Mars Astrobiology: Recent Status and Progress - Planetary Exploration and Science: Recent Results and Advances - page 190

8.5 New Theoretical Models for Mars Astrobiology

8.5.1 Possible Biosignatures

8.5.1.1 Methane

Trace amounts of methane in the atmosphere of Mars were discovered in 2003 and
verified in 2004. As methane is an unstable gas, its presence indicates that there must
be an active source on the planet in order to keep such levels in the atmosphere. It
is estimated that Mars must produce 270 t/year of methane, but asteroid impacts
account for only 0.8 % of the total methane production. Although geologic sources
of methane such as serpentinization are possible, the lack of current volcanism,
hydrothermal activity, or hot spots is not favorable for geologic methane.
It has been suggested that the methane was produced by chemical reactions
in meteorites, driven by the intense heat during entry through the atmosphere.
Although research published in December 2009 ruled out this possibility, research
published in 2012 suggests that a source may be organic compounds on meteorites
that are converted to methane by ultraviolet radiation (Fig. 8.24 ).
The existence of life in the form of microorganisms such as methanogens
is among possible, but as yet unproven sources. If microscopic Martian life is
producing the methane, it probably resides far below the surface, where it is still
warm enough for liquid water to exist.

Fig. 8.24 Distribution of methane in the atmosphere of Mars in the northern hemisphere during

summer ( http://astrobiology.gsfc.nasa.gov/research1.html - NASA/JPL-Caltech/MSSS, 2009)