The Moon may not have much of an atmosphere, largely due to its weak gravitational field (whether it had a substantial atmosphere billions of years ago is debatable). However, it is thought to currently retain its tenuous atmosphere, known as the exosphere, due to meteorite impacts.
Space rocks have been bombarding the moon for 4.5 billion years. Now researchers from MIT and the University of Chicago have discovered that lunar soil samples collected by astronauts during the Apollo era provide evidence that meteorites, from giant meteors to micrometeoroids no bigger than dust grains, have launched a steady stream of atoms into the exosphere.
Although some of these atoms escape into space and others fall back to the surface, the atoms that remain above the moon create a thin atmosphere that is continually replenished as more meteorites hit the surface.
“In the long term, evaporation from micrometeorite impacts is the main source of atoms in the lunar atmosphere,” the researchers said in a study published recently in Science Advances.
Ready for launch
When NASA sent its Lunar Atmosphere and Dust Environment Explorer (LADEE) orbiter to the moon in 2013, the mission was intended to determine the origin of the moon's atmosphere. LADEE observed more atoms in the atmosphere during meteor showers, suggesting that impacts had something to do with the atmosphere. However, it left open questions about the mechanism that converts impact energy into a diffuse atmosphere.
To find these answers, a team of researchers from MIT and the University of Chicago, led by professor Nicole Nie of MIT's Department of Earth, Atmospheric and Planetary Sciences, had to analyze the isotopes of elements in the lunar soil that are most sensitive to the effects of micrometeoroid impacts. They chose potassium and rubidium.
Potassium and rubidium ions are particularly sensitive to two processes: impact evaporation and ion sputtering.
Impact vaporization is the result of particles colliding at high speeds, generating extreme amounts of heat that excites atoms enough to vaporize the material they are in and send them flying. Ion sputtering involves high-energy impacts that free atoms without vaporizing them. Atoms released by ion sputtering tend to have more energy and move faster than those released by impact vaporization.
Both can create and maintain the Moon's atmosphere after meteorite impacts.
If atoms introduced into the atmosphere by ion sputtering have an energy advantage, why did the researchers find that most of the atoms in the atmosphere actually come from impact vaporization?
Back to bottom
Because the lunar soil samples provided by NASA had previously been quantified with the lighter and heavier isotopes of potassium and rubidium, Lie's team used calculations to determine which collision process is more likely to prevent different isotopes from escaping the atmosphere.
The researchers found that atoms transferred to the atmosphere by ion sputtering are sent off at such high energies that they often reach escape velocity—the minimum velocity needed to escape the moon's already weak gravity—and travel farther into space. After all, atoms that enter the atmosphere can also escape from the atmosphere.
The fraction of atoms that reach escape velocity after impact vaporization depends on the temperature of those atoms. Lower energy levels associated with impact vaporization result in lower temperatures, giving atoms a lower chance of escaping.
“Impact evaporation is the most important long-term source of the Moon's atmosphere, likely contributing more than 65 percent of the atmospheric [potassium] atoms, with ion sputtering taking care of the rest,” Lie and her team said in the same study.
There are other ways that atoms escape from the moon’s atmosphere. It is mainly lighter ions that tend to get stuck in the exosphere, with ions falling back to the surface if they are too heavy. Others are photoionized by electromagnetic radiation from the solar wind and often carried into space by solar wind particles.
What we’ve learned about the moon’s atmosphere from lunar soil could inform studies of other bodies. It’s already been discovered that impact vaporization launches atoms into Mercury’s exosphere, which is thinner than the moon’s. Studying Martian soil, which may land on Earth in future sample-return missions, could also provide more insight into how meteorite impacts affect the atmosphere.
As we enter a new era of manned lunar missions, the Moon can tell us more about where its atmosphere came from and where it's going.
Scientific Progress, 2024. DOI: 10.1126/sciadv.adm7074
