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Long-extinct bacteria that released methane that became trapped in ice or minerals in the upper crust. Changing temperature and pressure could stress the ice and release that ancient gas into today's atmosphere.

Bacteria that are actively producing methane to this day.

Abiological sources. Iron can combine with oxygen in terrestrial hot springs and volcanoes to create methane. This gas can also become trapped in solid forms of water or 'cages' called clathrate hydrates that can preserve it for a long time. Olivine, a common mineral on Earth and Mars, can react with water under the right conditions to form another mineral called serpentine. When altered by heat, water and pressure, such in environments such as hydrothermal springs, serpentine can produce methane.