Outgassing is the release of a gas that was dissolved, trapped, frozen or
absorbed in some material. Outgassing can include sublimation and evaporation
which are phase transitions of a substance into a gas, as well as
desorption, seepage from cracks or internal volumes and gaseous products of
slow chemical reactions. Boiling 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.
In a vacuum Outgassing is a challenge to creating
and maintaining clean high-vacuum environments. NASA 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
cells and obscure them. Materials not normally considered absorbent can
release enough light-weight molecules to interfere with industrial or scientific
vacuum processes. Moisture, sealants, lubricants, and adhesives are the most
common sources, but even metals and glasses can release gases from cracks or
impurities. The rate of outgassing increases at higher temperatures 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 spaceprobe suffered reduced image quality due to an unknown
contaminant that had condensed on the CCD sensor of the navigation camera. A
similar problem affected the Cassini spaceprobe's Narrow Angle Camera, but
was corrected by repeatedly heating the system to 4 °C. A comprehensive
characterisation of outgassing effects using mass spectrometers could be
obtained for ESA's Rosetta spacecraft. 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 Moon, and may evaporate or even boil at ambient
temperature. Materials on the lunar surface have completely outgassed and
been ripped away by solar winds long ago, but volatile materials may remain
at depth. Once released, gases almost always are less dense than the
surrounding rocks and sand and seep toward the surface. The lunar atmosphere
probably originates from outgassing of warm material below the surface. 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. In a closed environment
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
odourless material such as wood may build up a strong smell if kept in a
closed box for months. There is some concern that plasticizers and solvents
released from many industrial products, especially plastics, may be harmful to
human health. Some types of RTV sealants outgas the poison cyanide for weeks
after application. Outgassing toxic gases are of great concern in the design
of submarines and space stations, which must have self-contained recirculated
atmospheres. In construction
The outgassing of small pockets of air near the surface of setting concrete can
lead to permanent holes in the structure that may compromise its structural
integrity. See also
Volatile organic compound 1986 Lake Nyos Disaster
Materials for use in vacuum References
External links Outgassing Data for Selecting Spacecraft
Materials Online ESA Outgassing Data for Spacecraft
Materials according to ECSS-Q-70-02