Atmosphere of the Moon
Encyclopedia
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 planet
s 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 micrometeorite
s, the solar wind
, and sunlight
, in a process known as sputtering
. Gases that are released by sputtering can either:
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:
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 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.
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
is considered to be surrounded by vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. The elevated presence of atomic and molecular particles in its vicinity (compared to interplanetary medium
Interplanetary medium
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.-Composition and physical characteristics:...
), referred to as 'lunar
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
atmosphere' for scientific objectives, is negligible in comparison with the gaseous envelope surrounding Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and most planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
s of the Solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
- less than one hundred trillionth of Earth's atmospheric density at sea level.
One source of the lunar atmosphere is outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
: the release of gases such as radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
that originate from radioactive decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
within the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. Another important source is the bombardment of the lunar surface by micrometeorite
Micrometeoroid
A micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...
s, the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, and sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
, in a process known as sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
. Gases that are released by sputtering can either:
- be re-implanted into the regolithRegolithRegolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
as a result of the Moon's gravityGravitation of the Moonright|thumb|300px|Radial gravity anomaly at the surface of the Moon in [[Gal ]]The gravitational field of the Moon has been determined by the tracking of radio signals emitted by orbiting spacecraft...
; - be lost to space either by solar radiation pressureRadiation pressureRadiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
or, if the gases are ionizedIonizationIonization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
, by being swept away in the solar wind's magnetic fieldStellar magnetic fieldA stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively...
.
Composition
The elements sodium (Na)Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and potassium (K)
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
have been detected using Earth-based spectroscopic methods
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, whereas the isotopes radon
Radon
Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of uranium or thorium. Its most stable isotope, 222Rn, has a half-life of 3.8 days...
-222 and polonium
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
-210 have been inferred from data obtained by the Lunar Prospector
Lunar Prospector
The Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
. Argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
-40, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
-4, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and/or methane (CH4)
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, nitrogen gas (N2)
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and/or carbon monoxide (CO)
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, and carbon dioxide (CO2)
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
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
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
. 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
Electrostatic levitation
Electrostatic levitation is the process of using an electric field to levitate a charged object and counteract the effects of gravity. It was used, for instance, in Robert Millikan's oil drop experiment and is used to suspend the gyroscopes in Gravity Probe B during launch.Due to Earnshaw's...
dust. See moon dust for more details.