Outgassing (sometimes calledoffgassing, particularly when in reference toindoor air quality) is the release of agas that was dissolved, trapped,frozen, orabsorbed in some material.^[1] Outgassing can includesublimation andevaporation (which arephase transitions of a substance into a gas), as well asdesorption,seepage from cracks or internal volumes, and gaseous products of slowchemical reactions.Boiling is generally thought of as a separate phenomenon from outgassing because it consists of a phase transition of a liquid into avapor of the same substance.

Outgassing is a challenge to creating and maintaining clean high-vacuum environments.NASA andESA maintain lists of materials with low-outgassing properties suitable for use inspacecraft, as outgassing products can condense onto optical elements,thermal radiators, orsolar cells and obscure them. Materials not normally considered absorbent can release enough lightweightmolecules to interfere with industrial or scientific vacuum processes.Moisture,sealants,lubricants, andadhesives are the most common sources, but evenmetals andglasses can release gases from cracks or impurities. The rate of outgassing increases at highertemperatures because thevapor 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 of surfaces, or heating of individual components or the entire assembly (a process called "bake-out") can drive offvolatiles.

NASA'sStardustspace probe suffered reduced image quality due to an unknown contaminant that had condensed on theCCD sensor of the navigation camera.^[2] A similar problem affected theCassini space probe's Narrow Angle Camera, but was corrected by repeatedly heating the system to 4 °C.^[3] A comprehensive characterisation of outgassing effects usingmass spectrometers could be obtained for ESA'sRosetta spacecraft.^[4]

Natural outgassing is commonplace incomets.^[5]

Outgassing is a possible source of many tenuousatmospheres ofterrestrial planets or moons.^[6] Many materials are volatile relative to the extreme vacuum ofouter space, and may evaporate or even boil at ambient temperature. Materials on thelunar surface have completely outgassed and been blown away bysolar winds long ago, but volatile materials may remain at depth. Thelunar atmosphere probably originates from outgassing of warm material below the surface.

Once released, gases almost always are lessdense than the surroundingrocks andsand and seep toward the surface.Explosive eruptions of volcanoes result from water or other volatiles outgassed frommagma being trapped, for example by alava dome. At the Earth'stectonic divergent boundaries where newcrust is being created,helium andcarbon dioxide are some of the volatiles being outgassed frommantlemagma.Alpha decay ofprimordial radionuclides (and their decay products) produces the vast majority of the helium that continues to gas out of rocks on terrestrial planets.

Outgassing can be significant if it collects in a closed environment where air is stagnant or recirculated. For example,new car smell consists of outgassed chemicals released by heat in a closed automobile. Even a nearly odorless material such as wood may build up a strong smell if kept in a closed box for months. There is some concern thatplasticizers andsolvents released from many industrial products, especially plastics, may be harmful to human health.^[7] Long-term exposure to solvent vapors can causechronic solvent-induced encephalopathy (CSE). Outgassing toxic gases are of great concern in the design ofsubmarines andspace stations, which must have self-contained recirculated atmospheres.

The outgassing of small pockets of air near the surface of settingconcrete can lead to permanent holes in the structure (calledbugholes) that may compromise its structural integrity.^[8][9]

• Lake Nyos Disaster
• Materials for use in vacuum
• Volatile organic compound
• Comet

• Outgassing Data for Selecting Spacecraft Materials Online (NASA database)
• ESA Outgassing Data for Spacecraft Materials according to ECSS-Q-70-02