Hopes of finding life on Mars have been boosted by British scientists studying sources of methane on the planet.
They have ruled out any possibility of meteorites delivering the high levels of the gas detected in the Martian atmosphere.
That leaves only two alternatives. Either the methane is created by chemical reactions between volcanic rock and water, or it is being generated by living organisms.
On Earth methane is produced in large quantities both by microbes and the digestive processes of large animals such as cows.
The gas has a short lifetime of just a few hundred years on Mars because it is continuously being destroyed by sunlight in the planet's atmosphere.
Yet its levels remain constant, leading scientists to speculate about what might be topping it up.
Some experts believe the most likely source is extraterrestrial micro-organisms living in the Martian soil.
Their hopes have now been raised by the latest research, which has dismissed one of the leading alternative theories.
It was thought that meteorites burning up in the Martian atmosphere could be releasing the methane.
But scientists at Imperial College London have shown that the amounts of methane generated in this way are far too low to maintain the levels detected in the atmosphere.
Previous studies have ruled out the possibility of volcanic activity emitting the methane. The one remaining alternative to life being the source is chemical reactions between volcanic rock and water.
Dr Richard Court, from the Department of Earth Science and Engineering at Imperial College, said: "Our experiments are helping to solve the mystery of methane on Mars. Meteorites vaporising in the atmosphere are a proposed methane source but when we recreate their fiery entry in the laboratory we get only small amounts of the gas. For Mars, meteorites fail the methane test."
The findings, published in the journal Earth and Planetary Science Letters, will help American and European scientists planning a joint mission to the Red Planet in 2018 to investigate the source of Martian methane.
Co-author Professor Mark Sephton, also from Imperial College, said: "This work is a big step forward. As Sherlock Holmes said, eliminate all other factors and the one that remains must be the truth. 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 scientists reproduced the same searing conditions experienced by meteorites as they enter the Martian atmosphere.
An infrared beam was used to measure gases released by meteorite fragments heated to 1000C.
Applying the results to known data on the numbers of meteorites falling on Mars, the researchers calculated that only 10 kilograms of methane was being delivered to the planet from space each year. This was far below the 100 to 300 tonnes required to replenish methane levels in the Martian atmosphere.
In January scientists at the American space agency Nasa reported finding evidence of "multiple plumes" of methane on Mars concentrated in three regions of the planet just north of the equator.