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.
Highlights
The search for extraterrestrial life is fundamentally referenced to Earth as the only known and accessible benchmark for comparison, from the microscopic level up to the scale of our planet and its atmosphere, where life has perturbed planetary environments over long timescales ( Judson, 2017). ‘‘Life’’ is a complex phenomenon, and here we refer to it as we know it today—a self-organized, self-replicating, and metabolically active molecular system that is carbon based (Pace, 2001)
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
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, 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
Summary
All of Earth’s surface and subsurface waters have likely been in contact with microbes or their by-products since at least 3.5 Ga, when the first widely accepted traces of life appear in the geological record (cf., Schopf et al, 2018 and references therein). Over the eons during which water-rich Earth has been teeming with life, it is difficult to determine how an ‘‘uninhabited habitable planet’’ would appear when sampled directly or observed remotely. It has often been generally assumed that substances or objects in Earth’s near-surface environment might be abiotic unless there is definitive evidence of biological activity. The pervasiveness of life in Earth’s near-surface and subsurface environments indicates that, perhaps virtually everything might be biologically influenced unless an abiotic origin can be definitively established
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