Atmosphere of the Moon

The atmosphere of the Moon is a very sparse layer of gases surrounding the Moon, consisting only of an exosphere. For most practical purposes, the Moon is considered to be surrounded by vacuum. The elevated presence of atomic and molecular particles in its vicinity compared to interplanetary medium, referred to as "lunar atmosphere" for scientific objectives, is negligible in comparison with the gaseous envelopes surrounding Earth and most planets of the Solar System, and comparable to their exospheres. The pressure of this small mass is around 3×10−15 atm (0.3 nPa), varying throughout the day, and has a total mass of less than 10 metric tonnes. Otherwise, the Moon is considered not to have an atmosphere because it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment due to the high rate at which its gases are lost into space.

Roger Joseph Boscovich was the first modern astronomer to argue for the lack of atmosphere around the Moon in his De lunae atmosphaera (1753).

Sources

One source of the lunar atmosphere is outgassing: the release of gases such as radon and helium resulting from radioactive decay within the crust and mantle. Another important source is the bombardment of the lunar surface by micrometeorites, the solar wind, and sunlight, in a process known as sputtering.

Escape velocity and atmospheric hold

Gases can:

be re-implanted into the regolith as a result of the Moon's gravity;

escape the Moon entirely if the particle is moving at or above the lunar escape velocity of 2.38 km/s (1.48 mi/s), or 5,328 mph (8,575 km/h);

be lost to space either by solar radiation pressure or, if the gases are ionized, by being swept away in the solar wind's magnetic field.

Composition

What little atmosphere the Moon has consists of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars, or Venus.