Outgassing

Outgassing, also called offgassing when talking about indoor air quality, is when a gas that was dissolved, trapped, frozen, or absorbed in something is released. This can happen in many ways. It might come out through sublimation or evaporation, which are changes from a solid or liquid into a gas. It can also happen when gases slowly leak out from cracks or inside materials, or when chemical reactions create new gases.

Boiling is usually seen as different from outgassing because it is when a liquid turns into a vapor of the same substance. Outgassing is important to understand because it can affect the air we breathe and the way materials behave, especially in places like space or inside buildings.

In a vacuum

Outgassing can be a problem when trying to keep a very clean space with very little air, like inside a spacecraft. Organizations like NASA and ESA have lists of materials that don’t let out much gas, because these gases can stick to important parts like cameras or solar panels and make them work worse. Even things that don’t usually hold onto gases, like metals or glass, can release small amounts that interfere with scientific work.

The amount of gas released can go up when things get hotter, since heat makes molecules move faster. Ways to lower outgassing include cleaning surfaces or heating parts in a process called “bake-out” to drive off extra gases. Some space probes, like Stardust and Cassini, faced issues with their cameras because of gases that stuck to them, but these were fixed by heating the systems. Natural outgassing also happens in objects like comets.

From rock

Outgassing may help create thin atmospheres around planets and moons. Many materials can turn into gas in the empty space of outer space. On the Moon, materials close to the surface have been lost, but some may still be deep below.

Once gases are released, they are usually lighter than rocks and sand, so they move toward the surface. Volcanoes can erupt powerfully when trapped gases, like water, escape from melted rock. At places where new land forms on Earth, gases like helium and carbon dioxide come from deep, hot rock. Radioactive processes create most of the helium that slowly escapes from rocks on planets.

Some rocks, like those with radium-226, can release a gas called radon, which needs special steps to keep safe.

In a closed environment

Outgassing can be important in places where air doesn’t move much. For example, the smell in a new car comes from chemicals that are released when the air inside stays still. Even something that seems smell-free, like wood, can start to smell strong if it sits in a closed box for a long time. Some chemicals that come out of plastics and other products might not be good for our health if we breathe them in for a long time. This is especially important when building places like submarines and space stations, where the air has to be kept and reused.

In construction

When small pockets of air come out near the surface of drying concrete, it can leave permanent holes called bugholes. These holes might weaken the structure over time.