When you learn about the moon in school, you’re generally taught that its gravity is insufficient to capture and retain any significant atmosphere. The moon is nonetheless surrounded by a thin, ephemeral halo of gasses—an exosphere.
This surprising fact was first discovered using instruments carried by astronauts who visited the moon with the Apollo program. The moon’s weak gravity means that the exosphere’s constituent atoms are constantly draining away into space—and, as such, its continuous presence means that the supply of these atoms is being constantly replenished.
A new study published in Science Advances on August 2 looks at exactly how this replenishment happens. It examines a group of elements whose presence in the lunar atmosphere might come as a surprise to anyone who’s studied chemistry: alkali metals.
Alkali metals form the first group of the periodic table, and include lithium, sodium, potassium, rubidium, and caesium (along with francium, which is never found in macroscopic quantities because it’s so radioactive). Why is their presence a surprise? On Earth, they’re famous for their reactivity, as evidenced by the classic high school demonstration of what a piece of sodium does when it encounters water. On the moon, however, things are very different.
As Prof. Nicole Nie, lead author of the paper, tells Popular Science, “In lunar soils and rocks, alkali metals are bound in minerals, forming stable chemical bonds with oxygen and other elements. But when they are released from the surface, they usually become neutral atoms. There is no liquid water or substantial atmosphere [on the moon], so these metals can remain in their elemental form—[and] because the number of atoms in the lunar atmosphere is so small, the atoms can travel a long distance freely without colliding with one another.”
This does, however, raise the question of how the atoms are released from the surface in the first place. The paper seeks to answer this question—and, specifically, the relative contributions of three processes known collectively as “space weathering.” The uniting factor in these three processes is that they involve something striking the lunar surface and knocking the alkali metal elements out of the mineral compounds in which they’re bound. (These processes also release other elements, but the volatility of alkali metals makes them particularly easy to liberate.)
The first of these processes is micrometeorite impacts, where tiny pieces of space debris rain down with sufficient force to vaporize a small piece of the lunar surface and launch its component atoms into orbit. The second is ion sputtering, where charged particles driven by the solar wind strike the lunar surface. And finally there’s photon-stimulated desorption, where it’s high-energy photons from the sun that knock the alkali metals loose.
As the paper notes, while each process has been well-characterized, previous research has “not conclusively disentangled their [relative] contributions” to the lunar atmosphere. To go about doing this, Nie and her team went right back to the source of the question: the Apollo program. The various crewed missions to the moon in the late 1960s and early ‘70s brought back a total of 382 kg of lunar soil samples, and decades later, these samples are still revealing their secrets to researchers. Nie’s study involved examining 10 samples from five different Apollo missions, including several from Apollo 8, the first manned moon landing.
The team used these samples to look at the relative proportions of different isotopes of potassium and rubidium in the soil. (Sodium and cesium only have one stable isotope each, while lithium is less volatile than its heavier cousins.) As Nie explains to Popular Science, “Lighter isotopes of an element are preferentially released during these processes, leaving the lunar soils with relatively heavier isotopic compositions. For elements that are affected by space weathering, we would expect lunar soils to show heavy isotopic compositions, compared to deeper rocks that are not affected by the process.”
The different space weathering processes produce different ratios of isotopes, and the team’s results indicate that it appears that micrometeorite impacts make the largest contribution to the lunar atmosphere, “likely contributing more than 65% of atmospheric [potassium] atoms, with ion sputtering accounting for the rest.”
This provides a valuable insight into how the moon’s atmosphere has evolved over billions of years—while its composition may well vary over shorter timescales, these results suggest that in the long run, micrometeorite impacts play the dominant role in the constant replenishment of the atmosphere. The study also points to how similar research might be carried out on other objects similar to the moon, like Phobos, one of Mars’s two satellites.