Methane is a greenhouse gas that on Earth most often has a biological origin, although it can also come from volcanic activity. Because of the biological implications – and because any Martian methane will be destroyed by oxygen in the atmosphere in a geologically brief time (less than several hundred years) – finding methane there would be an exciting discovery.
Recently, teams of scientists using the Planetary Fourier Spectrometer on the Mars Express orbiter and Earth-based spectroscopic observations have claimed to detect methane in the Martian atmosphere. They have also claimed that the methane varies both in where it is found and on timescales as short as a few months. Now a paper published in the April 2011 Icarus by Kevin Zahnle (NASA Ames Research Center) and two colleagues casts doubt on both claims.
The Zahnle team points out that the Mars Express and Earth-based observations call for the lifetime for methane in the Martian atmosphere to be just weeks or months. While oxygen in the atmosphere does remove methane, they say, the claimed rate requires both a strong source for the methane and a similarly vigorous way of eliminating it. They review physical and chemical methods of removing methane from the atmosphere, but find no plausible mechanisms operating with such rapidity.
The researchers also examine the spectral detections themselves. They note that the Earth-based observations are extremely difficult because Martian methane’s spectral lines lie on top of vastly stronger terrestrial ones. This means scientists have to use varying motions between Earth and Mars to separate the lines using doppler shifts. The snag, says the Zahnle team, was that methane was detected only when Earth and Mars were approaching each other, and not when their relative motion was away from each other. The methane should have been detectable under both situations, not just one.