Inhabited or Uninhabited? Pitfalls in the Interpretation of Possible Chemical Signatures of Extraterrestrial Life

Abstract

The “Rare Earth” hypothesis—put forward by Ward and Brownlee in their 2000 book of the same title—states that prokaryote-type organisms may be common in the universe but animals and higher plants are exceedingly rare. If this idea is correct, the search for extraterrestrial life is essentially the search for microorganisms. Various indicators may be used to detect extant or extinct microbial life beyond Earth. Among them are chemical biosignatures, such as biomolecules and stable isotope ratios. The present minireview focuses on the major problems associated with the identification of chemical biosignatures. Two main types of misinterpretation are distinguished, namely false positive and false negative results. The former can be caused by terrestrial biogenic contaminants or by abiotic products. Terrestrial contamination is a common problem in space missions that search for biosignatures on other planets and moons. Abiotic organics can lead to false positive results if erroneously interpreted as biomolecules, but also to false negatives, for example when an abiotic source obscures a less productive biological one. In principle, all types of putative chemical biosignatures are prone to misinterpretation. Some, however, are more reliable (“stronger”) than others. These include: (i) homochiral polymers of defined length and sequence, comparable to proteins and polynucleotides; (ii) enantiopure compounds; (iii) the existence of only a subset of molecules when abiotic syntheses would produce a continuous range of molecules; the proteinogenic amino acids constitute such a subset. These considerations are particularly important for life detection missions to solar system bodies such as Mars, Europa, and Enceladus.

Deciphering Biosignatures in Planetary Contexts

“…Searches for extraterrestrial life often focus on biomarkers but determining whether a specific compound is of extraterrestrial biological origin and not a terrestrial contaminant is the crux of this problem. Abiotic chemistry must be understood sufficiently in detail such that when a signal is observed, there is a reasonable certainty as to whether it is biotically or abiotically produced versus being a terrestrial contaminant (Fox and Strasdeit, 2017). Extensive work in abiotic chemistry shows that a variety of biochemicals can be generated by abiological processes, for example, through atmospheric (Miller, 1953), hydrothermal (Hennet et al , 1992; Amend and Shock, 1998), or interstellar chemistry (Bernstein et al , 2005).…”

Molecules to Microbes

“…These false positive bio-signals may be of abiotic nature-for example, although porphyrins, in particular metalloporphyrins, are often thought to be of biological origin, the very same molecules could also be made abiotically [31,47,48], thus giving the impression of the presence of tentative cellular life forms. Since there are many such molecules (e.g., methane; see Kotsyurbenko below) that have an abiotic origin, these cannot be excluded, meaning that the risk of misinterpretations is ever-present [49]. Therefore, sophisticated combinations of analytical methods are necessary.…”

Molecules to Microbes

“…These false positive bio-signals may be of abiotic nature-for example, although porphyrins, in particular metalloporphyrins, are often thought to be of biological origin, the very same molecules could also be made abiotically [31,47,48], thus giving the impression of the presence of tentative cellular life forms. Since there are many such molecules (e.g., methane and see Kotsyurbenko below) that have an abiotic origin, these cannot be excluded, meaning that the risk of misinterpretations is continually ever-present [49]. Therefore, sophisticated combinations of analytical methods are necessary.…”