potentially constitute a recoverable record of the Earth’s atmosphere.
Nitrogen and noble gases have long been known to make the wind-borne journey up to the moon, but until Terada’s team got to work there has never been any evidence that oxygen could do likewise.
The finding was not unexpected, though, says planetary scientist Michael Summers, from George Mason University in Fairfax, Virginia.
“The Earth’s atmosphere has been thermally evaporating at a very slow rate since it was first formed,” he says.
“These atoms, ions and molecules have been dispersed widely in the solar system.”
But, by analysing data from the Kaguya lunar orbiter, at the point when the moon and the spacecraft were shielded from solar wind, Terada and his team found the first clear evidence that oxygen is reaching the lunar surface.
Most oxygen on Earth is generated by the biosphere, so the moon has been regularly contaminated with products generated by life for about 2.5 billion years.
The Earth preserves little to no information about its own deep history because constant tectonic activity recycles its crust. But on the calm surface of the moon its ancient atmosphere is very likely preserved.
Unfortunately, this doesn’t necessarily mean that the climatic information thus stored will be at all easy to retrieve.
“The authors are properly cautious in saying that there is no realistic means to separate the atmospheric gases embedded in the lunar surface from that embedded from the solar wind,” Summers says.
“There is no apparent means to get a timeline for what is deposited on the moon. So, unless there is a process we have yet to discover, we won’t be able to sample the ancient Earth’s atmosphere by studying the lunar soil.”
So the moon might be a time capsule full of the ancient secrets of the Earth – but it seems we might not be able to unlock them just yet.