What sustains moon's fragile exosphere? Being 'bombarded' by meteorites, study says

The moon may not have any breathable air, but that doesn't mean it has no atmosphere.

Astronomers have for decades been aware of the delicate atmosphere surrounding the moon that is so thin and fragile that it's better referred to as an exosphere. And while scientists have long been stumped as to how that exosphere has managed to hang on, one team of researchers believes they finally have an answer.

Turns out, our planet's singular natural satellite can thank the longevity of its exosphere to the space rocks that have reliably "bombarded" it throughout its 4.5 billion-year history, the team wrote in research published this month.

First it was massive meteorites that routinely crashed into the pock-marked moon. More recently, however, it's been smaller, dust-sized “micrometeoroids” that have been constantly colliding into the lunar surface – kicking up soil and lofting up vaporized atom particles into the air in a process the researchers call, "impact vaporization."

Some of those atoms are ejected into space. But, crucially, enough of them remain suspended over the moon to sustain its exosphere long enough for more meteorites to pelt the surface, according to the research.

"The (moon's) surface has been continuously bombarded by meteorites," lead authorer Nicole Nie, a geochemist at the Massachusetts Institute of Technology, said in a statement. "We show that eventually, a thin atmosphere reaches a steady state because it's being continuously replenished by small impacts all over the moon."

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LADEE orbiter provides data on moon's exosphere

The moon's heavily cratered surface serves as a testament to how the celestial object has become a figurative magnet for space rocks throughout its long lifespan.

But it wasn't until a NASA lunar orbiter investigated the moon in 2013 that astronomers began to suspect that the exosphere's existence depends on those space rock assaults. The team meticulously studied data from the orbiter, called the Lunar Atmosphere and Dust Environment Explorer (LADEE,) which ultimately spent seven months gathering intel about the moon's atmosphere and conditions near the surface.

Based on LADEE's discoveries, scientists have theorized that two processes could be behind shaping the lunar atmosphere: impact vaporization and “ion sputtering," a phenomenon in which solar wind carries energetic charged particles from the sun through space. When these particles hit the moon’s surface, they can transfer their energy to the atoms in the soil and flinging those atoms into the air. 

The new analysis that Nie and her colleagues performed led them to confirm that both processes are indeed playing a role.

"During meteorite showers, you see more atoms in the atmosphere, meaning impacts have an effect," Nie said in a statement. "But it also showed that when the moon is shielded from the sun, such as during an eclipse, there are also changes in the atmosphere's atoms, meaning the sun also has an impact."

Team studies lunar soil from Apollo missions

To determine which process bears more responsibility for the moon's exosphere, the team turned to soil samples collected by astronauts in the Apollo lunar program, which came to an end in 1972.

While studying the samples, the researchers were primarily looking for two elements both known to occur on the moon: potassium and rubidium. Because both elements are easily vaporized, the team reasoned that an analysis of their isotopes – variations of the same elements – would allow them to conclude whether meteorite strikes or solar sputtering contributed more to the moon's atmosphere.

Ultimately, the team determined that the soils contained mostly heavy isotopes of both potassium and rubidium. This told them impact vaporization was the most pivotal process in vaporizing atoms and ejecting them into the moon's atmosphere.

Could other celestial bodies exhibit a similar process?

The findings, the team contends, have implications far beyond determining the moon's atmospheric origins.

In fact, it's not unthinkable that similar processes could potentially be taking place at other celestial bodies in the solar system, including asteroids and other moons, said Justin Hu, a geophysicist at Cambridge University studying lunar soils, who was not part of the study.

“Without these Apollo samples, we would not be able to get precise data and measure quantitatively to understand things in more detail,” Nie concluded. “It’s important for us to bring samples back from the moon and other planetary bodies, so we can draw clearer pictures of the solar system’s formation and evolution.”

The team's research was published Friday in the journal Science Advances.

Eric Lagatta covers breaking and trending news for USA TODAY. Reach him at [email protected]