Deciphering Biosignatures in Planetary Contexts

Abstract

Microbial life permeates Earth's critical zone and has likely inhabited nearly all our planet's surface and near subsurface since before the beginning of the sedimentary rock record. Given the vast time that Earth has been teeming with life, do astrobiologists truly understand what geological features untouched by biological processes would look like? In the search for extraterrestrial life in the Universe, it is critical to determine what constitutes a biosignature across multiple scales, and how this compares with “abiosignatures” formed by nonliving processes. Developing standards for abiotic and biotic characteristics would provide quantitative metrics for comparison across different data types and observational time frames. The evidence for life detection falls into three categories of biosignatures: (1) substances, such as elemental abundances, isotopes, molecules, allotropes, enantiomers, minerals, and their associated properties; (2) objects that are physical features such as mats, fossils including trace-fossils and microbialites (stromatolites), and concretions; and (3) patterns, such as physical three-dimensional or conceptual n -dimensional relationships of physical or chemical phenomena, including patterns of intermolecular abundances of organic homologues, and patterns of stable isotopic abundances between and within compounds. Five key challenges that warrant future exploration by the astrobiology community include the following: (1) examining phenomena at the “right” spatial scales because biosignatures may elude us if not examined with the appropriate instrumentation or modeling approach at that specific scale; (2) identifying the precise context across multiple spatial and temporal scales to understand how tangible biosignatures may or may not be preserved; (3) increasing capability to mine big data sets to reveal relationships, for example, how Earth's mineral diversity may have evolved in conjunction with life; (4) leveraging cyberinfrastructure for data management of biosignature types, characteristics, and classifications; and (5) using three-dimensional to n -D representations of biotic and abiotic models overlain on multiple overlapping spatial and temporal relationships to provide new insights.

Biogenic Metal Nanoparticles: A New Approach to Detect Life on Mars?

“…In addition to this issue, we bias our analysis by grounding it on the single example provided by our own planet and its life-forms. Despite such limitations, it is based on such Earth-centric studies that we now know that traces of life can be detected on the crust of our planet and that life is present in many extreme locations previously considered sterile [48]. The study of such extreme environments, particularly those defined as terrestrial analogs, provide us with vital information for astrobiological exploration of other planets and moons [48][49][50].…”

“…Any detectable sign, matter, gas, or pattern that strongly suggests a biological origin and can thus be used as evidence indicating the existence, past or present, of living organisms is a biosignature [48,51]. By definition, they are objective, quantifiable characteristics of biological processes.…”

“…They can be atmospheric biosignatures if referring to the presence of one or several gas species, surface biosignatures if detectable by their light reflection characteristics which allows them to be detected by remote sensing, or bioindicators if they suggest a biological origin but can also be produced abiotically [51]. Biosignatures can be divided into three different categories (Table 4): substances, objects, and patterns [48]. Table 4.…”

“…The AuNPs and AgNPs formation are here proposed as a method to screen for possible traces of life. A positive result, on its own, can be considered a potential biosignature, which is also referred to as an ambiguous biosignature by Chan et al [48]. This type of biosignature leads to more research and data gathering in order to conclude whether there is presence or absence of life.…”

“…This type of biosignature leads to more research and data gathering in order to conclude whether there is presence or absence of life. Ambiguous signatures, in which the formation of AuNPs and AgNPs fit, are one of the main focuses of astrobiological studies [48]. Nevertheless, further investigation should still be done, by applying adequate characterization methods, confirming that any molecule/enzyme/protein responsible for the metal ions reduction has a biological source, and look for possible primary biomolecules or diagenetically altered biomolecules adhered to the NPs that might aid on the identification of any biogenic processes and exclude any false-positive results.…”