The Surprising Source of the Moon’s Delicate Atmosphere: A 4.5-Billion-Year Mystery Solved
Scientists have identified the primary process responsible for forming and sustaining the Moon’s tenuous atmosphere. This atmosphere, technically known as an “exosphere,” consists of a very thin layer of atoms and has been observed since the 1980s. Researchers from MIT and the University of Chicago, in a study published in Science Advances, have concluded that the lunar atmosphere is primarily a result of “impact vaporization.”
The team analyzed lunar soil samples collected by astronauts during NASA’s Apollo missions. Their findings indicate that over the Moon’s 4.5-billion-year history, its surface has been continuously bombarded—first by massive meteorites and more recently by smaller, dust-sized “micrometeoroids.” These impacts vaporize certain atoms upon contact, causing some particles to be ejected into space, while others remain suspended above the Moon’s surface, forming a delicate atmosphere that is constantly replenished by ongoing impacts.
“We provide a definitive answer that meteorite impact vaporization is the dominant process creating the lunar atmosphere,” said Nicole Nie, the study’s lead author and an assistant professor in MIT’s Department of Earth, Atmospheric, and Planetary Sciences. “The Moon is close to 4.5 billion years old, and through that time, the surface has been continuously bombarded by meteorites. We show that a thin atmosphere eventually reaches a steady state because it’s continually replenished by small impacts all over the Moon.”
Nie’s co-authors include Nicolas Dauphas, Zhe Zhang, Timo Hopp from the University of Chicago, and Menelaos Sarantos from NASA Goddard Space Flight Center.
In 2013, NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) orbited the Moon to investigate its thin atmosphere, surface conditions, and environmental influences on lunar dust. LADEE’s mission aimed to determine the origins of the Moon’s atmosphere. The data suggested that two space weathering processes might play a role: impact vaporization and “ion sputtering,” a phenomenon involving solar wind. When these energetic particles from the Sun hit the Moon’s surface, they can cause atoms in the soil to sputter into the air.
“Based on LADEE’s data, it seemed both processes are playing a role,” Nie explained. “During meteorite showers, more atoms appear in the atmosphere, indicating that impacts have an effect. However, when the Moon is shielded from the Sun, such as during an eclipse, changes in the atmosphere’s atoms suggest the Sun also has an impact. The results were not clear or quantitative.”
To precisely determine the lunar atmosphere’s origins, Nie and her colleagues analyzed 10 samples of lunar soil, each about 100 milligrams in size. They focused on isolating two volatile elements: potassium and rubidium. These elements exist in the form of isotopes, which are variations of an element with the same number of protons but a different number of neutrons.
The team reasoned that if the Moon’s atmosphere comprises vaporized atoms, lighter isotopes should be more easily lofted, while heavier isotopes would more likely settle back in the soil. The specific ratio of light to heavy isotopes in the soil would then reveal the main process contributing to the lunar atmosphere’s formation.
After crushing the soils into a fine powder and dissolving them in acids to purify solutions containing potassium and rubidium, the researchers used a mass spectrometer to measure the isotopes. They found that the soil samples contained mostly heavy isotopes of both elements. By comparing the ratios, they determined that impact vaporization is the dominant process, contributing around 70% to the formation of the Moon’s atmosphere, with ion sputtering accounting for the remaining 30%.
“With impact vaporization, most atoms would stay in the lunar atmosphere, whereas ion sputtering would eject many atoms into space,” Nie explained.
Justin Hu, a postdoctoral researcher at Cambridge University who studies lunar soils, praised the study. He noted, “The discovery of such a subtle effect is remarkable, thanks to the innovative idea of combining potassium and rubidium isotope measurements along with careful, quantitative modeling.”
Nie emphasized the importance of returning samples from the Moon and other planetary bodies to gain clearer insights into the solar system’s formation and evolution. “Without these Apollo samples, we would not be able to obtain precise data and measure quantitatively to understand things in more detail,” she said.
Source: The Surprising Source of the Moon’s Delicate Atmosphere: A 4.5-Billion-Year Mystery Solved
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The Surprising Source of the Moon’s Delicate Atmosphere: A 4.5-Billion-Year Mystery Solved