Chemical biosignatures: all that glitters is not gold
Abstract
Chemistry offers sophisticated analytical tools that allow the detection and identification of, for example, organic molecules even if present in only minute quantities. This makes chemical biosignatures attractive as means for life detection on other planets and moons. However, the various kinds of chemical biosignatures are not equally useful in the search for extraterrestrial life, and there are many potential pitfalls in interpreting them (Fox and Strasdeit, 2017).The most important type of misinterpretation is probably false positive results. It occurs when terrestrial biogenic or abiogenic contaminants are mistaken as extraterrestrial biosignatures or when products of natural abiotic syntheses are not recognized as such. Terrestrial contamination is a permanent problem in many astrobiology space missions, especially in those to the surfaces of solar system bodies. False negative results are also possible. They may occur, for example, when a chemical compound simultaneously has productive abiotic and less productive biological sources. In such a situation the biotic source may be overlooked. Methane on other planets and moons is a candidate for this kind of chemical compound."Strong" chemical biosignatures have a low risk of misinterpretation. Homochiral polymers of defined length and sequence, such as proteins and polynucleotides, undoubtedly belong to this group. Abiotic organic syntheses almost always produce a continuous spectrum of molecules, as, for example, in the case of α_-alkyl-α_-amino acids. However, when only a subset of these molecules is found, this strongly indicates biogenicity (McKay, 2004). Stability over long periods of time of a chemical biosignature (or at least of easily recognizable alteration products thereof) also increases its "strength." One reason that some molecules are regarded as good biosignatures is because no natural abiotic syntheses of them are known. However, when an abiotic pathway is discovered_as was the case with porphyrins (Fox and Strasdeit, 2013)_the picture changes. "Weak" chemical biosignatures are by no means useless, but they need a stronger integration into the context (for example, the geological one) to avoid misinterpretation.References:Fox, S. and Strasdeit, H. (2017) Inhabited or uninhabited? Pitfalls in the interpretation of possible chemical signatures of extraterrestrial life. Front Microbiol. 8: 1622; doi: 10.3389/fmicb.2017.01622.Fox, S. and Strasdeit, H. (2013) A possible prebiotic origin on volcanic islands of oligopyrrole-type photopigments and electron transfer cofactors. Astrobiology 13: 578_595; doi: 10.1089/ast.2012.0934.McKay, C. P. (2004) What is life_and how do we search for it in other worlds? PLoS Biol. 2: e302; doi: 10.1371/journal.pbio.0020302.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E3269S