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It’s easy to imagine the moon as an atmosphereless rock orbiting Earth. But despite having no breathable air, our planet’s faithful natural satellite does have a thin, tenuous atmosphere.
Scientists have long puzzled over the existence of this tenuous atmosphere, or “exosphere,” and have been searching for the key process that sustains it. However, new research suggests that this tenuous lunar atmosphere, or “exosphere,” owes its existence to renewal and replenishment caused by the moon’s heavy bombardment of space rocks.
The research team suggests that the moon’s atmosphere has been largely maintained by this onslaught for billions of years, creating a phenomenon called impact vaporization. This process occurs when impacts kick up the lunar soil, vaporizing materials that then escape into space or float above the moon, renewing the exosphere.
“We provide a definitive answer that evaporation from meteorite impacts is the dominant process creating the moon’s atmosphere,” team leader Nicole Nie, an assistant professor at the Massachusetts Institute of Technology (MIT), said in a statement. “The moon is nearly 4.5 billion years old, and during that time its surface has been continuously bombarded by meteorites. We show that eventually a thin atmosphere reaches a steady state because it is continuously replenished by small impacts all over the moon.”
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The History of Violence on the Moon
The moon’s pitted and battered surface is a stark geologic reminder that the moon has been littered with space rocks throughout its nearly 4.5 billion-year history.
Early in the moon’s life, the young solar system was violent and turbulent. As a result, the lunar surface was frequently hit by huge meteorites. As time went on, collisions between bodies in the solar system ground up much larger space rocks. This meant that as the moon grew older, the bombardment continued, but the attackers shrank to smaller “micrometeoroids,” particles from space smaller than a grain of sand.
However, these less dramatic impacts were enough to continue the evaporation of the impact and continually refresh the Moon’s atmosphere.
Scientists first began to suspect that the onslaught of space rocks on the moon was partly responsible for the creation of the exosphere when NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) in 2013 investigated the moon’s thin atmosphere, conditions on the surface and outside influences on the lunar dust.
This led them to highlight two processes that regenerated the exosphere. The first was impact vaporization, the other was “ion sputtering.” The latter process occurs when high-energy charged particles from the sun, known as the “solar wind,” hit the lunar surface and impart energy to atoms. This also causes those atoms to be flung into the exosphere.
“Based on the LADEE data, it seemed that both processes were at play,” Nie explained. “For example, it showed that during meteor showers, you see more atoms in the atmosphere, which means that impacts are having an effect.
“But it also showed that when the moon is shielded from the sun, like during an eclipse, there are also changes in the atoms of the atmosphere, which means the sun is also having an impact. So the results were not clear or quantitative.”
Nie and colleagues wanted to determine which process is primarily responsible for maintaining the moon’s atmosphere. To do this, they used lunar soil collected during NASA’s Apollo missions.
The answers lie in the dirt
The team was able to touch ten samples of lunar soil, each weighing just 100 milligrams. This amount is so small that Nie estimated it would 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 are easily evaporated by meteorite impacts and by solar sputtering caused by bombardment by the solar wind.
The team wanted to see the ratios of different “isotopes” of potassium and rubidium. An isotope is a variation of an element that has different numbers of neutrons in its nucleus. This means that isotopes with more neutrons (the number of protons cannot vary when changing from one element to another) are heavier than those with fewer.
The team predicted that light isotopes of potassium and rubidium would be more likely to float into the moon’s exosphere, while heavier isotopes would fall back to the lunar surface. However, impact vaporization and ion sputtering should be different in their effectiveness at throwing isotopes into the lunar atmosphere. That means looking at the abundance of heavy isotopes of these two elements in the lunar soil and comparing it to the abundance of lighter isotopes in the samples should reveal which of the two processes is more dominant.
“With impact vaporization, most of the atoms would remain in the moon’s atmosphere, while with ion sputtering, many atoms would be ejected into space,” Nie said.
Nie and colleagues found that the soil contained mostly heavy isotopes of both potassium and rubidium. This told them that impact evaporation was the dominant process by which atoms were vaporized and lifted upward to form the moon’s atmosphere. They found that 70% of the exosphere was generated by meteorite impacts and impact evaporation, with 30% attributed to the solar winds and ion sputtering.
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“The discovery of such a subtle effect is remarkable, thanks to the innovative idea of combining potassium and rubidium isotope measurements with careful, quantitative modeling,” said Justin Hu, a lunar soil researcher at the University of Cambridge who was not involved in the study. “This discovery goes beyond understanding the Moon’s history, as similar processes may occur and 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 simply would not have been feasible without the Apollo program, which ended in December 1972 with Apollo 17.
“Without these Apollo samples, we wouldn’t be able to get accurate data and 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 get clearer pictures of the formation and evolution of the solar system.”
The team’s research was published Friday (August 2) in the journal Science Advances.