The atmosphere of the Moon is very tenuous and insignificant in comparison with that of the Earth--less than one hundred trillionth the density of the Earth's atmosphere at sea level. For most practical purposes, the Moon is considered to be surrounded by a vacuum.
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Sources
One source of the lunar atmosphere is outgassing: the release of gases such as radon and helium 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.[1] Gases that are released by sputtering can either:
- be re-implanted 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.
Composition
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.[2] 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.[3]
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: less 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.[4] 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 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.
See also
References
- ^ P. Lucey and 17 coauthors (2006). "Understanding the lunar surface and space-Moon interactions". Reviews in Mineralogy and Geochemistry 60 (1): 83–219. doi:. http://rimg.geoscienceworld.org/cgi/content/extract/60/1/83.
- ^ S. Lawson, W. Feldman, D. Lawrence, K. Moore, R. Elphic, and R. Belian (2005). "Recent outgassing from the lunar surface: the Lunar Prospector alpha particle spectrometer". J. Geophys. Res. 110 (E9): E9009. doi:. http://www.agu.org/pubs/crossref/2005/2005JE002433.shtml.
- ^ S. Alan Stern (1999). "The Lunar atmosphere: History, status, current problems, and context". Rev. Geophys. 37 (4): 453–491. doi:. http://www.agu.org/pubs/crossref/1999/1999RG900005.shtml.
- ^ Adapted from Stern, S.A. (1999) Rev. Geophys. 37, 453
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