What is the moon's atmosphere made of? We have some clues. The Apollo 17 mission deployed an instrument called the Lunar Atmospheric Composition Experiment (LACE) on the moon's surface. It detected small amounts of a number of atoms and molecules including helium, argon, and possibly neon, ammonia, methane and carbon dioxide. From here on Earth, researchers using special telescopes that block light from the moon's surface have been able to make images of the glow from sodium and potassium atoms in the moon's atmosphere as they are energized by the sun. Still, we only have a partial list of what makes up the lunar atmosphere. Many other species are expected.
We think that there are several sources for gases in the moon's atmosphere. These include high energy photons and solar wind particles knocking atoms from the lunar surface, chemical reactions between the solar wind and lunar surface material, evaporation of surface material, material released from the impacts of comets and meteoroids, and out-gassing from the moon's interior. But which of these sources and processes are important on the moon? We still don't know.
With the discovery of significant ice deposits at the moon's poles by NASA's Lunar CRater Observation and Sensing Satellite (LCROSS) and Lunar Reconnaissance Orbiter (LRO) missions, and the discovery of a thin scattering of water molecules in the lunar soil by the Chandrayaan X-ray Observatory, another fascinating possibility has captured researchers' interest. The moon's atmosphere may play a key role in a potential lunar water cycle, facilitating the transport of water molecules between polar and lower latitude areas. The moon may not only be wetter than we once thought, but also more dynamic.
