Noble gases
Noble gases are the unreactive gases helium, neon, argon, krypton and xenon, whose isotopes record many events in a meteorite's history.
The noble gases are helium, neon, argon, krypton and xenon. They hardly react with other elements, so they are not locked into minerals by chemistry, and even tiny amounts can be measured. Heating a meteorite releases them, and a noble gas mass spectrometer can count atoms in very small numbers.
Noble gases in meteorites come from several sources. Some were trapped when the rock formed, from the solar nebula or from presolar carriers. Some were implanted by the solar wind into grains at an asteroid's surface. Some were made by cosmic rays during the journey through space, and some by radioactive decay inside the rock.
By separating these components, researchers obtain exposure ages, gas retention ages and evidence of which meteorites came from planetary surfaces. Most famously, gases trapped in shock glass in shergottites match the Martian atmosphere measured by the Viking landers, which confirmed that these rocks come from Mars.
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