Yes, the moon has an atmosphere – and it’s metal as hell

When you learn about the moon in school, you generally learn that its gravity is insufficient to capture and hold any meaningful atmosphere. The moon is nevertheless surrounded by a thin, short-lived halo of gases– an exosphere.

This surprising fact was first discovered using instruments from astronauts who visited the moon with the Apollo program. The moon’s weak gravity means that the atoms that make up the exosphere are constantly flowing out into space – and as such, the moon’s continued presence means that the supply of these atoms is constantly being replenished.

a new study published in Scientific progress on August 2, we will look at how exactly this addition is done. It examines a group of elements whose presence in the lunar atmosphere might come as a surprise to anyone who has studied chemistry: alkali metals.

Alkali metals make up the first group of the periodic table and include lithium, sodium, potassium, rubidium, and cesium (along with francium, which is never found in macroscopic quantities because it is so radioactive). Why is their presence a surprise? On Earth, they are known for their reactivity, as evidenced by the classic high school demonstration of what a piece of sodium does when it comes into contact with water. On the moon, however, things are very different.

As Prof. Nicole Nie, lead author of the article, explains 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 that these metals can remain in their elemental form –[and] Because the number of atoms in the moon’s atmosphere is so small, the atoms can freely travel a long distance without colliding with each other.”

However, this does raise the question of how the atoms are detached from the surface in the first place. The paper attempts to answer this question – and in particular the relative contributions of three processes collectively known as ‘space weathering’. The connecting factor in these three processes is that something hits the moon’s surface and knocks out the alkali metal elements from the mineral compounds in which they are bound. (These processes also release other elements, but the volatility of alkali metals makes them particularly easy to release.)

The first of these processes are micrometeorite impacts, in which small pieces of space debris rain down with enough force to vaporize a small patch of the moon’s surface and launch its constituent atoms into orbit. The second is ion sputtering, in which charged particles driven by the solar wind strike the moon’s surface. And finally, there is photon-stimulated desorption, where it is the high-energy photons from the sun that loosen the alkali metals.

As the article notes, while each process has been well characterized, previous research “has not convincingly disentangled the processes.” [relative] contributions” to the lunar atmosphere. To do this, Nie and her team went right back to the source of the question: the Apollo program. The several manned missions to the moon in the late 1960s and early 1970s brought back a total of 800 pounds (382 kg) of lunar soil samples, and decades later these samples are still revealing their secrets to researchers. Nie’s research involved examining ten samples from five different Apollo missions, including some 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 each have only one stable isotope, while lithium is less volatile than its heavier cousins.) As Nie explains Popular science, “Lighter isotopes of an element are preferentially released during these processes, giving the lunar soils relatively heavier isotopic compositions. For elements affected by space weathering, we would expect lunar soils to exhibit a heavy isotopic composition, compared to deeper rocks not affected by the process.”

The different weathering processes in space produce different isotope ratios, and the team’s results indicate that it appears that micrometeorite impacts are the largest contributors to the lunar atmosphere, “probably accounting for more than 65% of the atmospheric impact.” [potassium] atoms, with ion sputtering taking care of the rest.”

This provides valuable insight into how the moon’s atmosphere has evolved over billions of years. Although its composition may well vary on shorter time scales, these results suggest that long-term micrometeorite impacts play the dominant role in the ongoing replenishment of the atmosphere. The study also points out how similar research could be conducted on other objects similar to the moon, such as Phobos, one of Mars’ two satellites.