The Viking1 and Phoenix2 landers have revealed no organic materials (which are prerequisite for life) on the Martian surface. The Phoenix mission (see Figure 1) detected inorganic perchlorates (potent oxidizing substances) in the Martian soil.2 Their high-temperature oxidative properties may promote combustion of organics in pyrolytic experiments, such as those used on the Viking and Phoenix landers. Because the high-temperature conditions of the Viking instruments and the Phoenix Thermal and Evolved Gas Analyzer (TEGA) are not representative of the environment on Mars, this may compromise the ability of the experiments to detect organics.
I have posed the question as to whether organic materials can survive oxidation with perchlorates at less extreme temperatures than found in the TEGA and Viking instruments. I have surveyed the literature on perchlorate oxidation of various groups of organic materials. Several amino acids, notably glycine and alanine, are quite resistant to this process. The same is true for some heterocycles, namely certain purines and purimidines. These organic materials may survive in the natural environment on Mars, although the landers may not have been able to detect them.
Figure 1. Phoenix lander on the Martian surface in a vertical projection, combining multiple exposures taken by the Surface Stereo Imager camera. The black circle indicates where the camera is mounted. (Image courtesy of NASA/Jet Propulsion Laboratory-California Institute of Technology/University Arizona/Texas A&M University.)
The main work in this area is by Martinie and Schilt,3 who evaluated the effectiveness of wet perchloric-acid oxidation of 85 different organic substances. Such oxidation is used to destroy organic materials as much as possible, so that the aqueous residue can be analyzed for the presence of various elements, such as iron from blood, chrome from tanned leather, or sulfur from coal. Screening for organic residues, whose presence would indicate incomplete destruction, was done by a range of methods, including proton-nuclear-magnetic resonance, UV, IR, and mass spectrometry, and carbon microanalysis.
Certain samples were found to resist oxidation partially or completely. Approximately half of the samples retained measurable carbonaceous matter in solution. The table shows the data on amino acids. While some were destroyed, several survived, including glycine and alanine.3
Table 1. Amino-acid residues after wet perchloric-acid oxidation. (%C: Percentage carbonaceous matter.)
|Amino acids||%C||Identified organic residue|
|Alanine||67||Alanine, acetic acid|
|Glutamic acid||14|| |
The same was true for various heterocycles, some of which are important for life. Based on these results,3 I state that the organics that survive vigorous wet-perchlorate oxidation ought to survive on Mars in the presence of perchlorates under more moderate conditions than those of surface-based experiments performed to date. Thus, an article from 1976 is now of central relevance to the chemistry on Mars and must be acknowledged.
I plan to investigate further perchlorate oxidation of amino acids under moderate conditions (more in line with the Martian environment), so that I can estimate more precisely their possible survival on the planet's surface. I believe that more amino acids should be recovered and in larger amounts under conditions simulating those on Mars than in wet-oxidation experiments. If correct, this may influence the design of instruments for future Mars landers.
Vera M. Kolb
University of Wisconsin-Parkside
Vera Kolb is a chemistry professor who received her PhD in organic chemistry. She has worked in astrobiology with Leslie Orgel at the Salk Institute and Stanley Miller at the University of California at San Diego. She has over 130 publications, including three patents, in the fields of organic and medicinal chemistry, and astrobiology. She has been recipient of several research and education grants from the Wisconsin Space Grant Consortium, and was inducted into the Southeastern Wisconsin Educators' Hall of Fame in 2002.
2. M. H. Hecht, S. P. Kounaves, R. C. Quinn, S. J. West, S. M. M. Young, D. W. Ming, D. C. Catling, B. C. Clark, W. V Boynton, J. Hoffman, L. P. DeFlores, K. Gospodinova, J. Kapit, P. H. Smith, Detection of perchlorate and the soluble chemistry of Martian soil at the Phoenix lander site, Science 325, pp. 64-67, 2009.