The Moon is anything but a lifeless rock hanging in our sky. It has pockets of water ice trapped on its surface, caves in which astronauts might one day live, and even an incredibly tenuous atmosphere known as an exosphere.

That thin layer of atoms, which begins at the lunar surface and extends 100 kilometers (60 miles) into space, exists mostly because small micrometeoroids strike the surface, vaporizing atoms and lofting some into the void. That’s according to a new study published in Science Advances.

The Moon’s exosphere is made up of elements such as argon, helium, oxygen, and potassium. It has an extremely low density—just 100 molecules per cubic centimeter (0.06 cubic inch) compared with the 30 quintillion (1018) at Earth’s surface—but is substantial enough to glow in sunlight, as seen by spacecraft and even by some Apollo astronauts.

The origin of that exosphere had been uncertain since its discovery in the 1970s. Scientists had speculated that either micrometeoroids were to blame or solar wind hitting the lunar surface transferred energy into particles and ejected them upward in a process called sputtering.

Nicole Nie, a cosmochemist at the Massachusetts Institute of Technology, and colleagues believe they have now solved the mystery. “The meteorite impacts are the cause of more than 70% of the atoms being released into the air,” Nie said, “and the solar wind sputtering contribution is less than 30%.”

Micrometeoroids are minuscule pieces of dust, molecules to millimeters in scale, that drift through space. Their impact on the Moon is constant, as is the solar wind sputtering effect. Both replenish the atoms in the exosphere that then “stay there for maybe days or weeks,” Nie said, before they either fall back to the Moon’s surface or are lost to space.

Evidence for these two processes was detected by NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft in 2013. Data showed that sodium and potassium in the lunar atmosphere increased during meteor showers and dropped when the Moon was shielded from the solar wind inside Earth’s magnetic field.

These elements are linked to both processes, indicating that both micrometeoroids and solar wind added to the Moon’s exosphere. “The question was, Which one was the major contributor?” Nie said.

Moon Rocks

To find out, Nie and her colleagues examined rocks returned by the Apollo missions of the 1960s and 1970s. The group measured the ratios of light to heavy isotopes of both potassium and rubidium. “These isotopes are very sensitive to the meteoritic impact and solar wind sputtering,” Nie said.

Both processes would result in some lighter isotopes being ejected into space. Heavier isotopes would fall back to the surface, to be detected later in the Apollo samples. However, the exact ratio of heavy and light isotopes in the lunar soil allowed Nie and her team to work out which process was more dominant.

“You can calculate the contribution of each process,” she said, using a mathematical model the team developed. “Both scenarios will cause the lunar soils to be isotopically heavy, but the isotope ratios they cause are different.”

This kind of analysis was not possible until decades after the samples were collected. “Nowadays we have much better mass spectrometers,” instruments that can pick apart the composition of rock samples, Nie said.

Justin Hu, a planetary scientist at the University of Cambridge in the United Kingdom who was not involved in the study, said the work involved “very careful experimental design” that had a good understanding of the natural processes taking place on the Moon. The result gives a “convincing interpretation,” he said.

Other bodies in the solar system, including Mercury and Mars’s moon Phobos, also have slim exospheres that could be linked to impacts and solar wind sputtering. “We should think about the element loss that may happen on many other planets, moons, and asteroids,” Hu said. “It could affect their composition and be used to track their formation history.”

Nie said she is already hoping to repeat the experiment with samples set to be returned from Phobos in 2031 by a Japanese robotic probe called Martian Moons Exploration (MMX). “I’m going to measure the potassium,” she said, although rubidium will not be measurable because of the small amount of sample being collected, perhaps just 10 grams in total compared with the nearly 400 kilograms of Moon rock returned by the Apollo missions.

This article was originally published in Eos.