Between 1969 and 1972, 12 astronauts left footprints on the moon. But according to new research, our planet has been sending another sign of life to the lunar surface for billions of years: oxygen. And even though an estimated 4 trillion trillion trillion atoms of oxygen have become embedded in the lunar soil in the last 2.4 billion years or so, that won’t make settling the moon any easier.
A small bit of Earth’s air leaks into space each day. (Don’t worry, it’s only about 90 metric tons out of a total of about 5 quadrillion metric tons.) Some atoms and molecules near the top of our atmosphere are simply moving so fast they overcome Earth’s gravitational tug. Charged particles can be accelerated to even higher speed by our planet’s magnetic field. Once these émigrés escape our world, they remain inside a teardrop-shaped region of space surrounding Earth called the magnetosphere (whose rounded end is pointed toward the sun) and are eventually blown away from the sun by the solar wind and into interplanetary space.
For the largest part of each month, the moon is bombarded with high-speed, highly charged atoms spewing from the sun and carried by the solar wind. But for 5 days every month, Earth’s magnetosphere passes over the moon, shielding it from the solar particles and allowing slower speed particles from Earth to take their place, says Kentaro Terada, a cosmochemist at Osaka University in Toyonaka, Japan. Moon-orbiting probes experience the same conditions, he notes.
In 2008, sensors onboard Japan’s Kaguya moon-orbiting probe detected a dramatic change in the kinds of oxygen ions striking the craft during a narrow window each month. Those ions moved at slower speeds than those typically carried by the solar wind and sported just a single positive charge. They also arrived during an interval that fell solidly inside the 5-day period when Earth’s magnetosphere blocked the solar wind. All of these factors suggest the oxygen ions originated on Earth, Terada and his colleagues report online today in Nature Astronomy. During each burst of oxygen, an estimated 26,000 ions per second passed through each square centimeter of sensor, the researchers say.
The team suggests that earthly oxygen ions most likely originated in our atmosphere’s ozone layer, where certain wavelengths of sunlight break apart ozone into regular oxygen molecules and single atoms. Later, those single atoms filtered upward to higher layers of the atmosphere and then escaped into space.
Those atoms’ origin in the ozone layer might also help explain a longstanding mystery about some grains of lunar soil brought back by Apollo astronauts. A few of those grains sport higher-than-normal proportions of oxygen-17 and oxygen-18 isotopes (as compared with the universe’s predominant form of the element, oxygen-16). Notably, Terada and his colleagues say, previous studies have shown that the overall proportions of oxygen isotopes in the ozone layer also are skewed toward above-average concentrations of oxygen-17 and oxygen-18.
“No one has ever had a convincing explanation for how those anomalies could occur in lunar soil,” says Mahesh Anand, a cosmochemist at The Open University in Milton Keynes, U.K.
The data may be quite important for another reason, says Philippe Escoubet, a plasma physicist at the European Space Agency (ESA) in Noordwijk, Netherlands. He and his colleagues analyze information gathered by a group of ESA satellites whose looping orbits carry them from near Earth to about one-third of the way toward the moon. “We’ve seen these [oxygen ions] before, but we don’t have the data to know where in Earth’s atmosphere they come from,” he notes. Now, he and his team, as well as other scientists, may be able to get a better handle on processes taking place in Earth’s atmosphere and nearby space.
For instance, Escoubet suggests, he and his team can look at data from their Earth-orbiting satellites gathered at the same time as the Kaguya data to see whether they, too, show similar increases in singly charged oxygen ions streaming from Earth. Such analyses could lead to better models of atmospheric chemistry occurring at very high altitudes on the fringes of space.