The mystery of how the moon’s thin atmosphere is produced has been solved, according to scientists studying lunar samples brought back by the Apollo missions.
Discovered in the 1960s and 70s, when Nasa sent astronauts to the moon, the lunar atmosphere is far thinner than that of Earth, and was thought to arise from space weathering of the moon’s surface.
Now experts say they have unpicked the relative contributions of such processes to the lunar atmosphere, revealing meteorite impacts to be the major player.
“Our findings provide a clearer picture of how the moon’s surface and atmosphere interact over long timescales, [and] enhance our understanding of space weathering processes,” said Dr Nicole Nie, the co-author of the new study based at MIT’s department of Earth, atmospheric, and planetary sciences.
Writing in the journal Science Advances, Nie and her colleagues describe how the lunar atmosphere must be constantly replenished because its atoms are continuously being lost to space, primarily because of the moon’s weak gravity, or trapped on the lunar surface.
Ultraviolet photons from the sun can rerelease the latter, but the researchers say replenishment of the atmosphere is thought to rely on atoms being released from within lunar minerals – either via vaporisation by meteorite impacts, or by solar wind sputtering, a process in which charged particles from the sun hit the moon and eject atoms.
But which of the two factors dominates had been unclear, with data from Nasa’s lunar atmosphere and dust environment explorer, launched in 2013, suggesting both were at play.
Nie and colleagues unpicked the conundrum by studying the different forms, or isotopes, of potassium and rubidium in 10 samples of lunar soil from the Apollo missions.
The team say meteorite impacts and solar wind sputtering both favour the release of lighter forms of the elements, but that the actual proportion of heavy to light isotopes that end up in the lunar atmosphere and soil would differ depending on the process.
“After measuring the isotopic compositions of lunar soils, we built a mathematical model taking into account various space weathering processes, and solve for the contribution of each of them by matching the measured isotopic compositions,” said Nie.
The results suggest about 70% of the moon’s atmosphere is down to impact vaporisation and 30% to solar wind sputtering.
Dr Simeon Barber, a senior research fellow at the Open University who was not involved in the work, said the findings were another important piece in the puzzle of understanding how our moon works.
“Understanding how the thin atmosphere forms on moons and small planets helps us understand how these bodies have come to be so varied,” he said, adding that the way forward was to visit new places with spacecraft, take measurements there, and then return samples to Earth for analysis. “The moons of Mars, Phobos and Deimos, would be fascinating places to do this kind of study on next,” he said.
