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Outgassing is the release of a gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 that was dissolved, trapped, frozen
Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting....

 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. It can include sublimation and evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 which are phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

s of a substance into a gas, as well as desorption
Desorption is a phenomenon whereby a substance is released from or through a surface. The process is the opposite of sorption . This occurs in a system being in the state of sorption equilibrium between bulk phase and an adsorbing surface...

, seepage from cracks or internal volumes and gaseous products of slow chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

s. Boiling
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...

 is generally thought of as a separate phenomenon from outgassing because it consists of a phase transition of a liquid into a vapor made of the same substance.

Outgassing in a vacuum

Outgassing is a challenge to creating and maintaining clean high-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...

 environments. NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 and ESA maintains a list of low-outgassing materials to be used for spacecraft, as outgassing products can condense onto optical elements, thermal radiators, or solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....

s and obscure them. Materials not normally considered absorbent can release enough light-weight molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s to interfere with industrial or scientific vacuum processes. Moisture
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

, sealant
A sealant may be viscous material that has little or no flow characteristics and stay where they are applied or thin and runny so as to allow it to penetrate the substrate by means of capillary reaction...

s, lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...

s, and adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

s are the most common sources, but even metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

s and glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

es can release gases from cracks or impurities. The rate of outgassing increases at higher temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

s because the vapour pressure and rate of chemical reaction increases. For most solid materials, the method of manufacture and preparation can reduce the level of outgassing significantly. Cleaning surfaces or baking individual components or the entire assembly before use can drive off volatiles.

NASA's Stardust
Stardust (spacecraft)
Stardust is a 300-kilogram robotic space probe launched by NASA on February 7, 1999 to study the asteroid 5535 Annefrank and collect samples from the coma of comet Wild 2. The primary mission was completed January 15, 2006, when the sample return capsule returned to Earth...

 spaceprobe suffered reduced image quality due to an unknown contaminant that had condensed on the CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 sensor of the navigation camera. A similar problem affected the Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...

 spaceprobe's Narrow Angle Camera, but was corrected by repeatedly heating the system to 4 degrees Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

. A comprehensive characterisation of outgassing effects using mass spectrometers could be obtained for ESA's Rosetta
Rosetta (spacecraft)
Rosetta is a robotic spacecraft of the European Space Agency on a mission to study the comet 67P/Churyumov–Gerasimenko. Rosetta consists of two main elements: the Rosetta space probe and the Philae lander. The spacecraft was launched on 2 March 2004 on an Ariane 5 rocket and will reach the comet by...


Outgassing from rock

Outgassing is the source of many tenuous atmospheres of terrestrial planets or moons. Many materials are volatile relative to the extreme vacuum of space, such as around the 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...

's 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...

, and may evaporate or even boil at ambient temperature. Materials on the lunar surface have completely outgassed and been ripped away by 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...

s long ago, but volatile materials may remain at depth. Once released, gases almost always are less dense
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 than the surrounding rocks
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 and sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

 and seep toward the surface. The lunar atmosphere
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 , referred to as 'lunar atmosphere' for scientific objectives, is negligible in comparison with the gaseous envelope surrounding Earth and most...

 probably originates from outgassing of warm material below the surface. At the Earth's tectonic divergent boundaries
Divergent boundary
In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which produce rift valleys...

 where new 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...

 is being created, 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...

 and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 are some of the volatiles being outgassed from 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....

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...


Outgassing in a closed environment

Outgassing can be significant if it collects in a closed environment where air is stagnant or recirculated. This is, for example, the origin of new car smell
New car smell
New-car smell is the common term for the odor that comes from the combination of materials found in new automobiles. Some people regard it as a pleasant and desirable smell. However, there is some question about whether the chemicals in the odour pose a health risk.- Chemical composition :Both the...

. Even a nearly odourless material such as wood may build up a strong smell if kept in a closed box for months. There is some concern that softeners and solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

s that are released from many industrial products, especially plastics, may be harmful to human health. . Some types of RTV sealants outgas the poison cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....

 for weeks after application. These outgassing poisons are of great concern in the design of submarines and space stations.

See also

  • Volatile organic compound
    Volatile organic compound
    Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...

Note: VOCs don't outgas; they create a significant vapor pressure not because they have absorbed gas, but because they break down (depolymerize) due to time, light, and heat.
  • 1986 Lake Nyos Disaster

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