Examining old rocks with new technology reveals the source of the moon’s atmosphere: Meteorites bombarding the lunar surface over billions of years have kicked up dust that forms the satellite’s tenuous atmosphere, according to a report in Science Advances.
Why Scientists are Studying Moon Rocks
A team of scientists analyzed moon rocks collected during the Apollo missions that ran from 1968 to 1972. Those samples had been examined earlier.
“But the instruments weren’t very precise back then,” says the study’s lead author, Nicole Nie, an assistant professor at MIT.
In 2013, NASA launched the Lunar Atmosphere and Dust Environment Explorer to gather information about the moon’s thin atmosphere. That mission delivered two clues. First, potassium and rubidium concentrations in the moon’s atmosphere increased during meteor showers. Second, the levels of those elements changed during a lunar eclipse, which shielded it from Earth.
Those findings hinted that two processes combined to form — and constantly replenish — the moon’s atmosphere. Researchers suspected that two space weathering processes play a role in shaping the lunar atmosphere: impact vaporization and ion sputtering.
Read More: 7 Things You Didn’t Know About Moon Rocks
Space Weathering Processes
Impact vaporization happens when meteorites hit the surface and essentially kick up dust into the atmosphere. Ion sputtering occurs when the solar wind carries charged particles from the sun to the moon’s surface and transfer energy into the soil, sending those atoms sputtering into the air.
That information pointed the researchers in the right direction. But it didn’t provide conclusive data about what and how much of the potassium and rubidium found in the moon’s atmosphere arose from vaporization versus solar winds.
So, Nie and her team gained 10 samples from the Apollo rocks — each about the size of a single raindrop. They crushed the samples into powder, dissolved the powders in acids to isolate them into separate solutions containing potassium and rubidium. They then analyzed the solutions with a mass spectrometer to measure the various isotopes of both potassium and rubidium in each sample.
Essential Isotopes
Determining the isotopes presence was essential. An element’s isotopes have the same number of protons, but different amounts of neutrons.
Because both potassium and rubidium are volatile — meaning that sudden changes like, say, impact could morph them into different isotopes — measuring these atomic variations could show both what elements from the moon’s surface made it into the atmosphere, as well as provide a pretty sound explanation as to how.
If the moon’s atmosphere consists of atoms that have been vaporized and suspended in the air, lighter isotopes should be more easily kicked into the atmosphere, while heavier isotopes would likely settle into the soil.
Also, impact vaporization, and ion sputtering cause very different isotopic proportions. Therefore, the specific ratio of light to heavy isotopes that remain in the soil, for both potassium and rubidium, should show the process contributing to the lunar atmosphere.
Read More: 50 Years Later, NASA Just Handed Scientists Untouched Apollo Moon Rocks
The Moon’s Surface
The scientists’ analysis showed that about 70 percent or more of the moon’s atmosphere arose from meteorite impacts, while the rest was blown in from solar winds.
While the scientists weren’t surprised by their findings, since the orbiter predicted these elements’ presence, this study makes a pretty convincing case that most of the atmospheric elements came from the surface.
“The surface is being bombarded by all sorts of things,” says Nie. “You would expect these things would release atoms into the atmosphere.”
She is looking forward to seeing samples from lunar missions like Artemis that will bring soil samples from the moon’s south pole, as well as its dark side, because those samples could have different concentrations of the key elements. She’s also excited to see samples from one or both of Mars’ moons.
“Our framework involves a technique to analyze samples from other planetary bodies,” Nies says.
Read More: Why We See Only One Side of the Moon’s Surface
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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
