Atmosphere of the Moon
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 envelope surrounding Earth and most planets of the Solar system - less than one hundred trillionth of Earth's atmospheric density at sea level.
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. Gases that are released by sputtering can either:
- be re-implanted into the regolith as a result of the Moon's gravity;
- escape the moon entirely if the particle is moving upward at or above the lunar escape velocity of 2.38 km/s;
- 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.
The elements sodium (Na) and potassium (K) have been detected using Earth-based spectroscopic methods, whereas the isotopes radon-222 and polonium-210 have been inferred from data obtained by the Lunar Prospector alpha particle spectrometer. Argon-40, helium-4, oxygen and/or methane (CH4), nitrogen gas (N2) and/or carbon monoxide (CO), and carbon dioxide (CO2) were detected by in-situ detectors placed by the Apollo astronauts.
The average daytime abundances of the elements known to be present in the lunar atmosphere, in atoms per cubic centimeter, are as follows:
- Argon: 40,000
- Helium: 2,000-40,000
- Sodium: 70
- Potassium: 17
- Hydrogen: fewer than 17
This yields approximately 80,000 total atoms per cubic centimeter, marginally higher than the quantity posited to exist in the atmosphere of Mercury. While this greatly exceeds the density of the solar wind, which is usually on the order of just a few protons per cubic centimeter, it is virtually a vacuum in comparison with the atmosphere of the Earth.
In fact, the Moon is often considered to not have an atmosphere, as it cannot absorb measurable quantities of radiation, does not appear layered or self-circulating, and requires constant replenishment given the high rate at which the atmosphere is lost to space (solar wind and outgassing are not primary sources of the Earth's atmosphere, or of any stable atmosphere yet known).
See also 
- Sodium tail of the Moon
- Orders of magnitude (pressure)
- Lunar Atmosphere and Dust Environment Explorer (LADEE)
- Moon Storms
- P. Lucey and 17 coauthors, P. (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry 60 (1): 83–219. doi:10.2138/rmg.2006.60.2.
- S. Lawson, W. Feldman, D. Lawrence, K. Moore, R. Elphic, and R. Belian, Stefanie L. (2005). "Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer". J. Geophys. Res. 110 (E9): E9009. Bibcode:2005JGRE..11009009L. doi:10.1029/2005JE002433.
- S. Alan Stern, S. Alan (1999). "The Lunar atmosphere: History, status, current problems, and context". Rev. Geophys. 37 (4): 453–491. Bibcode:1999RvGeo..37..453S. doi:10.1029/1999RG900005.
- Adapted from Stern, S.A. (1999) Rev. Geophys. 37, 453