Mars methane not biological in origin
Disappointingly, scientists say they've been able to establish where the methane on Mars comes from - and it's not from living organisms.
When methane was discovered in Mars' atmosphere nine years ago, scientists drew paralles with Earth to conclude that it was probably produced either by volcanoes or - more excitingly - biological processes.
The planet appears to be generating about 200 to 300 tonnes of methane every year.
However, researchers at the Max Planck Institute for Chemistry in Mainz and the universities in Utrecht and Edinburgh have now shown that methane escapes from a meteorite if it's irradiated with ultraviolet light under Martian conditions.
Since carbon-rich meteorites and interplanetary dust from space are continually hitting the Martian surface, they conclude that this is where the methane's coming from.
"Methane is produced from innumerable, small micro-meteorites and interplanetary dust particles that land on the Martian surface from space," says atmospheric chemist Frank Keppler."The energy is provided by the extremely intense ultraviolet radiation."
Unlike Earth, Mars has no protective ozone layer which could absorb most of the UV radiation from space. Moreover, the Martian atmosphere is very thin, so that much less of the meteorite burns up.
Testing their theory involved irradiating samples of the Murchison meteorite, which fell to Earth in 1969 in Australia, with ultraviolet light.
"The meteorite contains several percent carbon and has a similar chemical composition to most of the meteoritic matter that lands on Mars," says cosmochemist Ulrich Ott.
When the researchers irradiated their samples under conditions identical to those on Mars, considerable quantities of methane escaped almost immediately.
The team concludes that carbonaceous compounds in the meteoritic matter are decomposed by the high-energy UV radiation, forming methane molecules in the process.
The warmer the conditions, the more methane was released - explaining why the largest concentration of methane has been found in Mars' equatorial region, its warmest area.
The findings don't necessarily mean there are no living creatures on Mars producing methane - but they make it less likely. The team says that Curiosity, the Mars Rover due to land in August, will provide more answers.