Solar wind regularly sends some of Earth’s oxygen to the surface of the Moon
It’s a bit hard to tell by looking at the rocks, trash and asteroid impact craters, but the Moon gets pummeled with high-powered particulate every day. Just about everything in the solar system does, because the Sun is always spewing out charged particles like electrons and protons, which are collectively referred to as solar wind. The Earth weathers this bombardment with help from the planet’s magnetic field, which helps deflect a significant portion of these particles. The Moon doesn’t have a protective field like that, and really only gets a break five days out of each orbit around the Earth when it hides in our shadow. At which point it starts getting hit by our particles.
Towards the outer “edges” of the Earth’s atmosphere, some atoms and molecules are energized enough that they can occasionally break out from the Earth’s gravitational pull. Others can actually be accelerated to those speeds by the same magnetic field that fends off most of the solar wind. Once they’re further out from the planet, that same solar wind can then push and rocket them deeper into space, where, when things are lined up correctly, brings them into contact with the Moon. In particular, this dynamic has put a lot of Earthly oxygen into the Moon’s surface— maybe around 4 x 1036 atoms in the last 2.4 billion years.
Samples from space and soil
This model is based largely on data from Japan’s Kaguya probe, which first dented regular spikes in the amount of oxygen ions hitting the Moon in 2008. Once an orbit, the probe would detect 26,000 oxygen ions per second, syncing up with the period of time when the Earth blocked direct impact from solar wind. This would also help explain some otherwise confusing soil samples that were gathered by Apollo astronauts in the 1970s. Those samples showed unusual amounts of oxygen-17 and -18 isotopes, which are found on Earth but are rare in the solar system at large.
If more satellites and sensors can confirm this, it will help us better understand the chemistry of our upper atmosphere. At the very least, it helps us understand how so much of the Earth ended up on the Moon in the first place.
Source: Earth is sending oxygen to the moon by Sid Perkins, Science