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(Viking / organic compounds / exobiology / astrobiology)
Departments of Chemistry, Anatomy, and Cell Biology, University of
Florida, Gainesville, FL 32611
Communicated by Leslie Orgel, The Salk Institute for Biological
Studies, San Diego, CA, December 13, 1999 (received for review November
4, 1998)
GC-MS on the Viking 1976 Mars missions did not detect organic
molecules on the Martian surface, even those expected from meteorite bombardment. This result suggested that the Martian regolith might hold
a potent oxidant that converts all organic molecules to carbon dioxide
rapidly relative to the rate at which they arrive. This conclusion is
influencing the design of Mars missions. We reexamine this conclusion
in light of what is known about the oxidation of organic compounds
generally and the nature of organics likely to come to Mars via
meteorite. We conclude that nonvolatile salts of benzenecarboxylic
acids, and perhaps oxalic and acetic acid, should be metastable
intermediates of meteoritic organics under oxidizing conditions. Salts
of these organic acids would have been largely invisible to GC-MS.
Experiments show that one of these, benzenehexacarboxylic acid
(mellitic acid), is generated by oxidation of organic matter known to
come to Mars, is rather stable to further oxidation, and would not have
been easily detected by the Viking experiments. Approximately 2 kg of
meteorite-derived mellitic acid may have been generated per m2 of Martian surface over 3 billion years. How much
remains depends on decomposition rates under Martian conditions. As
available data do not require that the surface of Mars be very strongly oxidizing, some organic molecules might be found near the surface of
Mars, perhaps in amounts sufficient to be a resource. Missions should
seek these and recognize that these complicate the search for organics
from entirely hypothetical Martian life.
Geology
The missing organic molecules on Mars
*
To whom reprint requests should be addressed. E-mail:
benner{at}chem.ufl.edu.
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