When we learn about the Moon in school, we are generally taught that the Moon’s gravity is insufficient to capture and retain an atmosphere. A thin, temporary ring of gas—The exosphere.
This amazing fact was first discovered using instruments carried by astronauts who visited the Moon during the Apollo missions. Because of the Moon’s weak gravity, the atoms that make up the exosphere are constantly leaking out into space, so the continued existence of the Moon means that the supply of these atoms is constantly being replenished.
a New research published Scientific advances The study, published August 2, takes a closer look at how this replenishment occurs. It examines a group of elements that anyone with a chemistry degree might be surprised to find present in the moon’s atmosphere: the alkali metals.
The alkali metals are in the first group of the periodic table and include lithium, sodium, potassium, rubidium, and cesium (including francium, which is so radioactive it’s never seen in amounts visible to the naked eye). Why are they surprising? On Earth, these metals are notoriously reactive. A typical high school demonstration We don’t know what happens when a lump of sodium comes into contact with water on the Moon, but the situation is completely different on the Moon.
Professor Nicole Nie, lead author of the paper, said: Popular Science“In lunar soils and rocks, alkali metals are bound to minerals and form stable chemical bonds with oxygen and other elements. But when they leave the surface, they typically become neutral atoms. There is no liquid water or substantial atmosphere. [on the moon]these metals can remain in their elemental form.[and] The number of atoms in the Moon’s atmosphere is so small that they can travel freely over long distances without colliding with each other.”
But this raises the question of how the atoms get released from the surface in the first place. This paper seeks to answer that question, in particular the relative contributions of three processes collectively known as “space weathering.” What these three processes have in common is that something impacts the lunar surface, knocking the alkali metal elements out of the mineral compounds they’re bound to. (These processes also release other elements, but the alkali metals are particularly easy to release because they’re so volatile.)
The first process is micrometeorite impacts, where tiny pieces of space debris rain down with enough force to vaporize a small portion of the lunar surface and launch its constituent atoms into orbit, ion sputtering, where charged particles driven by the solar wind bombard the lunar surface, and finally photon-stimulated desorption, where high-energy photons from the Sun knock off alkali metals.
The paper notes that while each process is well characterized, previous studies “have not conclusively elucidated those processes”. [relative] To understand the “contribution” to the Moon’s atmosphere, Nie and his team went back to the source of the question: the Apollo program. Various manned lunar missions in the late 1960s and early ’70s returned a total of 382 kg of lunar soil samples, and even decades later, these samples continue to reveal secrets to researchers. For Nie’s study, they examined 10 samples from five different Apollo missions, including several from Apollo 8, the first manned moon landing.
The team used these samples to examine the relative proportions of different isotopes of potassium and rubidium in the soil. (Sodium and cesium each have only one stable isotope, and lithium is less volatile than its heavier isotopes.) As Ni explains, Popular Science“During these processes, lighter isotopes of elements are preferentially released, leaving lunar soil with a relatively heavy isotopic composition. For elements affected by space weathering, lunar soil is expected to exhibit a heavier isotopic composition compared to deeper rocks that are not affected by this process.”
Different space weathering processes produce different isotope ratios, and the team’s findings suggest that micrometeorite impacts are the largest contributor to the moon’s atmosphere, “likely making up more than 65 percent of the atmosphere.” [potassium] The rest is due to ion sputtering.”
This provides valuable insight into how the Moon’s atmosphere has evolved over billions of years. Although its composition may vary on shorter timescales, these results suggest that in the long term micrometeorite impacts play a major role in the continuous replenishment of the atmosphere. The study also suggests that similar studies could be conducted on other Moon-like bodies, such as Phobos, one of Mars’ two moons.
