Ever wondered if the moon has an atmosphere? Surprisingly, it does, but it’s nothing like Earth’s. According to recent research, it’s actually extremely thin and almost ghost-like.

Published in Science Advances, the study reveals that the Moon’s atmosphere, known as an “exosphere,” is primarily sustained by meteorite impacts and has been around for billions of years. These impacts have been occurring since the Moon’s formation, vaporizing parts of its surface and creating this weak atmosphere. Unlike Earth’s dense and breathable atmosphere, the Moon’s exosphere is composed of atoms that are so sparse they rarely collide with each other.

Some of these atoms escape into space, but many remain, creating a thin, fragile atmosphere. This process, known as impact vaporization means atoms are being lofted upward and then settling back down, only to be kicked up again.

“We give a definitive answer that meteorite impact vaporization is the dominant process that creates the lunar atmosphere,” says the study’s lead author, Nicole Nie, 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 eventually, a thin atmosphere reaches a steady state because it’s being continuously replenished by small impacts all over the moon.” 

Solar wind is another main contributor. Called “ion sputtering,” it consists of charged particles from the Sun that knock atoms off the Moon’s surface. However, the study shows that meteorite impacts play a more significant role, contributing about 70% or more to the Moon’s atmosphere, with solar wind accounting for the rest.

The key to these discoveries lies in lunar soil samples brought back by the Apollo missions. Scientists crushed these samples into powder and analyzed the isotopes of the potassium and rubidium within them. Both elements are “volatile,” meaning that they are easily vaporized by impacts and ion sputtering. By examining the ratio of lighter to heavier isotopes, they determined that impact vaporization is the main process of forming the Moon’s atmosphere.

This isn’t the first time scientists have been interested in the Moon’s atmosphere. In 2013, NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) orbited the Moon to gather detailed data about its atmosphere. LADEE also showed that both meteorite impacts and solar wind contribute to the lunar exosphere.

“Based on LADEE’s data, it seemed both processes are playing a role,” Nie says. “For instance, it showed that during meteorite showers, you see more atoms in the atmosphere, meaning impacts have an effect. 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. So, the results were not clear or quantitative.”

Understanding the Moon’s atmosphere helps scientists learn more about its history and the processes shaping its surface. This research also has implications for other celestial bodies. It shows the importance of sample-return missions, which provide the detailed data needed to find the atmospheres of other cosmic objects.

“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,” says Justin Hu, a postdoc who studies lunar soils at Cambridge University, who was not involved in the study. “This discovery goes beyond understanding the Moon’s history, as such processes could occur and might be more significant on other moons and asteroids, which are the focus of many planned return missions.”