The Moon has almost no atmosphere, although it has no breathable air. Since the 1980s, astronomers have observed a very thin layer of atoms bouncing around on the Moon’s surface. This thin atmosphere, technically known as the “exosphere,” is likely the product of some cosmic weathering process. But it has been difficult to determine exactly what these processes might be.

Now scientists from the Massachusetts Institute of Technology and the University of Chicago say they have identified the key process that created the moon’s atmosphere and continues to maintain it today. In a study published Science DevelopmentsThe team reports that the lunar atmosphere is largely the product of “shock evaporation.”

In their study, the researchers analyzed lunar soil samples collected by astronauts during NASA’s Apollo missions. Their analysis shows that over the moon’s 4.5 billion-year history, its surface has been continuously bombarded, first by giant meteorites and more recently by smaller, dust-sized “micrometeorites.”

These constant impacts lifted the lunar soil, vaporizing certain atoms on contact and lifting particles into the air. Some atoms were ejected into space, while others remained suspended above the moon, creating a rarefied atmosphere that was constantly replenished as meteors continued to strike the surface.

Researchers have found that shock evaporation is the key process that has allowed the Moon to create and maintain its extremely thin atmosphere over billions of years.

“We provide a definitive answer that meteorite impact evaporation was the dominant process that created the Moon’s atmosphere,” said lead author of the study Nicole Nee, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“The Moon is about 4.5 billion years old, and during that time its surface has been continuously bombarded by meteoroids. We show that the thinned atmosphere eventually reached a stable state because it is constantly being replenished by small shocks all over the Moon.”

Ni’s co-authors include Nicholas Dofas, Zhe Zhang and Timo Hopp of the University of Chicago, and Menelaos Sarantos of NASA’s Goddard Space Flight Center.

The roles of weather conditions

In 2013, NASA sent an orbiter to the moon to conduct a detailed study of the atmosphere. The Lunar Atmosphere and Dust Explorer (LADEE, pronounced “kid”) was tasked with remotely gathering information about the moon’s thin atmosphere, surface conditions, and environmental influences on lunar dust.

The LADEE mission was designed to determine the origin of the Moon’s atmosphere. Scientists hoped that the probe’s remote measurements of the composition of the soil and atmosphere could be linked to specific space weathering processes, which could then explain how the lunar atmosphere formed.

Researchers suspect that two space-weathering processes play a role in shaping the lunar atmosphere: shock evaporation and “ion sputtering,” a phenomenon associated with the solar wind, which carries energetic particles from the Sun through space. When these particles hit the moon’s surface, they can transfer their energy to atoms in the soil, knocking them out and into the air.

“Based on the LADEE data, it appears that both processes play a role,” Nee says.

“For example, during meteor showers, more atoms are seen in the atmosphere, which means the impacts are effective. However, it has also been shown that when the Moon is protected from the Sun, for example during an eclipse, there are changes in the atoms in the atmosphere, meaning the Sun also has an effect.

Answers in the soil

To more precisely determine the origin of the moon’s atmosphere, Nee looked at samples of lunar soil collected by astronauts during NASA’s Apollo missions. He and colleagues at the University of Chicago took 10 samples of lunar soil, each containing about 100 milligrams—the small amount he estimated would fit in a single raindrop.

Nee first tried to isolate two elements from each sample: potassium and rubidium. Both elements are “volatile,” meaning they evaporate easily during impacts and ion sputtering.

Each element exists in several isotope forms. An isotope is a variant of the same element that has the same number of protons but a slightly different number of neutrons. For example, potassium can exist as one of three isotopes, each with one more neutron and slightly heavier than the previous one. Similarly, rubidium has two isotopes.

The team reasoned that if the moon’s atmosphere consists of atoms that evaporate and remain suspended in the air, the lighter isotopes of these atoms should be more easily removed, while the heavier isotopes are more likely to settle back into the soil.

Scientists also predict that shock evaporation and ion sputtering should have led to very different isotope ratios in the soil. For both potassium and rubidium, the specific ratio of light and heavy isotopes remaining in the soil should reveal the underlying processes that contributed to the formation of the lunar atmosphere.

With all this in mind, Nee analyzed the Apollo samples by first grinding the soil into a fine powder and then dissolving the powders in acids to purify and isolate solutions containing potassium and rubidium. He then ran these solutions through a mass spectrometer to measure the different potassium and rubidium isotopes in each sample.

The team found that the soils contained mostly heavy isotopes of potassium and rubidium. The researchers were able to measure the ratio of heavy to light isotopes of both potassium and rubidium, and by comparing the two elements, they found that shock evaporation was most likely the dominant process by which atoms vaporized and rose to form the moon’s atmosphere.

“With shock evaporation, most of the atoms will remain in the lunar atmosphere, while with ion sputtering, many atoms will be ejected into space,” Ni says.

“From our work we can now quantify the role of both processes; we can say that the relative contribution of shock evaporation to ion sputtering is about 70:30 or more.” That is, 70% or more of the Moon’s atmosphere is the product of meteor impacts, with the remaining 30% being the result of solar winds.

“Finding such a subtle effect is remarkable because of the innovative idea of ​​combining measurements of potassium and rubidium isotopes with careful quantitative modeling,” says Justin Hu, a postdoctoral researcher at the University of Cambridge who studies lunar soils and is not affiliated with the research. teaching.

“This discovery goes beyond understanding the history of the Moon, as such processes may occur and be more important on other moons and asteroids, which are the focus of many planned return missions.”

“Without these Apollo samples, we wouldn’t have been able to get accurate data and make quantitative measurements to understand events in more detail,” Nee says. “It’s important for us to bring back samples from the Moon and other planets so we can paint clearer pictures of the formation and evolution of the Solar System.”