By Dr David Whitehouse
BBC News Online science editor
Methane has been found in the Martian atmosphere which scientists say could be a sign that life exists today on Mars.
Is there life beneath the soil?
It was detected by telescopes on Earth and has recently been confirmed by instruments onboard the European Space Agency's orbiting Mars Express craft.
Methane lives for a short time in the Martian atmosphere so it must be being constantly replenished.
There are two possible sources: either active volcanoes, none of which have been found yet on Mars, or microbes.
The spectral signature of the gas was seen by the Infrared Telescope on Hawaii and the Gemini South Observatory in Chile.
Scientists operating the Mars Express Planetary Fourier Spectrometer (FPS) have announced they have detected the presence of methane in the Red Planet's atmosphere, too.
Scientists see two possibilities, both of them scientifically important, but one of them is sensational
The world's largest telescope, the twin Keck facility on Hawaii, has looked but has yet to report its findings.
But further evidence of methane on Mars will be presented at a meeting next month by a consortium of astronomers using the Canada-France-Hawaii telescope.
Methane is not a stable molecule in the Martian atmosphere. If it was not replenished in some way, it would only last a few hundred years before it vanished.
Scientists see two possibilities, both of them scientifically important, but one of them is sensational.
It is possible that the methane is being produced by volcanic activity. Lava deposited on to the surface, or released underground, could produce the gas.
Nasa's Infrared Telescope detected methane last year
This explanation has some difficulties, however. So far, no active volcanic hotspots have been detected on the planet by the many spacecraft currently in orbit.
If active volcanism were responsible then it would be a major discovery with important implications. The heat released by any volcanism would melt the vast quantities of sub-surface ice discovered on the planet, producing an environment suitable for life.
Life on Mars?
On Earth, there are organisms called methanogens - microbes that produce methane from hydrogen and carbon dioxide. These organisms do not need oxygen to thrive, and they are thought to be the type of microbes that could possibly live on Mars.
The twin US space agency rovers that landed on the Red Planet in January will be unable to answer the question of the methane's origin as they are designed for geological work.
But future missions could include sensors to analyse the methane to determine where it came from.
The failed Beagle 2 mission had a device that could have sniffed the Martian atmosphere for methane.