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 (10−14) of Earth's atmospheric density at sea level. Otherwise the Moon is 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. The solar wind and outgassing are not primary sources of the Earth's atmosphere, or of any stable atmosphere.
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.2
Gases 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.
Until recently, most everyone accepted the conventional wisdom that the moon has virtually no atmosphere. Just as the discovery of water on the moon transformed our textbook knowledge of Earth's nearest celestial neighbor, recent studies confirm that our moon does indeed have an atmosphere consisting of some unusual gases, including sodium and potassium, which are not found in the atmospheres of Earth, Mars or Venus. The moon's atmosphere consists of an infinitesimal amount of air, when compared to Earth's atmosphere. At sea level on Earth, each cubic centimeter of the atmosphere contains approximately 10,000,000,000,000,000,000 molecules; by comparison the lunar atmosphere contains fewer than 1,000,000 molecules in the same volume. This is considered to be a very good vacuum on Earth. In fact, the density of the atmosphere at the moon's surface is comparable to the density of the outermost fringes of Earth's atmosphere where the International Space Station orbits.3
The elements sodium (Na) and potassium (K) have been detected in the moon's atmosphere 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.4 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.5
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.6 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.
- Atmosphere of Mercury
- Lunar Atmosphere and Dust Environment Explorer (LADEE)
- Orders of magnitude (pressure)
- Sodium tail of the Moon
- 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