Using three martian years worth of Mars Global Surveyor (MGS) data, scientists presenting their results at the European Planetary Science Congress in Rome this week find that methane in the red planet's atmosphere follows an annual cycle.

Mars' atmosphere is predominantly carbon dioxide, with small amounts of nitrogen and argon, and trace amounts of oxygen, water and methane. But methane is of particular interest since, on Earth, it is produced by geological or biological activity.

"Only small amounts of methane are present in the martian atmosphere, coming from very localised sources," says Sergio Fonti of the Università del Salento. "We've looked at changes in concentrations of the gas and found that there are seasonal and also annual variations. The source of the methane could be geological activity or it could be biological – we can’t tell at this point. However, it appears that the upper limit for methane lifetime is less than a year in the martian atmosphere."

Fonti and colleague Giuseppe Marzo of NASA Ames analysed data from the MGS Thermal Emission Spectrometer (TES) collected between July 1999 and October 2004, which equates to three martian years, pulling out methane spectra from some three million observations.

The highest levels of methane – 70 parts per billion – peak in northern hemisphere autumn, although methane is detected all over the planet at the same time during this period. The team also note a sharp decrease in methane in winter months, with only a faint, lingering band located at 40-50 degrees north. As the hemisphere emerges from winter into spring there is a gradual rise in methane, which takes a firmer hold in summer, spreading across the planet.

"One of the interesting things that we’ve found is that in summer, although the general distribution pattern is much the same as in autumn, there are actually higher levels of methane in the southern hemisphere," says Fonti. "This could be because of the natural circulation occurring in the atmosphere, but has to be confirmed by appropriate computer simulations."

Fonti and Marzo identify three methane hotspots, with the highest concentrations observed over the Tharsis volcanic region, where volcanic, hydrothermal and geothermal processes could be occurring. High concentrations are also observed over Elysium, another volcanic region, and Arbia Terrae, where buried water ice is known to exist.

"It's evident that the highest concentrations are associated with the warmest seasons and locations where there are favourable geological – and hence biological – conditions such as geothermal activity and strong hydration," says Fonti. "The higher energy available in summer could trigger the release of gases from geological processes or outbreaks of biological activity."

Although methane has been detected in the martian atmosphere for years, this latest study is the first to monitor its behaviour over an extended period.