The Moon has its own extremely thin atmosphere

The Concept of a Surface Boundary Exosphere

In everyday terms, an atmosphere is imagined as a dense blanket of gas where molecules constantly collide with one another, generating atmospheric pressure, distributing heat, and creating weather. On Earth, at sea level, a cubic centimeter of air contains roughly ten quintillion molecules, which collide billions of times per second. By contrast, the Moon resides in what planetary scientists classify as a surface boundary exosphere. In this regime, the gas is so dilute that individual atoms and molecules virtually never collide with each other. Instead, their paths are governed entirely by gravity and interactions with the lunar regolith below.

Under the strict physical definition of an exosphere, particles travel along ballistic trajectories, launching upward from the surface and falling back down without experiencing the gas-phase collisions that define standard fluid dynamics. The total mass of this gaseous envelope is tiny, estimated at around 25,000 kilograms across the entire planetary body. If compressed to standard Earth surface temperature and pressure, the Moon's entire atmosphere would occupy a volume smaller than a typical warehouse. Its daytime surface pressure sits near a few tenths of a nan Pascal, which is trillions of times lower than the atmospheric pressure experienced on Earth.

Because of this extreme rarity, the lunar atmosphere changes dramatically between day and night. During the scorching lunar day, thermal energy excites surface particles and drives higher vapor pressures, while during the frigid two-week lunar night, many volatile elements freeze out directly onto the surface regolith. This means the structure and density of the exosphere are continuously fluctuating, tethered directly to the temperature of the underlying rocks and dust.