Until recently, most everyone accepted the conventional wisdom that the moon has virtually no atmosphere. Just as the discovery of water on the moon transformed our textbook knowledge of Earth’s nearest celestial neighbor, recent studies confirm that our moon does indeed have an atmosphere consisting of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars or Venus. It’s an infinitesimal amount of air when compared to Earth’s atmosphere. At sea level on Earth, we breathe in an atmosphere where each cubic centimeter contains 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere has less than 1,000,000 molecules in the same volume. That still sounds like a lot, but it is what we consider to be a very good vacuum on Earth. In fact, the density of the atmosphere at the moon’s surface is comparable to the density of the outermost fringes of Earth’s atmosphere where the International Space Station orbits.
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.