On Earth, wind can pick up grains of sand on the beach, and rain can turn the ground into mud. On the Moon, however, the weather is much more exotic.
Charged particles from the sun called solar wind stream down onto the lunar soil, kicking particles off into space. Thousands of tiny meteorites bombard the Moon, melting and vaporizing their points of impact. Sunlight makes molecules jump off the surface.
Space weather causes so much disturbance to the moon’s surface that it creates an extremely thin atmosphere, called an exosphere, made out of atoms from the lunar soil. This exosphere is bound to the moon by its weak gravitational field, and it has such low density that the atoms never collide. (What happens to the moon’s exosphere when it hits Earth’s magnetic field?)
Since so many different processes attack the lunar soil and fuel the moon’s exosphere, it has long been unclear what exactly is the main source. By analyzing samples taken during the Apollo missions, Nicole Xike Nie, a cosmochemist at MIT, and her colleagues have shown that micrometeorite impacts contribute the most atoms to the moon’s atmosphere in Science Advances.
“Using samples brought back from the Apollo missions is both an honor and a unique scientific opportunity. These samples represent humanity's first direct exploration of another celestial body,” says Nie. “Despite being collected over 50 years ago, the Apollo samples remain invaluable for scientific research.”
Mining moon rocks
The moon rocks and lunar soil samples that Apollo astronauts returned to Earth forever changed our understanding of the moon, and major advances in sample analysis methods have renewed the samples as essential scientific data. Nie’s team used portions of 10 different samples, from five different landing sites, totaling just 50 milligrams of moon rock powder. Even small amounts of returned samples “provide a wealth of information,” says Nie.
In the powder, the team searched for the chemical fingerprints of different types of space weathering in the form of potassium and rubidium isotopes—two elements that are especially sensitive to the space weathering seen on the moon. Isotopes are elements with the same number of protons, but different numbers of neutrons. The atoms share many chemical and physical properties, but have slightly different masses.
The exosphere likely contains lighter isotopes of potassium and rubidium compared to the lunar soil, and each weathering process leaves a different mix of heavy isotopes in the soil. Based on the mix of isotopes in the samples, the researchers traced what process influenced the exosphere the most.
Past work has shown that ultraviolet light from the sun recycles exosphere atoms that return to the surface but doesn’t contribute much to the exosphere, so Nie and her team focused on micrometeorites and solar wind.
Space weather patterns
Micrometeorite impacts appeared to be the biggest contributor. These tiny meteorites usually weigh less than a gram and have broken off of much larger objects. Micrometeorites constantly pummel the moon’s surface everyday. These strikes are so intense that they heat up the point of impact to temperatures ranging from 2000 to 6000 degrees Celsius. The impact melts and vaporizes lunar soil into the exosphere, like water turning into steam. (See a meteor hitting the lunar surface during a ‘blood moon’.)
The second most influential process involves solar wind, which flows out from the sun in a stream of high-energy particles, colliding with everything in its path. While a strong magnetic field protects Earth, the moon isn’t as lucky. Except during lunar eclipses, the moon is under a firehose of this plasma, mainly protons. Upon colliding, these protons transfer their energy to the atoms in the lunar soil and cause them to fly off the surface.
Nie and her colleagues’ work demonstrates, for the first time, that meteorite impacts are the major contributor to the exosphere, “accounting for more than 70% of its composition, while solar wind sputtering contributes about 30% or less,” she says.
“The study enhances our understanding of the Moon's atmospheric dynamics and surface evolution and also contributes to a broader body of research on the surfaces of near-Earth objects,” says Dani Mendoza DellaGiustina, a planetary scientist at the University of Arizona who also leads NASA’s OSIRIS-APEX mission. “There is a growing body of evidence that micrometeorite impacts drive the lion's share of the space weathering observed on objects residing in the inner solar system—not just on the Moon, but on asteroids as well.”
Next, Nie wants to study other isotopes in the lunar soil. The same methods could also be applied to new lunar samples like those returned by China’s Chang’e-6 mission, as well as samples returned from other objects in the solar system, like Mars’ moon Phobos, which is the target of a Japanese mission set to launch in 2026.
“Understanding the space environments of different planetary bodies is essential for planning future missions and exploring the broader context of space weathering,” says Nie. “This knowledge will be particularly important if humanity decides to establish a presence on other planetary bodies in the future.”
