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Outgassing is the release of a gas trapped or frozen in some material. This is not to be confused with sublimation or boiling which are phase transitions of a substance into a gas.
Outgassing on spacecraft
Outgassing is a problem encountered in high-vacuum applications. For example, NASA maintains a list of low-outgassing materials to be used for spacecraft that have sensitive optics, as outgassing can cause deposits on mirrors, optical lenses, thermal radiators, or solar cells. Materials not normally considered absorbent can release enough light molecules to interfere with industrial or scientific processes. Even metals and glasses can release gases from cracks or impurities, but sealants, lubricants, and adhesives are the most common cause. The rate of outgassing increases at higher temperatures. 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 spaceprobe suffered reduced image quality (http://stardust.jpl.nasa.gov/news/vision.html) due to an unknown contaminant that had condensed on the CCD sensor of the navigation camera. A similar problem affected the Cassini-Huygens spaceprobe's Narrow Angle Camera (http://saturn.jpl.nasa.gov/news/press-releases-02/20020723-pr-a.cfm), but was corrected by repeatedly heating the system to 4 degrees Celsius.
Outgassing from rock
Outgassing can also refer to the source of the tenous atmospheres of terrestrial planets or moons. Gases from any material which is above its boiling point tend to be of lighter density than surrounding material and flow toward the surface. In a vacuum, such as on Earth's Moon, the boiling point is quite low and many materials are volatile, but material on the surface outgassed long ago. The Moon's atmosphere is probably from below the surface which is outgassing from warm material. At the Earth's tectonic divergent boundaries where new crust is being created, helium and carbon dioxide are some of the volatiles being outgassed from mantle magma.
See Also
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