The Moon may not have much of an atmosphere due to its weak gravitational field (it is unclear whether it had one billion years ago). Debatable), however, it is believed that due to meteorite impacts, it currently maintains a thin atmosphere (also known as the exosphere).
The Moon has been subjected to violent bombardment by space rocks for the last 4.5 billion years, and researchers from MIT and the University of Chicago have found evidence in lunar soil samples collected by Apollo astronauts that meteorites, from giant meteorites to tiny, dust-sized objects, have been constantly spewing atoms into the exosphere.
Some of these atoms escape into space and others fall to the surface, but those that remain on the Moon create a thin atmosphere that continues to be replenished as more meteorites hit the surface.
“On long time scales, evaporation due to micrometeorite impacts is the dominant source of atoms in the lunar atmosphere,” the researchers wrote in a study recently published in the journal Science Advances.
Ready for launch
When NASA sent the lunar orbiter LADEE (Lunar Atmosphere and Dust Environment Explorer) to the Moon in 2013, the mission was aimed at understanding the origins of the Moon’s atmosphere. LADEE observed an increase in atmospheric atoms during meteor showers, suggesting that impacts had something to do with the atmosphere. But questions remained 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 Nee of the MIT Department of Earth, Atmospheric and Planetary Sciences, needed to analyze isotopes of elements in lunar soil that are most likely to be affected by micrometeorite impacts: they chose potassium and rubidium.
Potassium and rubidium ions are particularly susceptible to two processes: impact evaporation and ion sputtering.
Impact evaporation occurs when particles collide at high speed, generating a large amount of heat, which excites atoms, causing the material inside to evaporate and fly away. Ion sputtering is a phenomenon in which atoms are released by high-energy impacts without evaporating. Atoms released by ion sputtering tend to have higher energy and move faster than atoms released by impact evaporation.
Both could create and maintain a lunar atmosphere after a meteorite impact.
So if atoms ejected into the atmosphere by ion sputtering have an energetic advantage, why did researchers find that most of the atoms in the atmosphere actually come from impact evaporation?
Return to Landing
Light and heavy isotopes of potassium and rubidium had already been quantified in lunar soil samples provided by NASA, so Lee’s team used calculations to determine which impact processes were likely to prevent the various isotopes from escaping the atmosphere.
The researchers found that atoms released into the atmosphere by ion sputtering are propelled with so much energy and speed that they often reach escape velocity (the minimum speed needed to escape the moon’s already weak gravity) and continue flying into space, where they may end up being lost.
The fraction of atoms that reach escape velocity after impact evaporation depends on the temperature of those atoms: the lower the energy levels associated with impact evaporation, the lower the temperature and the less likely the atoms are to escape.
“Impact evaporation is the major long-term source of the Moon’s atmosphere, potentially accounting for more than 65% of the atmosphere. [potassium] “Atomic sputtering accounts for the rest, and ion sputtering accounts for the rest,” Li and her team wrote in the same study.
There are other ways that atoms are lost from the Moon’s atmosphere: mainly light ions remain in the exosphere, while ions that are too heavy fall to the surface, and other ions are photoionized by electromagnetic radiation from the solar wind, often being carried into space by solar wind particles.
What we learn about the lunar atmosphere from lunar soil could have implications for studies of other celestial bodies: we already know that impact evaporation can eject atoms into the lunar atmosphere. mercuryMars’ soil is thinner than that of the Moon, and studying it, which may be landed on Earth during a future sample return mission, could provide greater insight into how meteorite impacts affect the Martian atmosphere.
As we approach a new era of human lunar exploration, the Moon may be able to tell us more about where our atmosphere comes from and where it’s going.
Science Advances, 2024. DOI: 10.1126/sciadv.adm7074
