What is a trace gas?

A trace gas makes up less than 1% by volume of a planet’s atmosphere. Trace gases in the martian atmosphere include methane, water vapour, nitrogen dioxide and acetylene. Although making up a very small amount of the overall atmospheric inventory, methane in particular holds key clues to the planet’s current state of activity.

On Earth, living organisms release much of the planet’s methane. It is also the main component of naturally occurring hydrocarbon gas reservoirs, and a contribution is also provided by volcanic and hydrothermal activity. Because of the key role natural biology plays in Earth’s methane production, confirming the existence of methane on Mars, and distinguishing between its potential sources, is a top priority of the ExoMars Trace Gas Orbiter.