One possibility for the source of Martian methane has been ruled out
The methane found on Mars is not brought to the planet by meteor strikes, scientists say.
Meteoritic material subjected to high temperatures did not release enough methane to account for the amount believed to be released on Mars.
The researchers argue that the methane must therefore be created by geologic or chemical processes, or it is a by-product of microbial life.
The work appears in Earth and Planetary Science Letters.
The origin of the methane on Mars has remained a mystery since it was first detected in 2004.
Because methane has a limited lifetime in the Martian atmosphere before degrading, some process must be pumping hundreds of tonnes of it into the Martian atmosphere annually to keep it at the levels that have been detected.
Scientists at Imperial College London say they have now ruled out the possibility that the methane is being constantly deposited by meteorites landing on the planet.
They heated meteorite fragments to 1,000C, quantifying the gases produced by measuring how much they absorbed an infrared laser.
Sunlight quickly breaks down methane in the Martian atmosphere
They then calculated, based on estimates of how many meteorites impact Mars annually, how much methane would be released.
They found that just 10kg of methane is produced from meteors each year, in contrast to the 100-300 tonnes that must be produced to keep the atmospheric concentration at its current levels.
That suggests that a number of other possibilities, many of which are based on chemical reactions of the rocks that form the planet's crust.
Alternatively, the gas may be produced by volcanoes or life that survives beneath the crust - or it may be trapped in chemical cages, having been produced long ago.
"As Sherlock Holmes said, eliminate all other factors and the one that remains must be the truth," said study co-author Mark Sephton.
"The list of possible sources of methane gas is getting smaller and excitingly, extraterrestrial life still remains an option. Ultimately, the final test may have to be on Mars."
The US space agency Nasa will launch the Mars Science Laboratory in 2011, which will be able to study the methane more closely.
In November, the European and American space agencies signed an agreement to collaborate on Mars missions that will return to the planet, starting in 2016.