This is an interesting twist on the view that Mars is a dead planet. A recent six-year study of methane levels in Mars' atmosphere shows the planet is actually far from dead, but whether the activity is merely geological or microbial is still unknown.
A team of researchers based in Italy looked at billions of measurements taken by NASA's Mars Gobal Surveyor to compile seasonal maps of methane gas, which appears in minute quantities in the carbon-dioxide rich atmosphere. The researchers found that methane concentrations are highest in autumn and drop off dramatically in winter. Levels build up again in spring and climb rapidly in summer, which causes the gas to spread across the planet.
Ultraviolet light from the sun breaks methane down, so something is happening either on or in the planet that is replenishing the gas. Also, another mystery is the speed at which methane is being depleted. The seasonal changes are quite unexpected from a planet that is believed to not have much left going on.
Researchers found three regions in the planet's northern hemisphere with consistently higher concentrations of methane – Tharsis, Elysium, and Arabia Terrae. The first two are home to the largest volcanoes on Mars, while Arabia Terrae has large quantities of subterranean frozen water.
Lead researcher Sergio Fonti, from Italy's Universita del Salento, says the seasonal nature of the methane levels rules out the possibility of cosmic ray bombardment or meteorite impacts causing the changes. "It could be geology or biology, but it is not coming from another source," claimed Fonti.
On Earth, colonies of bacteria that consume methane have been found living right along species that produce it, and many scientists believe that there could easily be a similar situation on Mars. Still, some skeptics believe that some simple geologic explanation such as the release of methane trapped in frozen water during seasonal melting could help explain the phenomena.
NASA and Europe are planning a joint mission in 2016 to draft more detailed maps of Mars' methane. NASA's Mars Science Laboratory, scheduled for launch next year, also has an instrument that will detect atmospheric methane.