Scientists Have Unveiled the Origin of the Mysterious Gas Enveloping the Moon. The Culprit: Very Tiny Rocks

On the Moon, there's no "atmosphere," at least not the way we understand the word on Earth. For one, it lacks a magnetic field to shield its surface from cosmic and solar radiation, leaving the moon exposed to the vacuum of space. However, researchers at Massachusetts Institute of Technology have recently made a breakthrough in understanding the mysterious layer of gases surrounding the Moon. In short, this layer of gases is similar to Earth’s atmosphere.

Rocks from outer space. As it turns out, that thin and tenuous atmosphere on the moon is what scientists know as the exosphere. The issue is that the moon doesn’t have a magnetic field like Earth, so its exosphere should’ve been removed by solar activity long ago.

It was evident that something was “replenishing” these gases as they were being depleted. The new MIT study reveals that the source of this replenishment is micrometeorites—tiny rocks the size of dust grains—that continuously crash into the lunar surface. This impact kicks up and vaporizes lunar dust, releasing atoms into the space around the moon.

Official news release. As MIT geochemist Nicole Nie explained in a statement, “We give a definitive answer that meteorite impact vaporization is the dominant process that creates the lunar atmosphere... The moon is close to 4.5 billion years old, and through that time, the surface has been continuously bombarded by meteorites.”

Additionally, according to the researcher, the study has also shown that over time, a thin atmosphere reaches a stable state “because it’s being continuously replenished by small impacts all over the moon,” Nie said.

The issue of the lunar “atmosphere.” The mysterious gas enveloping the moon posed a challenge for researchers because it was too vague. Despite the detection of different atomic components by instruments the Apollo missions left behind, scientists struggled to determine its exact origin.

In their new paper, the MIT researchers propose that micrometeorite impacts and a process called “ion sputtering” may be key factors in the formation of the lunar atmosphere. Ion sputtering involves atoms being ejected from the lunar surface when bombarded with charged particles carried by solar wind.

The lunar orbiter was key. The scientists carried out a new analysis to reach the conclusion about micrometeorites. They carefully focused on the data from a lunar orbiter called the Lunar Atmosphere and Dust Environment Explorer (LADEE), which operated for seven months between 2013 and 2014. “Based on LADEE’s data, it seemed both processes are playing a role,” Nie said. “For instance, during meteorite showers, you see more atoms in the atmosphere, meaning impacts have an effect.”

Furthermore, the analysis also indicated that when the Moon is shielded from the Sun, such as during an eclipse, there are changes in the atoms in the atmosphere, “meaning that the sun also has an impact. So, the results were not clear or quantitative,” the researcher added.

To delve deeper, the MIT scientists examined actual samples of lunar dust, collected during the Apollo program, for two elements: potassium and rubidium, both known to be present on the Moon and which vaporize easily.

Results. The study team found that these two elements and their characteristics were crucial. When solar particles or micrometeorites struck the lunar surface, the rubidium and potassium present there vaporize. However, because they’re heavier elements, they quickly fall back to the lunar surface. Essentially, the proportions of the isotopes (a variation of the same element, with the same number of protons and a different number of neutrons) vary depending on whether they’re vaporized by micrometeorite impact or by ion sputtering.

The next step involved grinding the lunar dust into a thin layer and analyzing the results using a mass spectrometer. The team discovered that both processes contribute to the creation of the lunar exosphere, with micrometeorites contributing more than twice as much as the solar wind.

Implications. According to Nie, the study allows scientists to “quantify the role of both processes, to say that the relative contribution of impact vaporization versus ion sputtering is about 70:30 or larger.” What’s more, this result not only helps us understand one of the moon’s great enigmas but also has implications beyond that. For instance, if similar processes are occurring elsewhere in the solar system, such as on asteroids and other moons, scientists may be able to detect them in samples.

“Measuring potassium and rubidium isotopes in the regolith of those objects will help us understand how they were affected by meteoroid bombardments and solar wind sputtering on geological timescales and how space weathering differs across the solar system,” the study concludes.

This article was written by Miguel Jorge and originally published in Spanish on Xataka.

Image | NASA

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