8. Methane on Mars
Discovery: Methane in the martian atmosphere, produced from specific surface regions and confirmed by repeated observation.
Significance: Must be produced by an active process, geochemical or biological.
Image: Mumma et al (2009)
Methane was tentatively detected in the martian atmosphere in 2003, primarily by ground-based IR instruments, possibly by ESA’s Mars Express. The concentrations were at the lower limit of the instruments’ abilities.
Mumma, M. J. et al. (2003). Bull. Am. Astron. Soc. 35,
937.
Krasnopolsky, V. A. et al. (2004). Detection of methane in the martian
atmosphere: Evidence for life? Icarus 172, 537–547.
Formisano, V et al.
(2004). Detection of methane in the atmosphere of Mars. Science 306,
1758–1761.
Image: NASA/Trent Schindler
Methane is not stable in the martian atmosphere; UV photodissociation should destroy it. But its predicted lifetime was several centuries, long enough for any release to distribute evenly around the planet. Instead, careful analysis showed that methane was localized in specific areas. Its concentration there increased from martian spring through summer, then dropped. This argues for destruction by UV- produced peroxides or other rapid chemical oxidants on the surface or entrained on airborne dust. It also indicates real-time release of methane from an active source.
Mumma, M. et al. (2009). Strong release of methane on Mars in
northern summer 2003. Science 323, 1041–1045.
Lefevre, F. and Forget, F.
(2009). Observed variations of methane on Mars unexplained by known atmospheric
chemistry and physics. Nature 460, 720.
Image: Mumma et al. (2009)
The results suggest either
geologic or biological activity. Methane could be produced by unexpected
volcanism, by reaction of olivine rock with groundwater and subsurface heat
(serpentinization), or – potentially – by subterranean microbial life. On Earth,
living creatures preferentially take up lighter element isotopes (C-12 over
C-13, H-1 over H-2), so terrestrial biogenic and geologic methane can be
distinguished by their isotope signatures. Both the Mars Science Laboratory
(2011) and ExoMars (2018) will carry instruments to detect and analyze martian
methane.
Mahaffey, P. R. (2009). Sample analysis at Mars (SAM) instrument
suite for the 2011 Mars Science Laboratory. Paper contributed to the 40th Lunar
and Planetary Science Conference, March 23-27 2009, The Woodlands TX. Sponsored
by the Lunar and Planetary Institute, Houston TX. Retrieved from
http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1088.pdf.
E.g.: Potter E.G. et
al. (2009). Isotopic composition of methane and inferred methanogenic substrates
along a salinity gradient in a hypersaline microbial mat system. Astrobiology
9(4), 383-390 and references cited therein.
