Ideal Biomarker

Ideal Biomarker

Astrobiology still ranks as the only data-free discipline in science.

Part of the problem is that in their search for signs of present or past existing life beyond planet Earth, astrobiologists have been attempting to detect biomarker molecules that already are known to yield ambiguous results. For example, they looked for and found methane emission from the Martian crust. While several terrestrial microbes produce methane in their metabolic activities, there are several completely abiotic (nonliving) chemical and geological processes that are also capable of releasing significant amounts of methane. Thus, the detection of methane release from the Martian regolith (loose rock layer) produced an equivocal result.

The same kind of ambiguity arises from searches for amino acids in dense interstellar molecular clouds. Though no undisputed detections of amino acids have yet to be achieved in such clouds, a future success would not prove the past or present existence of life there since a number of purely nonbiological mechanisms likewise can explain the existence of amino acids.

In response to the ambiguous biomarker problem a team of researchers from universities and museums in Montana and North Carolina have proposed that astrobiologists abandon their search criteria. Instead of extraterrestrial amino acids, lipids, methane, ammonia, hydrogen cyanide, glycoaldehydes, and polyaromatic compounds, all of which can be produced abiotically, researchers propose the detection of a class of molecules called porphyrins. Porphyrins are characterized by four nitrogen atoms linked together via methane bridges into a macrocycle (a closed loop of molecules that form a cavity with the four nitrogen atoms faced toward the cavity). Porphyrins include such well known biomolecules as chlorophyll and hemoglobin.

The research team points out a number of advantages of porphyrins to astrobiologists:

  1. They cannot be produced by nonbiological means under natural conditions in any measurable amounts.
  2. They are widely distributed among all living organisms on Earth, including micro-organisms.
  3. They are significantly more stable outside of the protection of living organisms than traditional biomarkers such as amino acids.
  4. Their chemical identification signatures are distinct. That is, chemists can clearly distinguish porphyrins from other molecular structures.

The disadvantage of porphyrins to astrobiologists has been how difficult they are to detect. Currently, there is no way any kind of spectrometer attached to a telescope can make a positive, unambiguous identification of the spectrum for any of the porphyrins. The point of the research team’s published article, however, was its demonstration that in time-of-flight secondary ion mass spectrometry experiments performed in the lab they could straightforwardly perform such identifications. Such experiments, however, do require tangible samples. So, while astronomers will not be able to identify porphyrins with their telescopes, space vehicles landing on various solar system bodies that are equipped with these sophisticated mass spectrometers will be able to determine whether or not porphyrins are present in soil and rock samples.

Though not discussed in the research paper, porphyrins would provide an excellent tool for testing competing models on the origin and evolution of life. Nontheistic models, based on the belief that the abundance, diversity, and duration of life on Earth can be explained by purely natural means, would predict that life, at least in unicellular form, also must exist or have existed in those environments on various solar system bodies where liquid water either exists or has existed. Remains of that life would leave behind measurable porphyrin signatures in proportion to the amount and duration of liquid water in the environment.

On the other hand, a biblical model would predict that porphyrins beyond Earth would be found only on those bodies where efficient transport by meteors of Earth soils containing the remains of Earth life has occurred. Also, the quantities of porphyrins found would be proportional to the efficiency of transport. That is, transport efficiency from Earth, not the presence of liquid water, would be the determining factor.To be specific, a biblical model would predict that porphyrins would be found in the greatest abundance in the regolith of the Moon, to a much lesser extent on Mars, and would be virtually non-existent elsewhere beyond Earth. A diligent search for porphyrins on solar system bodies beyond Earth will provide evidence in addition to that which is already available for determining which model for life is true and which models are false.