The Earth may be leaving its mark on the moon in more ways than previously thought.
A little bit of oxygen from our planet's atmosphere is escaping and ending up on the lunar surface, according to new research – and this may have been happening for billions of years.
Researchers analyzing data from the Japanese lunar orbiter SELENE, nicknamed Kaguya, picked up on what appears to be terrestrial oxygen ions that had been transported from the Earth's atmosphere to the moon.
As Earth's oxygen is thought to be almost entirely a result of photosynthesis, this suggests that the signature of life on Earth is ending up on the moon, the scientists say in a paper published Monday in the journal Nature Astronomy.
And this detection could help resolve a longstanding moon mystery.
The sun spews highly charged material that is carried by solar winds to bombard the Earth-moon system. While the Earth is largely protected by its magnetosphere, the moon is more vulnerable. This led scientists to assume that these high-energy ions end up buried in the lunar soil and therefore that the moon should bear an isotopic signature matching the solar wind.
Using that logic, more than a decade ago some researchers used moon rocks to estimate the composition of the sun. Some of those sediments were poor in oxygen-16, one of the stable isotopes of oxygen.
Theoretically, that should mean that the solar winds were also oxygen-16-poor. But solar wind samples collected by NASA's Genesis spacecraft revealed that it was actually rich in oxygen-16.
So something else must explain the lunar soil samples' oxygen composition. And that might just be the Earth's atmosphere, according to Kentaro Terada, a professor of isotope cosmochemistry and geochemistry at Osaka University in Japan and lead author of the new Nature Astronomy paper, and colleagues.
"One of the things that appears to have been a mystery is the oxygen isotope composition for the lunar soil," says Kathleen Mandt, a senior research scientist at the Southwest Research Institute studying planetary atmospheres who was not involved in the research. "They are providing a potential explanation for it."
How exactly does oxygen in the Earth's atmosphere get to the moon?
The Earth's magnetosphere – its protective blanket of sorts – doesn't envelop our planet evenly. Instead, it is more teardrop-shaped, with the rounded end toward the sun and the rest of it streaming away from the sun as the charged atoms of the solar wind distort it on its way out into space.
Because the moon is orbiting the Earth, it passes through the planet's magnetosphere for about five days in every orbit. During that time our satellite is also shielded from the bombardment of charged material from the sun.
But that doesn't mean the moon is safe from all streams of charged particles. When it passed through the Earth's magnetosphere, it also passed through the Earth's plasma sheet. That stream of charged material extends out in the middle of the magnetotail, as the extended part of the magnetosphere is called, and contains relatively high-energy oxygen ions, according to SELENE's detections.
Dr. Terada and his colleagues suggest that these oxygen ions have been whisked out of the upper atmosphere to be transported to the moon's location. And their model suggests the oxygen would be deposited in the lunar soils in compositions consistent with the oxygen-16-poor measurements.
"That's neat because it's kind of coming together in terms of perhaps why that measurement is the way it is," says Justin Simon, a cosmochemist who leads the Center for Isotope Cosmochemistry and Geochronology at NASA's Johnson Space Center and who was not involved in the research.
"The logical thing we could do would be to make more measurements of lunar materials," he tells The Christian Science Monitor in a phone interview, particularly as "it is very unfair to say that one measurement is representative of the moon."
The surface of the moon is certainly not uniform, as it is constantly being "gardened," as researchers call it, by the bombardment of meteorites.
Signatures of early life on Earth, on the moon
As oxygen is a key signature of life on Earth, if Earth's atmospheric oxygen has been ending up in the lunar soil for billions of years, as Terada and colleagues suggest, then the history of life on Earth might be tied up in those moon rocks too.
This detection suggests that the "Earth and Moon system have been coevolving not only "physically" but also “chemically (transportation of materials)”, in spite that the distance between earth and moon is [about 240,000 miles]," Terada writes in an email to the Monitor. "This is so exciting to me."
But that doesn't mean we should turn our moon into the next edition of history books, Dr. Mandt tells The Christian Science Monitor. "I think it would require a lot of work to really untangle that," she says.
In addition to the surface of the moon not being static thanks to impacting space rocks and other surface geological processes, researchers would have to have reliable models of the Earth's atmosphere and magnetosphere to understand how oxygen has been transported from Earth to the moon over the system's entire history.
"I'm not saying it's impossible," Mandt says. "It's just a big, complicated project."
Moon hopping for space travel
Understanding the composition of the moon's surface isn't just important for learning about our own planet. This information could help astronauts travel outside our own planet's system in the future, Dr. Simon suggests.
Oxygen is key for people to breathe, so perhaps future space travelers would be able to mine some from the moon or this transport process to be able to convert it to a useful form.
In addition to oxygen detections, Terada and colleagues also studied the hydrogen isotopes that were being transported to the moon. Hydrogen can be a fuel, Simon adds, or could be combined with oxygen to become water.
"These are all thing that one needs to further exploration of the planets and maybe to use the moon or some other body as a launching point to even further exploration," Simon says. And this sort of hopping from planet to planet, using resources found along the way, could help astronauts travel farther into our solar system.