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Ammonia may have been found in Mars' atmosphere which some scientists say could indicate life on the Red Planet. <br />Researchers say its spectral signature has been tentatively detected by sensors onboard the European Space Agency's orbiting Mars Express craft. <br /><br />Ammonia survives for only a short time in the Martian atmosphere so it must be getting constantly replenished. <br /><br />There are two possible sources: either active volcanoes, none of which have been found yet on Mars, or microbes. <br /><br />Excitement <br /><br />"Ammonia could be the key to finding life on Mars," one US Space Agency (Nasa) scientist told BBC News Online. <br /><br />Spectral evidence of the gas was seen by the Planetary Fourier Spectrometer (PFS) on Mars Express. <br /><br />Professor Vittorio Formisano, principal investgator for the instrument, is expected to release details of new findings from the PFS at an international conference being held next week in Paris. <br /><br />The PFS is sensitive to radiation in the spectral region of 1.2 - 5 microns and 5 - 50 microns - a region rich with important molecules such as water and carbon dioxide. Ammonia has a spectral line at 10 microns. <br /><br />Although Mars Express has been in orbit around Mars since December 2003, scientists have so far only analysed a fraction of the data the PFS has produced. <br /><br />Researchers say this is because they are still coming to terms with the complexities of the PFS as well as coping with some nagging power problems on Mars Express. <br /><br />So far the PFS has observed a depletion of carbon dioxide and an enrichment of water vapour over some of the large extinct volcanoes on Mars. <br /><br />But it is the detection of minor compounds, possible in the sensors high-resolution mode, that is causing excitement. <br /><br />The detection of ammonia comes just a few months after methane was found in the Martian atmosphere. Methane is another gas with a possible biological origin. <br /><br />Ammonia is not a s