On the moon, there's no air to breathe, no breezes to make the flags planted there by the Apollo missions flutter. However, there is a very, very thin layer of gases on the lunar surface that can almost be called an atmosphere. Technically, it's considered an exosphere.
In an exosphere, the gases are so spread out that they rarely collide with one another. They are rather like microscopic cannon balls flying unimpeded on curved, ballistic trajectories and bouncing across the lunar surface. In the moon's atmosphere, there are only 100 molecules per cubic centimeter. In comparison, Earth's atmosphere at sea level has about 100 billion billion molecules per cubic centimeter. The total mass of these gases is about 55,000 pounds (25,000 kilograms), about the same weight as a loaded dump truck.
Several elements have been detected in the lunar atmosphere. The Lunar Reconnaissance Orbiter recently detected helium. Detectors left by Apollo astronauts have detected argon-40, helium-4, oxygen, methane, nitrogen, carbon monoxide and carbon dioxide. Earth-based spectrometers have detected sodium and potassium, while the Lunar Prospector orbiter found radioactive isotopes of radon and polonium.
One of the sources for the moon's atmosphere is outgassing, the release of gases from the lunar interior, usually due to radioactive decay. Outgassing events may also occur during moonquakes. After being released, lighter gases escape into space almost immediately. Outgassing replenishes the tenuous atmosphere.
The impact of sunlight, the solar wind and micrometeorites hitting the moon's surface can also release gases that were buried in the lunar soil — a process called sputtering. These gases either fly off into space or bounce along the lunar surface. Sputtering may explain how water ice collected in lunar craters. Comets hitting the moon may have left some water molecules on the surface. Some of the molecules then accumulated in dark polar craters, forming beds of solid ice.
Ultraviolet sunlight affects the released gases by ejecting electrons, which gives them an electrical charge that can cause the particles to levitate more than a mile into the sky. At night, the opposite occurs. Atoms receive electrons from the solar wind and settle back down near the surface.
This floating fountain of moon dust travels along the boundary between night and day, creating a glow similar to Earth sunsets. Known as the Lunar Horizon Glow, it was observed several times during Apollo missions.
Apollo astronauts described moon dust as gritty, abrasive and clingy. It can wreak havoc on equipment and computers. Moonwalkers were coated in it and their spacesuits were almost threadbare when they returned to Earth. Much more will need to be learned about lunar dust before NASA returns astronauts to the moon.
— Tim Sharp, SPACE.com Reference Editor
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