Searching for Signs of Life


Exactly what kinds of compounds could signal the presence of life?  Certainly water would be first on the list.  From geologic clues, scientists are convinced  that water once existed in equilibrium with the atmosphere on the surface of Mars.  Water currently existing on Mars  is primarily in the solid or vapor state due to the low temperature and low atmospheric pressure on Martian surface.  (12)  Other compounds then rise to the top of the list.  The identification of carbonate and sulfate  deposits could signal the possibility of  a past water source.  In Earth's environment, carbonate deposits are the result of aqueous sedimentation.  It is hypothesized that perhaps fossilized microorganisms may also be found in the carbonate deposits.(8) Some scientists hypothesize that amino acids may be present on or just below the Martian surface.  On the surface, conditions are highly oxidizing and would be hostile to organic molecules.  Some scientists anticipate that by drilling just below the surface, the chance of finding protein residues, amino acids, would increase. (13)
        In situ analysis of the Martian soil by Raman Spectroscopy would yield important information about the inorganic and organic compounds that exist there.  Once relayed to earth, scientists can analyze the spectra in order to determine if the "chemical fingerprint" of specific compounds are present.  Three of these molecules have been chosen for computational analysis by WebMo. Each molecule was composed in editor.  Geometries were optimized first followed by vibrational frequency calculations.  Each were run with  B3LYP theory at a basis set of 6-311+G(d,p).  The resulting vibrational frequencies were then  reviewed , compared against published results and an approximation of group vibration was assigned.



Metal Carbonates

Figure 5: The Raman Active Carbonate Stretch

Metal carbonates give a very intense Raman signal ranging from 1086-1098 cm-1 depending on the metal involved. (14).  WebMo results for CO3-2 indicated a vibrational frequency at  1009.6 cm-1.  The stretch  at this frequency is displayed  to the left.  This stretch results in a change in polarizability and is Raman  active. As a result of this strong signal, the presence of carbonate deposits on Mars could easily be verified by Raman Spectroscopy.   WebMO output (2326) can be viewed here.





Phenylalanine is an amino acid that is present in many proteins.  The possible identification of the existence of amino acids on or near the surface of Mars would yield strong evidence for the possibility of past life on the planet.   This amino acid has multiple functional groups including a phenyl group attached to the basic backbone of the acid.  The molecular model may be viewed on the right.  The most predominant Raman signal from Phenylalanine occurs at  1000 cm-1.  This stretch is the result of the stretch of the benzene ring (ring breathing) and is a key marker for this amino acid. (8)  The diagram below illustrates a ring breathing mode for phenylalanine. 

Figure 7: The benzene ring breathing vibration

Figure 6: The structure of Phenylalanine

WebMo analysis of phenylalanine resulted in numerous vibrational frequencies.    WebMO output (2553) can be viewed here.      Reference Raman vibrational frequencies and Raman spectra for phenylalanine can be viewed here. (17)   An analysis of WebMo results versus published values for Raman vibrations showed excellent agreement.  In general, the two sets of values were linearly related  with a correlation coefficient (R2)  equal to 0.9993.   WebMo results were consistently higher in energy by an average factor or 30 cm-1 Analysis of results can be viewed here.



Figure 8: The structure of Glycine

     Glycine, a very simple amino acid, could be yet another marker indicating the possible presence of life on Mars.  WebMo output (2455) can be viewed hereReference Raman vibrational frequencies and Raman Spectra for glycine can be viewed here.  (17)  An analysis of WebMO vibrational frequencies versus published values indicated a strong correlation between the two sets of values.  The correlation coefficient equaled .9955 with a slope of .9971.  The y incept was 60cm-1 indicating that WebMo values for glycine were consistently 60cm-1 higher than the reference.  Analysis of results can be viewed here.     An example of a Raman active stretch for glycine is displayed below.  WebMO reported this stretch at 3014cm-1.  This was matched to the reference spectra at 2973 cm-1. (17, 18) From the diagram, it can be seen that the two hydrogens on the amine group (nitrogen is atom number 6) as well as the CH2 group are symmetrically stretching at this frequency.

Figure 9: The Raman active stretch at 3014 cm-1





          In addition to these simple chemical species,  complex molecules such as chlorophyl or even entire organisms such as cyanobacteria emit very specific, identifiable Raman spectra.  (2)  The spectrophotometer used for these missions must be able to withstand the vigor of interplanetary travel as well as function robotically on a remote planet.  Click on "The Raman Spectrophotometer" for more information.



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