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It’s easy to imagine the Moon as an atmosphere-less hunk of rock orbiting Earth. But despite lacking breathable air, our planet’s faithful natural satellite companion has a thin, faint atmosphere.
Scientists have long wondered about the existence of this thin atmosphere, or “exosphere,” and have searched for the main process that sustains it, but new research suggests that this thin lunar atmosphere, or “exosphere,” owes its existence to the renewal and replenishment caused by the Moon’s intense bombardment of space rocks.
The team behind the research suggests that the Moon’s atmosphere is primarily sustained by this onslaught, which causes a phenomenon called “impact evaporation,” and has been so for billions of years. This process occurs when impacts lift the lunar soil into the air, vaporizing material that escapes into space or remains suspended on the Moon, thus replenishing its exosphere.
“We provide a definitive answer that the dominant process that formed the lunar atmosphere was meteorite impact evaporation,” team leader Nicole Nie, an assistant professor at the Massachusetts Institute of Technology (MIT), said in a statement. “The Moon is about 4.5 billion years old, and its surface has been continuously bombarded by meteorites during that time. We show that a thin atmosphere eventually reached a stable state because it was constantly replenished by small impacts all over the moon.”
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The Moon’s violent history
The Moon’s pockmarked and scarred surface is a stark and obvious geological reminder that it has been littered with space rocks throughout its nearly 4.5 billion-year history.
Early in the moon’s life, the infant solar system was violent and turbulent. As a result, the moon’s surface was frequently collided with large meteoroids. As time progressed, collisions between solar system bodies crushed many large space rocks. This meant that as the moon aged, the bombardment continued, but the attackers shrank into smaller “micrometeorites” – space particles smaller than a grain of sand.
But these less dramatic effects were enough to keep the impact evaporation going and the moon’s atmosphere constantly renewed.
Scientists began to suspect that the impact of space rocks on the Moon was partly responsible for the formation of the exosphere, when NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) studied the Moon’s thin atmosphere, surface conditions and environmental influences on lunar dust in 2013.
This led them to highlight two processes that replenish the exosphere. One was impact evaporation, the other was “ion sputtering.” This last process occurs when high-energy charged particles from the sun, known as the “solar wind,” strike the lunar surface and transfer energy to atoms. This also causes these atoms to be ejected into the exosphere.
“Based on LADEE’s data, it seems that both processes play a role,” Nie explained. “For example, it showed that during meteor showers you see more atoms in the atmosphere, meaning that the impacts have an effect.
“However, it showed that when the moon is shielded from the sun, for example during an eclipse, there are also changes in the atoms of the atmosphere, meaning the sun also has an influence. So the results were not clear or quantitative.”
Nie and his colleagues wanted to determine what process is primarily responsible for sustaining the moon’s atmosphere. To do so, they turned to lunar soil collected during NASA’s Apollo missions.
The answers are in the soil
The team was able to tap ten samples of lunar soil weighing just 100 milligrams each—an amount so small that Nie estimated it could fit in a single raindrop.
The researchers set out to isolate two elements in these samples: potassium and rubidium. Both elements are “volatile,” meaning they can be easily vaporized by solar flares caused by both meteorite impacts and solar wind bombardment.
The team wanted to see the ratios of different “isotopes” of potassium and rubidium. An isotope is a variant of an element that has a different number of neutrons in its atomic nucleus. This means that isotopes with more neutrons (the number of protons cannot change when changing the element to another) are heavier than those with fewer neutrons.
The team predicted that the lighter isotopes of potassium and rubidium would be more likely to remain suspended in the moon’s exosphere, while the heavier isotopes would fall back to the lunar surface. However, impact evaporation and ion sputtering would have to be differently effective at launching isotopes into the lunar atmosphere. This means looking at the amount of heavy isotopes of these two elements in the lunar soil and comparing it to the amount of lighter isotopes in the samples would reveal which of the two processes is more dominant.
“With impact evaporation, most of the atoms will remain in the lunar atmosphere, while with ion sputtering, a large number of atoms will be ejected into space,” Nie said.
Nie and his colleagues found that the soils contained mostly heavy isotopes of potassium and rubidium. This told them that impact evaporation was the dominant process by which atoms were vaporized and lifted up to form the moon’s atmosphere. They found that 70% of the exosphere was formed by meteor impacts and impact evaporation, and 30% by solar winds and ion jets.
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“The discovery of such a subtle effect is remarkable thanks to the idea of combining potassium and rubidium isotope measurements with careful, quantitative modeling,” said Justin Hu, a lunar soils researcher at the University of Cambridge who was not involved in the study. “This discovery goes beyond understanding the history of the moon, because such processes can occur and may be more important on other moons and asteroids, which are the focus of many planned return missions.”
Nie also acknowledges that the team’s findings would not have been possible without the Apollo program, which ended with Apollo 17 in December 1972.
“Without these Apollo samples, we can’t get definitive data and make quantitative measurements to understand things in more detail,” Nie concluded. “It’s important for us to bring back samples from the Moon and other planetary bodies so we can paint clearer pictures of the formation and evolution of the solar system.”
The team’s research was published in the journal Science Advances on Friday (August 2).