The man in the Moon knows a lot more about us than we think. For instance, it's keeping tabs on the air we breathe by collecting samples of it.
Kaguya, the nickname for Japanese spacecraft SELENE, collected data with its particle sensors while orbiting 100km above the Moon from 2007 to 2009. The Moon, like Earth, gets a daily treatment of high-energy solar radiation. But for five days a month, it sits in the shadow of the Earth's magnetic field. Japanese scientists looking at Kaguya data have now learned that oxygen, which could have come from plants on Earth, hits the orbiter during this time.
How could oxygen from the Earth be making its way to the Moon? Well, our planet's magnetic field leaves a long funnel of electrically-charged particles in its wake. A much slimmer funnel with a higher particle density, called the plasma sheet, sits at the centre. The study, published today in Nature Astronomy, found higher-energy O+ ions — oxygen atoms minus one electron — when Kaguya passed into the plasma sheet. This high-energy oxygen was not detected when the Moon was outside Earth's magnetosphere.
After their analysis, the team was certain these O+ ions didn't come from the solar wind — they came from Earth's atmosphere, which means that plants probably produced them during photosynthesis. The ions also probably get stuck in the lunar dirt. "A consequence of this finding is that the entire lunar surface can be contaminated with biogenic terrestrial oxygen, which has been produced by photosynthesis over a few billion years."
We already knew there's Earth crap on the Moon, and not just the stuff our humans left there. But we haven't seen this process of Earth-Moon exchange for our favourite gas, oxygen, and it's pretty incredible that Kaguya was able to take a whiff from so far away.
Other scientists found the analysis reasonable. Saturn's moon Iapetus also gathers material collected from other moons, Alexander Mustill, a postdoc in theoretical physics at the Universidad Autónoma de Madrid, told Gizmodo. Still, "the very exciting part of this paper is in the possible implications: the possibility of reconstructing from deposits on the Moon the history of the Earth's atmosphere," he said in an email. However, both Mustill and the Japanese scientists acknowledge that at the present, there's not really a way to tell when an oxygen ion hit the Moon, so it would be hard to paint a picture of the Earth's atmospheric history with this data alone.
But just think, all this time the Moon's been watching us, collecting intel on our atmosphere. What else does it know about us? Might be time to change the Earth's password or something.