Methane has been detected in the Martian atmosphere, reigniting speculation about the possibility of life on the planet. On Earth, methane is a common byproduct of single-celled organisms, so its presence in the Martian atmosphere could be a sign of bacteria still living on the planet.
That's just one possible explanation, says Vittorio Formisano of the Institute of Physics of Interplanetary Space in Rome, who helped to confirm the finding. The methane could be produced by purely geological processes, such as volcanic activity.
Methane was first seen on Mars by astronomers led by Michael Mumma of Nasa's Goddard space flight centre in Maryland. They used Earth-based telescopes to detect gaps in the spectrum of infrared light coming from Mars. These gaps occur at wavelengths where methane absorbs radiation. The findings have been confirmed by Mars Express, the European Space Agency craft that released the ill-fated Beagle 2 lander.
Mars Express is orbiting Mars, carrying an instrument called the planetary fourier spectrometer (PFS), which found the spectral fingerprint of methane. "We have detected methane at concentrations of 10 parts per billion," says Formisano.
Methane cannot exist in the Martian atmosphere for longer than about 300 years. The molecules are split apart by sunlight, and the fragments escape the planet's gravity.
So researchers say there must be a present-day source of methane on Mars. Formisano's suspicion is that rather than coming from martian life, the methane may come from volcanic processes. Although there are no known active volcanoes, the gigantic Mons Olympus was active until 100,000 years ago - very recent in geological terms. So it might be emitting small amounts of methane now.