Methane has been found in the Martian atmosphere which scientists say could be a sign of present day life 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 ways to do this. Either active volcanoes, but none have yet been found on Mars, or microbes.
Astronomers have reported seeing the spectral signature of methane in the Martian atmosphere using several of the world's largest telescopes.
The Infrared telescope on Hawaii and the Gemini South observatory in Chile detected the gas last year, it has been revealed.
Also scientists operating the Mars Express Planetary Fourier Spectrometer (FPS) have announced that they have found the spectral signature of methane in the Martian atmosphere.
In addition, the world's largest telescope, the twin Keck telescopes on Hawaii, have looked but have yet to report its findings.
Further evidence of methane on Mars will also 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 it would only last a few hundred years before it vanished.
This means that the methane detected must be being replenished in some way.
Scientists see two possibilities, both of the scientifically important, one of them sensational.
It is possible that the methane is being produced by volcanic activity. Lava being deposited onto the surface, or released underground, could produce the gas.
That explanation has some difficulties, however. So far no active volcanic hotspots have been detected by the many spacecraft currently orbiting Mars.
If active volcanism is responsible then it is a major discovery with important implications. The sub-surface heat released by the volcano would melt the vast quantities of sub-surface ice discovered on Mars producing an environment suitable for life.
Life on Mars?
On Earth bacteria produce methane from hydrogen and carbon dioxide. Terrestrial microbes that produce methane do not need oxygen to thrive, and these are thought to be the type of microbes that could possibly live on Mars.
The twin rovers that landed on Mars in January will be unable to answer the question of the methane's origin as they are designed for geological work.
Future missions could include sensors to analyse the methane that may be able to determine if it is of biological origin.
Nasa's infrared telescope detected methane last year
The failed Beagle 2 mission had a device that could have sniffed the Martian atmosphere for methane.
[Edited on 29-3-2004 by rezwan1977]