Atmosphere of the Moon

The atmosphere of the Moon is very tenuous and insignificant in comparison with that of the Earth. One source of the lunar atmosphere is outgassing: the release of gases such as radon that originate 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.cite journal |last=P. Lucey and 17 coauthors |title=Understanding the lunar surface and space-Moon interactions |url=http://rimg.geoscienceworld.org/cgi/content/extract/60/1/83 |journal=Reviews in Mineralogy and Geochemistry |volume=60 |issue=1 |pages=83–219 |date=2006 |doi=10.2138/rmg.2006.60.2] Gases that are released by sputtering can either:
* be reimplanted into the regolith as a result of the Moon's gravity;
* 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 element radon-222 and polonium-210 have been inferred from data obtained by the Lunar Prospector alpha particle spectrometer. [cite journal |last=S. Lawson, W. Feldman, D. Lawrence, K. Moore, R. Elphic, and R. Belian |title=Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer |url=http://www.agu.org/pubs/crossref/2005/2005JE002433.shtml |journal=J. Geophys. Res. |volume=110 |issue=E9 |pages=E9009 |doi=10.1029/2005JE002433 |date=2005] 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-suit detectors placed by the Apollo astronauts. [cite journal |last=S. Alan Stern |title=The Lunar atmosphere: History, status, current problems, and context |url=http://www.agu.org/pubs/abs/rg/1999RG900005/tmp.html |journal = Rev. Geophys. |volume=37 |issue=4 |year=1999 |pages=453–491 |doi=10.1029/1999RG900005]

The average daytime abundances of the elements known to be present in the lunar atmosphere, in atoms per cubic centimeter, are as follows: H <17, He 2-40x10³, Na 70,K 17, Ar 4x10⁴, yielding ~8x10⁴ total atoms per cubic centimeter, marginally higher than the quantity posited to exist in the atmosphere of Mercury. [Adapted from Stern, S.A. (1999) Rev. Geophys. 37, 453] . It is worth noting however, that while this greatly exceeds the density of the solar wind (a component of the lunar atmosphere), which is usually on the order of just a few protons per cubic centimeter, the lunar atmosphere is less than one trillionth the density of the Earth's atmosphere at sea level. The Moon is usually 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 outgasing are not primary components of the Earth's, or any stable atmosphere yet known).

The Moon may also have a tenuous "atmosphere" of electrostatically-levitated dust. See moon dust for more details.

References