The presence of ammonia in the atmosphere of Mars was tentatively identified in 2004 by the Planetary Fourier Spectrometer (PFS) onboard the European Space Agency's orbiting Mars Express craft. This discovery follows closely that of methane in the martian atmosphere (see methane on Mars). It is considered by some scientists to be a strong indication of the presence of life, although other explanations, such as mild volcanic activity, can't yet be ruled out. Since the initial statement of the finding of ammonia to journalists by the PFS principal investigator Vittorio Formisiano, the PFS team has indicated that it has no firm evidence yet of this gas in the martian atmosphere although the search for it continues.

 

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.

 

The importance of ammonia is that it is a compound of nitrogen and hydrogen. Nitrogen is rare in the martian environment but because no form of terrestrial life can exist without it, the presence of ammonia may indicate that martian microbial life is hoarding it. In addition, ammonia is not a stable molecule in the martian atmosphere. If it was not replenished in some way, it would only last a few hours before it disappeared.