The moon has an atmosphere (sort of). Now astronomers have figured out why and how

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The moon has an atmosphere (sort of). Now astronomers have figured out why and how

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The moon may be our constant companion, but there is still much we don’t know about it.

For example, it was only during the Apollo missions in the ’60s and ’70s that we discovered that it had an atmosphere, albeit a very thin one.

“People don’t even know the moon has an atmosphere,” said Nicole Nie, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences and an author of the new lunar study.

“Technically speaking, the lunar atmosphere is not a true atmosphere. [As] Scientists call it the exosphere, simply because it is very, very thin.”

But it’s there, and scientists theorize about what creates this thin atmosphere, which is made up of helium, argon, neon, ammonia, methane, and carbon dioxide, as well as sodium, potassium, and rubidium.

Now No and co-authors new study Published in the journal Science Advances, the study provides further evidence supporting the theory that meteorites are responsible for at least part of the country’s atmosphere.

Getting into the dirt

The main factor influencing the moon’s atmosphere is thought to be something called cosmic weathering, which involves the evaporation of the moon’s surface when meteorites hit the moon. Another factor is “ion sputtering,” which comes from the solar wind, a flow of particles that moves outward into the solar system at about 1.6 million km/h.

Scientists believe that when charged particles carried by the solar wind reach the moon, they hit the surface and then transfer energy to the atoms in the soil, causing them to scatter, ultimately leading to the formation of a thin atmosphere.

Some of this information was collected via NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite, which orbited the moon in 2013–14.

But Nie and his co-authors wanted to get down to Earth and look at the moon’s regolith, which is sometimes called soil (although soil tends to contain organic matter). Scientists were able to analyze 10 samples from the Apollo missions.

A satellite is seen in orbit around the Moon.
Artist’s concept of NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) approaching lunar orbit. (NASA)

It weighed only 100 grams, so there was no room for error (you don’t want to make a mistake and waste Apollo’s precious samples). In fact, the process was so difficult that Nie said it took her three years just to develop a method for testing the samples, which involved crushing them and dissolving the tiny remaining powders in acids.

The scientists looked specifically at potassium and rubidium, two elements that evaporate easily from both ion sputtering and meteorite impacts.

Here’s where the science gets a little deeper: Each of these elements—potassium and rubidium—comes in different forms, called isotopes. There can be lighter and heavier isotopes. The researchers’ theory was that the lighter ones would likely float upwards, while the heavier ones would stay in the soil.

They concluded that the regolith contains mostly heavy isotopes of both elements, and that evaporation of these rocks is likely the main process by which the atoms are shot upward (imagine a fast, hot rock hitting another rock).

And because the moon is constantly bombarded by even small meteorites — called micrometeorites — this thin atmosphere is simply constantly being replenished.

Black and white photo of an astronaut on the Moon holding a silver container.
One of the Apollo 12 astronauts is seen on the surface holding a container of lunar soil; a second astronaut can be seen reflected in his helmet. Apollo 12 returned safely to Earth on November 24, 1969, with 34 kilograms of rock samples. (NASA/MSFC)

What does this mean for the future

This is important because in the case of ion sputtering, most of these atoms would escape into space. However, in the case of meteorites vaporizing rocks, most would remain. In fact, a recent study found that 70 percent of the lunar atmosphere is the result of these meteorite impacts.

He’s not thrilled about what this means for studying samples collected from other bodies, such as asteroids. For example, samples from the 4.5-billion-year-old asteroid Bennu were returned to Earth last September.

“I think this provides a framework for future research,” she said. “We provide a mathematical model that people can use, and then they can analyze samples, and then they can use our model to understand space weathering processes on other bodies. Because for each body the processes may be different.”

Myriam Lemelin, an assistant professor of applied geomatics at the University of Sherbrooke who was not involved in the study but is involved in several lunar missions (including the planned Canadian rover), said she is excited about the prospect of future analyses of other sites on the moon.

“The samples analyzed in this study, as in previous studies, were mainly concentrated in the equatorial region of the moon,” she said.

“The upcoming missions will target the south polar region. Based on what we can see in the orbital data sets, we think that space weathering is less intense in the polar region than in the equatorial region. So the samples that will be brought back from the south polar region can certainly be used to study the same isotopes and see if we can measure different things.”

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