Methane has been found in the Martian atmosphere which scientists say could be a sign that life exists today on Mars.
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.
---
Volcanic explanation
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.
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.
http://news.bbc.co.uk/1/hi/sci/tech/3577551.stm