You might not have thought so, but although the Moon doesn't have breathable air, it does possess a thin, almost imperceptible atmosphere known as an 'exosphere'.
Scientists have been studying this exosphere for many decades, but how this mysterious atmosphere came to be and how the Moon is able to sustain it has remained unknown.
A team of researchers may have finally discovered the answer to this mystery. They published their findings in the peer-reviewed journal Science Advances, and we'll talk about them in this article
A breathtaking view of our moon trough a telescope - (Image Credit: Gregory H. Revera via Wikimedia Commons)
What Created the Moon's Atmosphere?
For their study, the MIT and University of Chicago-based research team looked at soil samples from the Moon collected by astronauts during NASA's Apollo missions.
From 1969 to 1972, six Apollo missions collected and returned 382 kilograms (842 pounds) of lunar material, including rocks, core samples, pebbles, sand, and dust gathered from the Moon's surface. The soil samples revealed that the Moon's surface has been persistently bombarded over its 4.5-billion-year history.
The Moon formed likely as a result of a giant impact between Earth and a Mars-sized protoplanet called Theia. This period was characterized by intense bombardment as the solar system was still forming and full of debris. As a result, the Moon was initially bombarded by massive meteorites and, more recently, by smaller, dust-sized "micrometeoroids."
These ongoing impacts have stirred up the lunar soil, vaporizing specific atoms on contact and propelling particles into the air. Some atoms are expelled into space, while others remain suspended above the Moon, creating a thin atmosphere that is continually replenished by ongoing meteorite impacts.
Meteorite Impacts VS Solar Wind
Up until now, the solar wind has been credited as a major driver behind the Moon's atmosphere. The idea is that the Solar wind, which consists of charged particles like electrons and protons, continuously bombards the lunar surface, causing atoms from the surface materials to be ejected into the atmosphere. This process is known as ion sputtering.
However, the research team isolated and studied isotopes of potassium and rubidium retrieved from the lunar soil samples. Their analysis showed that heavier isotopes tended to settle back into the soil while the lighter ones remained in the exosphere.
This finding supports the idea that meteorite impacts, rather than solar wind, play a more significant role in maintaining the Moon's atmosphere.
Why Meteorite Impacts Matter More for the Moon's Atmosphere Than Solar Wind
So why do the researchers propose that meteorite impacts make a greater contribution to the maintenance of the Moon’s atmosphere?
When a meteorite hits the Moon's surface, it generates a tremendous amount of energy, causing the lunar soil to vaporize. During this vaporization process, lighter isotopes of elements like potassium and rubidium are more likely to be lofted into the atmosphere because they are less massive and can be more easily lifted by the energy of the impact. Heavier isotopes, being more massive, are less likely to be lofted and tend to settle back into the soil after the initial vaporization event.
In contrast, ion sputtering caused by the solar wind involves energetic particles from the Sun striking the lunar surface and ejecting atoms into space without strongly discriminating between lighter and heavier isotopes. Both light and heavy isotopes can be ejected into space relatively equally during ion sputtering.
If the solar wind were the main factor, the soil would have a more balanced mix of light and heavy isotopes because the solar wind affects both types equally. However, the fact that the actual soil of the Moon has more heavy isotopes suggests that meteorite impacts, which selectively leave heavier isotopes behind, are the primary process responsible for the Moon's thin atmosphere.
The solar wind: A constant outflow of solar material streams out from the Sun (Image Credit: NASA)
Based on their new insights, the team estimated how much of the Moon's atmosphere is created by meteorite impacts compared to the effects of the solar wind. They found that about 70 percent of the Moon's atmosphere comes from meteorite impacts, while the remaining 30 percent is from the solar wind.
The Importance of Bringing Back Samples from Space Missions
Justin Hu, a postdoc in lunar soils at Cambridge University (not associated with the study) stated that this discovery goes further than understanding the history of our Moon, as similar processes could occur and might be more significant on other moons and asteroids. They are the focus of many planned return missions.
Nicole Nie, co-author of the study, stated that without the Apollo samples, the team would not have been able to get precise data and measure quantitatively to understand things in more detail.
Nie: “It’s important for us to bring samples back from the moon and other planetary bodies, so we can draw clearer pictures of the solar system’s formation and evolution.”
This discovery not only sheds light on the Moon's atmospheric history because similar processes could occur and might be more significant on other moons and asteroids. Since these are the focus of many planned return missions, we are excited to see what will be uncovered next!
If you are interested in further details about the study be sure to check out the paper published in Science Advances listed below.
Sources and further reading of our Moon:
The Moon is shrinking, and that could be an issue for future astronauts (Universal-Sci)
Four surprising technological innovations that came out of the Apollo moon landings (Universal-Sci)
Lunar Soil Record of Atmosphere Loss over Eons (Science Advances)
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