Not everyone knows it, probably, but the Moon also has an atmosphere. Much thinner than the Earth’s, obviously. In fact: so thin as to be, for all intents and purposes, negligible. Planetologists, however, would like to know its origin and evolution precisely. And a new study from the Massachusetts Institute of Technology (MIT) seems to have the answer: the lunar atmosphere would in fact be the result of the continuous impact of meteorites on the surface of the satellite, which vaporizes the lunar regolith and constantly plumps up the gaseous layer that surrounds it.
The research, published in the journal Science Advances, was carried out by analyzing a sample of lunar soil collected during NASA’s Apollo program. By studying its atomic composition, in fact, the researchers hoped to be able to evaluate the contribution that the two main erosion phenomena to which the satellite’s surface is subjected offer to the formation of the lunar atmosphere: the impact of meteorites and micrometeorites, and ion sputtering, a mechanism linked to the action of solar winds that hit the soil and, by transferring their energy to the atoms that compose it, propel a certain quantity of it upwards.
According to the Lunar Atmosphere and Dust Environment Explorer (or Ladee) sent by NASA into orbit around the Moon in 2013, both phenomena contribute to the creation of the rarefied blanket of gas that surrounds the satellite. Since a significant portion of these atoms is destined to escape the weak gravitational attraction of the Moon, to get lost in open space, the authors of the new study wanted to understand if this happens more often following meteor impacts or ion sputtering, and therefore if one of the two phenomena contributes more to the formation of the atmosphere.
“Based on the data collected by Ladee, both phenomena appear to be involved,” explains Nicole Nie, a planetary scientist at MIT who participated in the new study. “They show, for example, that during meteor showers you see more atoms in the atmosphere, which shows that impacts have an effect. But they also show that when the Moon is shielded from the Sun’s rays, as during eclipses, there are changes in the atomic composition of the atmosphere, which means that solar winds also play a role. But the results were neither clear nor quantitative.”
To obtain more reliable results, the MIT researchers studied the lunar soil samples collected by the Apollo missions, and in particular, the proportion of potassium and rubidium ions present in them. Being volatile elements, in fact, they are both easily vaporized by meteorite impacts and ion sputtering. And once in the atmosphere – the researchers reasoned – the heavier ions will tend to return more easily to the ground, while the lighter ones will have a greater chance of remaining in suspension.
The analyses confirmed the presence of a greater than normal quantity of heavy ions in the lunar soil, and by comparing the results obtained for the two elements the researchers believe they have demonstrated the exact contribution that each of the two phenomena of space erosion provides to the formation of the lunar atmosphere: the impacts of meteorites would produce 70% of the atoms present in the atmosphere, while ion sputtering would be responsible for the remaining 30%.
“We have shown that vaporization caused by meteorite impacts is the main process that produces the lunar atmosphere,” Nie concludes. “The Moon is about 45 billion years old, and during that time the surface has been continuously bombarded by meteorites. We have shown that, eventually, a thin atmosphere reaches an equilibrium state, because it is continuously replenished by small impacts all over the lunar surface.”