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 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-situ 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: 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.^[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

Moon portal

• Sodium tail of the Moon
• Orders of magnitude (pressure)

References

v t e The Moon Physical features Internal structure Gravity field Topography Magnetic field Atmosphere Ion tail Moonlight Orbit Orbit of the Moon Phases Solar eclipse Lunar eclipse Tide Lunar surface Selenography Near side Far side Lunar mare (List) Craters (List) Mountains Valleys South Pole–Aitken basin Shackleton crater Water Soil Peak of eternal light Space weathering Transient lunar phenomenon Lunar science Geology (timescale) Giant impact hypothesis Moon rocks Lunar meteorites KREEP ALSEP Lunar laser ranging Late Heavy Bombardment Observing the Moon Exploration Exploration of the Moon Apollo program Robotic exploration Future missions Lunar colonization Moon Landing hoax accusations Other topics Calendar Month The Moon in art and literature The Moon in mythology Moon illusion Lunar effect Craters named after people Man in the Moon See also Solar System Natural satellite Double planet

v t e Atmospheres Major Sun Venus Earth Mars Jupiter Saturn Titan Uranus Neptune Triton Pluto HD 209458 b Tenuous Mercury Moon Io Europa Ganymede Callisto Enceladus Dione Rhea See also Stellar atmosphere Coma (cometary) Extraterrestrial atmospheres