Chapter 8: Searching for Life Beyond Earth
Abstract
The search for life beyond Earth necessitates a rigorous and comprehensive examination of biosignatures, the types of observable imprints that life produces. These imprints and our ability to detect them with advanced instrumentation hold the key to our understanding of the presence and abundance of life in the universe. Biosignatures are the chemical or physical features associated with past or present life and may include the distribution of elements and molecules, alone or in combination, as well as changes in structural components or physical processes that would be distinct from an abiotic background. The scientific and technical strategies used to search for life on other planets include those that can be conducted in situ to planetary bodies and those that could be observed remotely. This chapter discusses numerous strategies that can be employed to look for biosignatures directly on other planetary bodies using robotic exploration including those that have been deployed to other planetary bodies, are currently being developed for flight, or will become a critical technology on future missions. Search strategies for remote observations using current and planned ground-based and space-based telescopes are also described. Evidence from spectral absorption, emission, or transmission features can be used to search for remote biosignatures and technosignatures. Improving our understanding of biosignatures, their production, transformation, and preservation on Earth can enhance our search efforts to detect life on other planets.
One of the greatest challenges in the search for life on other planets is identifying features that are known to be uniquely associated with life. Features attributed to past and present life on Earth are often used as a basis for indicators of possible extinct or extant extraterrestrial life. These features, which range from chemical, isotopic, mineralogical, textural, or even technological signatures, are called biosignatures.
Biosignatures are molecules, elements, or features of biological origin that can be used as evidence for past or present life and can be distinguished from an abiogenic background (i.e., a background not formed by living organisms; Des Marais and Walter, 1999). They are the resulting expressions in the environment made by life's processes, such as metabolism, growth, reproduction, and evolution (Neveu et al., 2018). Biosignatures associated with Earth-based life (here referred to as terran biosignatures) include both features that are specific to life on Earth and those that are thought to be potentially common to all life-forms in the universe. While these expressions can offer clues to the presence or provenance of life in a particular context, they also vary in their quantity (abundance), survivability (how long they last), reliability (measurable differences from expected abiotic signals), ubiquity (whether they are found in all known life-forms), and diversity (how many types of biosignatures are expressed). Because of these varying factors, it is critical to consider multiple lines of evidence to confirm a phenomenon as life.
Since Earth is the only planet known to harbor life, our knowledge of detectable biosignatures comes from this single, limited example. Furthermore, the burden of proof necessary to verify that a given feature is a biosignature is determined not only by the likelihood that it was produced by a biological process but also by the unlikelihood that it was produced by nonbiological (or abiotic) processes (Des Marais et al., 2008; National Academies of Sciences, Engineering, and Medicine, 2019). Any interpretations of signs of life must only be accepted if there are no satisfactory explanations that involve solely abiotic processes. As said in Sagan et al. (1993): “life is the hypothesis of last resort” (see Chapter 8.5).
This chapter discusses the types of biosignatures that stem from our knowledge of terran life. We explore the search for biosignatures present on the surfaces or in atmospheres of Solar System bodies that could be in situ (Chapter 8.1) and also those that can be detected remotely (Chapter 8.3). In addition, a review of instrumental techniques and detection strategies that have been developed to detect the broad range of biosignatures is also presented (Chapter 8.2 and Chapter 8.4). We also discuss the definitiveness of a potential life-detection event or measurements, or lack thereof, and how to increase confidence in any resulting interpretation (Chapter 8.5). New perspectives on expanding the search for life to look for universal biosignatures without presupposing the existence of certain biochemical systems (i.e., without presupposing terran biochemistry), also known as “agnostic biosignatures,” are reviewed in more detail in Chapter 9.4.
8.1. In situ Biosignatures
Biosignatures that can be detected in situ can be chemical or physical. Proposed chemical biosignatures for both extant and extinct life include specific organic molecules, isotopic fractionation, homochirality, biominerals, biogases, and disequilibrium in chemical systems. Physical biosignatures of extant life include evidence of motility, cellular structures, and evidence of growth or replication, whereas physical biosignatures of extinct life include stromatolites or fossils. In both cases, biosignatures need to fulfill several criteria; namely, they must be stable and abundant enough to be detected, and they must be uniquely associated with life (Neveu et al., 2018). Below we describe chemical and physical biosignatures in this context.
8.1.1. Chemical biosignatures
8.1.1.1. Molecular biosignatures
Molecular biosignatures (or biomarkers) are chemicals that are either part of a biosynthetic pathway or molecular fossils for which the origin can be traced to known biological compounds (Summons et al., 2008). All the essential building blocks of life on Earth (i.e., nucleic acids, amino acids, carbohydrates, lipids, and metabolites; see Chapter 2.2.3) can constitute molecular biosignatures. However, many of these compounds can also be produced abiotically (see Chapter 3.3.1 and Chapter 4.2.3). Furthermore, the rapid degradation of certain classes of biomolecules (i.e., DNA, RNA, and proteins) by abiotic and biotic processes limits direct detection capabilities to only extant (or recently extinct) life (Mitterer, 1993; Simoneit, 2004). A widely studied group of molecular biosignatures are membrane lipids that have diagnostic structures that are stable over billion-year timescales (Eglinton et al., 1994; Brocks et al., 1999; Summons et al., 1999). These membrane lipids are particularly important biomolecules that are found in all terran life-forms (see Chapter 2.2.3.3), contribute to numerous cellular functions (see Chapter 4.2.4.4), and allow some life-forms to survive extreme conditions (see Chapter 6.3.1.2).
Molecular biosignatures are often used to reconstruct Earth's history and understand the chemistry and processes for the transition of biosignatures from extant life (i.e., biomolecules) to extinct life (i.e., geomolecules). The fossilization process of biomolecules to form geomolecules is part of a process called diagenesis (Killops and Killops, 2013), during which biomolecules lose chemical functionality and leave a molecular skeleton behind (i.e., molecular fossilization). The chemical changes caused by diagenesis vary based on environmental conditions (e.g., aqueous chemistry during deposition, temperature history, redox state of the sedimentary environment) that become imprinted in the structure, or in the distribution, of molecular fossils, and reveal the provenance and history of the molecular biosignature of interest (Brocks and Pearson, 2005). Some examples of molecular biosignatures and how they can be arranged into a “Biomarker Tree of Life” are shown in Fig. 8.1 (see Chapter 2.3 for discussion of the Tree of Life).
8.1.1.2. Isotopic biosignatures
Isotopes are elements that differ only in the numbers of neutrons in the atomic nucleus. They interact at different rates with enzymes and the environment (Urey, 1947; Sharp, 2017), and the partitioning of different isotopes within a system is referred to as isotopic fractionation. Enzymes in biological systems tend to prefer lighter isotopes of carbon, hydrogen, oxygen, nitrogen, and sulfur in metabolic processes, resulting in a preferential enrichment of the heavier isotopes in the surrounding environment (Freeman et al., 1990; Hayes, 2018). As certain stable isotope pairs do not undergo decay over geologic timescales, their ratios can be used to study biological processes such as carbon fixation in ancient microfossils (House et al., 2000) or to determine the sources of organic material in particular ecosystems (Hage et al., 2007). The most commonly considered stable isotope systems for astrobiology are 13C:12C (Summons et al., 2008) and 15N:14N (Stüeken et al., 2016), although, for example, 18O:16O, 34S:32S, and 56Fe:54Fe have also contributed to the understanding of ancient life on Earth (Canfield and Raiswell, 1999; Pavlov and Kasting, 2002).
While the isotope ratio in a given environment may be indicative of life, it must first be established that the fractionation is significantly different than that produced by abiotic processes (Rothschild and Des Marais, 1989; Cavalazzi and Westall, 2019). Abiotic isotopic fractionation can occur through gravitationally driven diffusion, where the heavier isotopes are preferentially enriched toward the center of mass, or through various chemical and physical processes (Lammer and Bauer, 2003). Additionally, isotopic analyses of specific compounds can elucidate terrestrial contamination sources when investigating extraterrestrial samples (Elsila et al., 2011). Thus, simply detecting isotopic fractionations may not itself be conclusive evidence of a biological system. Understanding the environmental and geochemical context of an isotopic biosignature and the isotopic signature of the original reservoir of material is critical.
8.1.1.3. Structural and compositional biases as biosignatures
As described in Chapter 2.2.3.5, terran biomolecules are effectively homochiral, such that L-chiral amino acids and D-chiral sugars dominate. However, abiotic systems tend to form enantiomers of chiral molecules (Fig. 8.2a) in equal abundances. A large enantiomeric excess is thus considered to be a biosignature of terran life and may serve to identify other forms of life as well.
Other structural biases include stereoisomer preference (stereoisomers are molecules that contain the same atoms that are linked in the same order, but their spatial configuration is different). Many metabolites are biosynthesized as a single stereoisomer (Peters et al., 2004), although hundreds of possibilities may exist. For example, the cholesterol molecule has 8 stereocenters and 256 possible configurations, though only one is naturally produced by terran life (Fig. 8.2b). While specific stereoisomers can be exceptionally diagnostic biosignatures, they are also inherent to enzymes that specifically evolved in terran life-forms and may not necessarily be applicable to the search for life elsewhere.
The distribution of lipid molecules can also serve as biosignatures. On Earth, most lipid classes are produced from two different biochemical pathways: the 2-carbon building block (acetogenic) pathway and the 5-carbon building block (isoprenoid) pathway. Acetogenic lipids (e.g., fatty acids, wax esters, long-chain hydrocarbons, etc.) contain an even number of carbons and are among the most common biogenic lipids (Summons et al., 2006). Since abiotically synthesized lipids have randomly distributed numbers of carbons, the even-over-odd preference of acetogenic lipids could be used as an indication of biogenicity (Fig. 8.2c; McCollom et al., 1999). Similarly, isoprenoid lipids can be cyclic compounds that exhibit complex structures and patterns that are very unlikely to occur abiotically; thus these can be highly diagnostic of terran life (Fig. 8.2b).
8.1.1.4. Minerals as biosignatures
Expressions of life may also be recorded or preserved in nonbiological materials. Biominerals form as a result of the activity of living organisms either directly through active precipitation (e.g., bones or shells) or indirectly through changes to the surrounding environmental chemistry that leads to mineral precipitation (passive precipitation). The distribution of minerals on Earth is markedly distinct from the distribution of minerals on other presumably lifeless bodies such as the Moon or Mercury; thus researchers think the presence and distribution of certain minerals (e.g., calcium carbonate, calcium phosphate, manganese oxides, and iron oxides) may be a robust biosignature (Banfield et al., 2001; Hazen and Ausubel, 2016; Hays et al., 2017). Organic minerals and oxidized minerals can contain especially important biosignatures (Johnson et al., 2020). However, it is difficult to distinguish minerals that were formed in the presence of biology from those that were produced purely abiotically (Golden et al., 2001).
When searching for mineral biosignatures on other planets, comparisons with previously described terran biominerals are of limited use because some minerals are different in their distributions solely due to differing geochemical and geophysical conditions of their formation. Using minerals as a biosignature requires rigorous contextual evidence to prove biogenicity, as well as rejection of the possibility that such a process could take place abiotically.
8.1.1.5. Gases as biosignatures
A biosignature gas is a volatile chemical species produced as a result of biological processes, typically as waste products during metabolism, that could be detected in a planetary atmosphere. Simultaneous spectroscopic detections (see Chapter 8.3.1) of species like oxygen (O2), nitrous oxide (N2O), and methane (CH4) could indicate terran-like processes, or even less abundant biogenic gases like methanethiol (CH3SH; Pilcher, 2004) or biogenic phosphine (Chapter 8.3.1.4) have also been proposed as a biosignature gas for other worlds. However, the detection of biosignature gases still requires supporting contextual evidence that such gases cannot be produced by the abiotic geochemical environment being explored (Cockell et al., 2021). The potential biogenicity of methane (CH4) detections on Mars has been a significant focus of exploration in the past decade. Further details on candidate biosignature gases, combination of gases, and the processes that produce them are reviewed in detail by Schwieterman et al. (2018).
8.1.1.6. Chemical and thermodynamic disequilibrium as a biosignature
Biological catalysis typically occurs with input of energy, such as in redox reactions (see Chapter 6.2.1). The maintenance of redox gradients for metabolism requires not only physical and chemical separation between reductants and oxidants but also that these compounds be in relatively close proximity in space and time to be harnessed by life. Some microbes are able to perform extracellular electron transfer (EET) to exploit redox potentials of solid-phase materials outside the confines of their cells to produce an electrochemical disequilibrium (see Chapter 6.2.1.2). It is possible that life beyond Earth could possess similar energetic mechanisms. Electrochemically detecting the transfer of electrons between redox-active molecules offers a powerful tool for life-detection efforts, assuming such redox reactions are inconsistent with abiotic geochemical processes (i.e., those that can happen spontaneously in the absence of life).
8.1.2. Physical biosignatures
Physical expressions of life on Earth can range from cellular fossils and microbially induced fabrics to larger sedimentary structures, and these offer detectable evidence of life on micro- and macroscales. As organisms grow, divide, and proliferate, they exhibit morphological patterns that can be recognized as life, or they can influence their surrounding environments by means of chemical or physical changes. These latter expressions are often imprinted in rocks and minerals and could potentially be observed with appropriate instruments carried by surface-based landers and rovers to other planetary bodies. A variety of physical biosignatures are discussed in the following sections.
8.1.2.1. Microscopic physical biosignatures
Microscopic physical characteristics can be used to identify dynamic activities or static imprints of life and thus could be used to detect active (living) and/or inactive (dead) life. Motility is frequently used as a sign of life in standard microbiology practices and was one of the first biosignatures to be suggested for use in Mars missions (National Research Council, 1966; Nadeau et al., 2016). Motility can be defined as the deliberate and directed motion of a microbe in terms of velocity and directionality that can be distinguished from the random movement of a particle, or Brownian motion (Lindensmith et al., 2016). Evidence of growth and replication could also be gleaned from observing cells, or cell-like structures, at different stages of their growth and replication (Neveu et al., 2018).
Microfossils that are formed when microorganisms die and become preserved in the rock record through biomineralization processes are another example of microscopic evidence for past life (Brasier and Armstrong, 1980; Onstott et al., 2019). They can be (i) three-dimensional cellular structures, including organelles, preserved in a mineral matrix, (ii) two-dimensional flattened structures preserved in fine-grained sediments (Cady et al., 2004; Qu et al., 2015), or (iii) trace fossils such as preserved tracks, paths, or marks an organism made when it was alive. Trace fossils can include microscopic burrowing features and tunnels formed by the movement of microorganisms (McLoughlin et al., 2009; Johnson et al., 2020). Biomolecules or diagenetically altered biomolecules can also serve as corroborating evidence that the observed structure stemmed from life (see Chapter 8.1.1.1). The oldest evidence for putative microfossils on Earth dates back to 3.95 Ga (see Chapter 4.1.5). While the range of shapes, dimensions, and internal structures of various microorganisms can be used as potential biosignatures, some abiogenic structures can also be misconstrued as organelles or biologically mediated cellular structures (e.g., morphologic structures debated during the study of the Allan Hills 84001 meteorite; see Chapter 8.2.5.2 in the supplementary material).
8.1.2.2. Macroscopic physical biosignatures
Macrofossils are large structures (e.g., bones, shells, or textures) most commonly preserved in clays or shales. Their potential to preserve macrofossils over geologic timescales makes clays and shales compelling targets to search for evidence of past macroscopic life (Butterfield, 1990; McMahon et al., 2016). Morphological evidence of microorganisms at the macroscale includes biofabrics, biofilms, microbial mats, and microbially induced sedimentary structures (MISS). Fossilized excrement of an organism, referred to as coprolite, may also be found.
Biofabrics are loosely defined as features or textures formed by the presence of microorganisms, such as bundles of microbial filaments or layered structures of microbial organisms (Pullan et al., 2008). Cohesive layers of microbes, known as “biofilms,” can attach to the surfaces of rocks within rivers and streams and can be preserved through fossilization (Fig. 8.3; Donlan, 2002; Noffke and Awramik, 2013; Johnson et al., 2020). These microbial communities can vary in composition, structure, thickness, physiology, and morphology. The most commonly known microbialites, stromatolites, are organo-sedimentary laminated structures as large as a meter thick formed by layered growth of microorganisms and the trapping and binding of sediment particles (Fig. 8.3c and 8.3d; Flügel, 2004; Cady and Noffke, 2009; Noffke and Awramik, 2013). The oldest macroscopic evidence for life on Earth is represented by stromatolites in Australia dating back to 3.25–3.5 Ga (see Chapter 4.1.5). MISS contain a variety of biologically mediated structures and have been observed in numerous aquatic environments. They range in lateral dimensions from millimeters to kilometers (Fig. 8.3a; Noffke, 2010). However, they differ from microbialites because they form with little, if any, mineral precipitation and instead only grow laterally (Noffke and Awramik, 2013; Hays et al., 2017).
When identifying the difference between a geologic structure and MISS or microbialites, researchers must take care to accurately interpret the two- and three-dimensional structure of the formation of interest. Interpretation can be aided by obtaining contextual information using complementary techniques (e.g., fluorescence microscopy, spectroscopy, etc.) on the morphology and geochemistry of the laminated structures prior to assigning them as biologically mediated (Allwood et al., 2018).
8.2. In situ Biosignature Detection Methods
On Earth, a wide variety of techniques can be used to determine the presence or absence of life within a sample. However, for in situ planetary missions aimed at detecting specific biosignatures, limitations from the spacecraft and instruments on board must be taken into consideration to ensure successful scientific measurements. Mission design (i.e., the conception, planning, construction, and implementation of space missions) is driven by the scientific objectives and the target body of interest. All designs must be concordant with planetary protection requirements (see Chapter 10) and the technical constraints imposed by extreme conditions during the cruise phase to and in the environment of another planetary body.
As for all space missions, biosignature detection instrument designs must carefully consider numerous factors including (though not limited to) device size, power consumption, target environment, mission architecture, material choice, and onboard capabilities for data collection, storage, and transmission. The technological maturity of instruments or their components is often assessed by the Technology Readiness Level (TRL) scale that ranges from 1 to 91. For example, TRL 1 indicates an initial “back of the napkin” conception (initial drawings or blueprint of a potential instrument design), 4 indicates successful demonstration of the instrument in a laboratory setting, 7 indicates successful demonstration of the instrument in a relevant space environment, and 9 ultimately indicates that the device is flight-proven through successful mission operations (Mankins, 1995; Hirshorn and Jefferies, 2016). TRL assessment also allows for the distinction between low heritage (components that exist and are considered for flight but have not yet matured to flight readiness) versus high heritage technology (components that have been tested in relevant space environments and/or have flown on previous missions). Flight readiness and mission heritage help scientists and engineers assess the need for instrument maturation (or critical component maturation) prior to its selection for a mission.
In situ instruments, typically designed for lander or rover-based spacecraft, can collect “ground truth” information (i.e., empirical evidence), which may help inform remote observations of a planetary surface. In situ instruments come in a variety of types that search for biosignatures by accurately detecting trace amounts of target molecules or observing morphological features at the scale and resolution of interest. In situ biosignature detection can also benefit from sample return (i.e., bringing samples back to Earth to be investigated), which allows for planetary materials to be analyzed with more sophisticated instruments than those that have been miniaturized and deployed on spacecrafts. In this section, we focus on techniques and instruments that have been proposed, or used, for in situ exploration beyond Earth.
8.2.1. Separation techniques
Environmental samples contain a mixture of compounds. As such, they are often complex and difficult to analyze, and therefore separation techniques are used to isolate compounds for individual analysis. Separation techniques include chromatography, where a sample (gas or liquid) is passed through a stationary phase that affects the travel time of each compound specifically, resulting in physical separation into individual components. These separated compounds can then be introduced into various detection platforms for chemical, structural, isotopic, or spectroscopic analyses (see Chapter 8.2.2).
8.2.1.1. Gas chromatography
Gas chromatography (GC) is used to separate compounds by first thermally volatilizing them (heating into the gas phase) and then moving them through a capillary tube using an inert gas. While in the capillary tube, they interact with a stationary chemical phase and are separated based on their size and polarity. GC is a widely used analytical chemistry technique with high flight heritage for planetary exploration and a low detection limit (in parts per million to parts per billion). Several missions, including the Viking Mars landers (Novotny et al., 1975), the Mars Science Laboratory on board the Curiosity rover (Mahaffy et al., 2012), and the Cassini-Huygens descent probe (Niemann et al., 2002), have successfully flown miniaturized gas chromatographs. However, the requirement to volatilize samples for separation poses practical challenges, as many compounds of astrobiological interest (e.g., amino acids or large macromolecules) are inherently difficult to volatilize by conventional GC. Wet chemistry pretreatment steps can be implemented, such as derivatizing (or chemically converting a compound to a similar compound called a derivative) the samples with chemical agents that aid in volatilization and stability, and thus detectability, of the compounds (Rodier et al., 2001; Buch et al., 2006).
8.2.1.2. Liquid chromatography
Liquid chromatography (LC) uses a liquid mobile phase for separation, in which compounds are dissolved in a solvent or mixture of solvents and passed through a stationary column with specific affinity to the compounds being separated (analytes). LC techniques can be useful for detecting amino acids and nucleobases because of their high solubility in liquid solvents. Modern high-performance liquid chromatography (HPLC) is capable of efficient high-pressure separation by forcing analytes at high pressure through a porous column and can be used to distinguish molecular enantiomers (Ilisz et al., 2008). Despite the difficulties in keeping liquid phases from freezing in planetary exploration conditions, efforts have been made to miniaturize LC-based instruments for space exploration (Getty et al., 2013).
8.2.1.3. Capillary electrophoresis
Capillary electrophoresis (CE) is an electrokinetic method where compound separation occurs in a solution based on differences in mobility induced by an applied voltage (electrophoretic mobility). Electrophoretic mobility is a function of the molecular structure, charge, and viscosity of the background solution used. Parts per trillion limits of detection can be achieved with CE techniques. Unlike GC and a few LC cases, CE techniques currently have lower flight heritage (Creamer et al., 2017).
8.2.2. Detection instruments
Direct observations of motility, morphology, growth, and replication would allow for the identification, visualization, and characterization of potential life-forms and could help select targets for additional life-detection experiments. This section reviews several detection instruments that have been flown or are in development for planetary exploration missions.
8.2.2.1. Mass spectrometry
Mass spectrometry (MS) is a technique used to study molecules by analysis of their mass. Compounds are first volatilized, and in some cases fragmented, and then ionized and accelerated into the instrument using electrostatic lenses. The ions are then sorted by their mass-to-charge ratios (denoted as m/z) using a mass analyzer (e.g., quadrupole, time-of-flight, magnetic sector, etc.), and their abundance then measured using an ion counting detector. Ionization techniques (e.g., electron impact) can cause molecules to fragment, and fragmentation patterns can be diagnostic of specific molecules. Furthermore, mass spectrometers have high sensitivity and low detection limits and, in some cases, even enable the detection of isotopic compositions of atoms and molecules in a sample.
As illustrated in Fig. 8.4, numerous orbiters, landers, and rovers have carried mass spectrometers to assess habitability characteristics of various planetary environments (e.g., the Curiosity rover on Mars, the Cassini-Huygens probe on Titan; Niemann et al., 2002; Mahaffy et al., 2012), to understand the distribution of organic molecules on extraterrestrial bodies (e.g., Rosetta-Philae on comet 67P/Churyumov-Gerasimenko; Quirico et al., 2016), and to search for signs of life (i.e., the Viking Mars landers; Biemann et al., 1977). Mass spectrometers are often coupled to separation techniques (e.g., GC; see Chapter 8.2.1) to first resolve complex mixtures of compounds before introducing them into the instrument for compound detection and identification. Due to their high heritage in space exploration, mass spectrometers will play a key role in future in situ missions and mission concepts, such as the Rosalind Franklin rover on the ExoMars mission and the Dragonfly rotorcraft (see review by Chou et al., 2021).
8.2.2.2. Microscopy
Microscopy techniques, including optical microscopy, electron microscopy, and atomic force microscopy, provide direct imaging of samples and hold potential for use in astrobiological in situ exploration. Optical techniques enable the direct detection of cells (morphology or motility), circumventing the need to culture (i.e., grow) and isolate cells from a sample. Scanning electron microscopy (SEM) can be used to image surface topography, crystal structures and orientation, elemental composition, and other potential biosignatures, and it is currently being miniaturized for flight (Gaskin et al., 2010). Atomic force microscopy (AFM) has also been proposed as a tool to look for biosignatures by scanning a functionalized probe tip over biomolecules, biofilms, microbes, fossils, and mineral surfaces and measuring the change in forces on a piconewton (pN) scale (Anderson, 2019).
Fluorescent imaging techniques can be used to observe naturally fluorescing compounds that are electronically excited when they absorb UV light and subsequently emit visible light when they relax (e.g., polycyclic aromatic hydrocarbons [PAHs] or chlorophyll). Compounds that do not naturally fluoresce can be labeled using fluorescent dyes (fluorescent molecules that attach to specific compounds) prior to microscopy (Nadeau et al., 2008; Storrie-Lombardi and Sattler, 2009), although this requires prior knowledge of the compounds of interest. Techniques such as fluorescence in situ hybridization (FISH), for example, use fluorescent dyes as probes for nucleic acid sequences specific to a particular microorganism (Huber et al., 2018).
Microscopy techniques are a promising tool for life detection in environments where obtaining liquid samples is feasible (e.g., melting an icy sample). However, sample preparation (e.g., preparing thin slices in situ for transmission optical microscopy of solids), illumination and stabilization of the microscope and sample, as well as environmental sensitivity and complex environmental backgrounds, present significant technical challenges when using microscopy as an in situ detection technique.
8.2.2.3. Spectroscopy
Spectroscopy is the study of how electromagnetic radiation (e.g., light) interacts with matter. Energy associated with specific wavelengths can excite molecular resonances that correspond to particular vibrational or rotational motions of the molecule (e.g., bond stretching). Because the energy states associated with these molecular motions are discrete, absorption or emission of light at a particular wavelength can be used to identify the composition of a material. Raman, visible near-infrared (VISNIR), and laser-induced breakdown (LIBS) spectroscopy are the most commonly used types of spectroscopy for in situ observations and applications to astrobiology. Spectroscopy is also used in the remote detection of life on exoplanets (see Chapter 8.4.3).
Raman spectroscopy uses a laser to excite molecular vibrations in a material and gathers information about the types of intramolecular chemical bonds based on the inelastically scattered light spectrum. Raman spectroscopy has been used to verify the presence of biominerals and preserved microbial filaments at several (terrestrial) martian analog sites (Edwards et al., 2007). It is a payload of two rover missions to Mars (i.e., the Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals [SHERLOC] instrument on NASA's Perseverance rover and the Raman Laser Spectrometer [RLS] on ESA's Rosalind Franklin rover, to be launched in 2028).
Visible and near-infrared (VISNIR) spectroscopy can be used to detect compounds that absorb in the visible wavelength range (400–700 nm), such as biological pigments (see Chapter 8.3.2) or minerals that can preserve biosignatures, such as clays (see Chapter 8.1.2.2). Currently, this type of instrument is used on a variety of space-based instruments (e.g., Mastcam on NASA's Curiosity rover and the MicrOmega-IR and Infrared Spectrometer for ExoMars [ISEM] instruments on ESA's upcoming Rosalind Franklin rover).
Laser-induced breakdown spectroscopy (LIBS) uses a laser to turn a material into a plasma that, upon cooling (relaxation), emits different wavelengths of light depending on chemical elements present. Recently, LIBS has been shown to be capable of identifying a wide range of terran-based microbes based on their characteristic atomic composition (Singh et al., 2018). LIBS is currently being used as part of the ChemCam instrument of NASA's Mars Science Laboratory on board the Curiosity rover, where it focuses on identifying potentially habitable martian environments within Gale impact crater (Wiens et al., 2015). The Perseverance rover, part of NASA's Mars 2020 mission, is utilizing an instrument called SuperCam, which is able to perform LIBS (1064 nm laser at distances of 7 m), Raman spectroscopy (532 nm laser at distances of 12 m), and VISNIR reflectance spectroscopy (400–900 nm, 1.3–2.6 μm).
8.2.2.4. Electrochemistry detector
As discussed in Chapter 8.1.1.6, an electrochemistry detector could detect biosignatures of extant life based on redox chemistry. One such technique is cyclic voltammetry, wherein an electric potential is introduced into a system that can detect if components are being reduced or oxidized (Enrico et al., 2008; Bard et al., 2022). Recent work has demonstrated that electron transfer measurements at the cell-electrode contact are able to help infer cellular respiration rate and metabolic activities on the single-cell level (Gross and El-Naggar, 2015). Electrochemical methods have been proposed as life-detection experiments, such as on ocean worlds (Thomson et al., 2020).
8.2.2.5. Nucleic acid sequencing
Nucleic acid sequencing is a tool that has allowed us to probe into the nature of our genetic code and was revolutionized by the invention of sequencing methods such as Sanger Sequencing and Next-Generation Sequencing. Recent advancements in nanopore sequencing technology have allowed for rapid analysis of single nucleic acid molecules and are highly promising in biosignature searches. Nanopore sequencers are also capable of detecting non-standard bases (Carr et al., 2017) and other biologically relevant molecules (Niedzwiecki et al., 2020). These devices have been successfully tested for tolerance in a variety of space conditions (e.g., radiating environments) that are relevant to exploration on other worlds such as Mars or Europa (Sutton et al., 2019).
8.2.3. Detection techniques
8.2.3.1. Isotope labeling
Isotope labeling (or tracing) involves measuring the activity level of potential life-forms using specific compounds (called tracers) that can be detected due to their difference in atomic mass or radioactivity. For example, bacterial growth and death rates in ecosystems are routinely determined by observing if an isotopically labeled compound is being consumed and used to form biomass (Li, 1984; Rousk and Bååth, 2011). Broadly utilized tracers include oxygen-labeled water (H218O; Schwartz et al., 2016), radio-labeled carbon (14C), hydrogen (tritium, 3H), and the stable isotopes of carbon (13C), hydrogen (deuterium, 2H), and nitrogen (15N) (Dumont and García, 2019). The use of isotope tracers was famously employed during the Viking missions of 1976 (see Chapter 8.2.5.1 in the supplementary material).
8.2.3.2. Immunoassays
Immunoassay techniques are used to detect organic biosignatures by probing for biological signatures using an antibody-based detection system or detecting strains of microorganisms in the sample of interest (Parro et al., 2005; Sephton et al., 2013). Immunoassay “chips” contain specific antibodies that induce an observable change (e.g., through electrical transduction or fluorescence) when bound to a specific target (Sims et al., 2005). While this technique is highly sensitive to biological compounds, it is also limited to compounds with known antibody pairs and thus is limited to the detection of life that is similar or identical to terran life. Furthermore, current chips are specifically designed and can only detect a limited number of targeted compounds (∼ thousands), whereas instruments like mass spectrometers are able to detect many compounds that are generally nontargeted (Chapter 8.2.2.1). The Life Marker Chip and Life Detector Chip are immunoassay-based detection systems that have been proposed for Mars missions (Sims et al., 2012) and have been tested in various analog sites (Parro et al., 2011).
8.2.4. Sample return
The search for in situ biosignatures on other planetary bodies is limited by technological challenges, such as (i) designing instruments that can withstand extreme space and extraterrestrial planetary conditions, (ii) miniaturizing the payload for cost effectiveness, (iii) performing automated robotic tasks on difficult extraterrestrial terrains, (iv) communicating during narrow windows dictated by planetary alignments, and (v) difficulty in interpreting low-resolution data. These factors affect the definitiveness of detection (see Chapter 8.5). Therefore, one of the best solutions to a life-detection task may be to perform a sample return such as that proposed by the Mars sample return campaign (Mattingly and May, 2011).
Sample return enables access to the full suite of analytic methods and instruments at the highest resolutions available or are not flight ready. For example, nuclear magnetic resonance spectroscopy (NMR), near-edge X-ray absorption fine structure (NEXAFS), or even nanoscale secondary ion mass spectrometry (NanoSIMS) could be used to provide the multiple lines of evidence needed to increase the definitiveness of detection and robustness of a potential life detection (Neveu et al., 2018). Many of these techniques are nondestructive or require only minute amounts of sample material (Nealson et al., 2008), thus allowing for the same sample (or various samples from the same parent material) to be analyzed with multiple complementary techniques. However, sample return suffers from its own limitations, including difficulties in retrieving samples from distant locations, alteration during the return phase, and potential terrestrial contamination during sample handling and curation (see Chapter 10 for more details on planetary protection).
8.2.5. Case studies
More details about how life-detection techniques were used by the Viking landers on Mars and the 1996 announcement of a life detection based on observations of “microfossils” in the Mars Allan Hills 84001 meteorite are further detailed in the supplementary material.
8.3. Remote Biosignatures
The quest for discovering extraterrestrial life expands beyond visiting the surface of other bodies in the Solar System, as biosignatures present in distant planetary systems can be studied remotely using Earth-based or space-based techniques. Because life itself is likely a planetary-scale process, the presence and abundance of specific metabolisms can dramatically alter the atmosphere and surface of a planet. To devise a robust biosignature search strategy, a general understanding of the biological processes that produce remotely observable biosignatures, as well as the abiotic processes that can contribute to false-positive detections and the geological and temporal context, must also be well understood (Meadows, 2017; Catling et al., 2018; Schwieterman et al., 2018). This section describes three main types of remote biosignatures and their key spectral features that can be linked to surface or near-surface life processes: atmospheres, biological pigments, and technosignatures. Although subsurface life may create biosignatures (e.g., Parnell et al., 2016), they are unlikely to be detectable remotely (Krissansen-Totton et al., 2018).
8.3.1. Biosignature gases and atmospheric chemical disequilibrium
Many gases, alone or in combinations, have been proposed as remote biosignatures (Fig. 8.5). While this section is focused on a few key gases and atmospheric disequilibria, a thorough evaluation of biosignature gases is given in Seager et al. (2016) and Schwieterman et al. (2018).
8.3.1.1. Oxygen
Molecular oxygen (O2) is an important atmospheric target species for assessing the potential presence of life in a planetary environment because it could be a direct by-product of photosynthetic life (Fig. 8.6), the dominant form of life on Earth (see Chapter 6.2). Oxygen is the second most abundant species in modern Earth's atmosphere and produces strong spectral features. Additionally, ozone (O3), a photochemical by-product of O2, can be detectable even at low levels due to its prominent spectral features in the UV-visible wavelength region (Schwieterman et al., 2018). However, relying on oxygen detections alone could lead to false-negative conclusions, since research has shown that O2 would be detectable to a remote observer of Earth only in the last ∼500 million years of the planet's history (see Chapter 5.2.2), despite life having arisen at least 3.5 billion years ago (Chapter 4.1.5).
While O2 can serve as a biosignature gas for planets that may have evolved oxygenic photosynthesis, there are planetary cases where large abundances of O2 can also be generated by abiotic processes (e.g., through atmospheric water loss; Luger and Barnes, 2015). In these cases, detection of O2 could be considered a false positive, and additional information about the planetary environment would be required to interpret whether or not O2 could be considered a true biosignature (Meadows, 2017; Meadows et al., 2018). For example, although O+ and O2+ are present within Titan's atmosphere and Mars has ∼0.1% O2 in its atmosphere, these occurrences are thought to be produced abiotically by photochemistry (Hörst et al., 2008) or due to the presence of oxidants on the surface (Trainer et al., 2019), respectively.
8.3.1.2. Methane
Because biology is a dominant source of CH4 in Earth's atmosphere, CH4 has been considered a potential biosignature in the search for life elsewhere. However, CH4 can also be generated abiotically via geological processes (Judd, 2000). For this reason, CH4 is often referenced as a compelling biosignature in a scenario where it is observed alongside O2 or another oxidizing atmospheric species (discussed further below). Methane has various absorption features throughout the VISNIR wavelengths, but they are relatively weak at modern Earth abundances. Methane has stronger absorption features in the infrared (between ∼7 and 8 μm), but this absorption feature overlaps with H2O, which makes CH4 detections at low spectral resolution potentially challenging (Schwieterman et al., 2018). On early Earth, CH4 may have been present at much higher concentrations (1000 ppm or even 1%) due to widespread methanogenic bacteria (see Chapter 5.2), which suggests that higher CH4 concentrations may be detectable in planetary atmospheres similar to that of early Earth's (Fig. 8.6; Arney et al., 2016).
8.3.1.3. Phosphine
Phosphine (PH3) recently received significant attention from the astrobiology community when an absorption feature in the clouds of Venus was attributed to PH3 (Greaves et al., 2020), although this conclusion is broadly contested (Snellen et al., 2020; Lincowski et al., 2021; Thompson, 2021). Phosphine is often associated with life where it is found on Earth and would be actively destroyed by the venusian environment. Thus, its detection would imply active production and could represent a biosignature both on Venus and exoplanets (Sousa-Silva et al., 2020).
8.3.1.4. Chemical disequilibria
Life on Earth actively interacts with the environment, and this has significantly impacted the composition of the atmosphere and ocean, driving the whole system out of equilibrium. Researchers have hypothesized that life on other planets would similarly shift its environment into disequilibrium; thus remote detection of two or more incompatible chemical species (species that normally react together and so should not be detected together) would be indicative of this disequilibrium and thus the presence of life (Lederberg, 1965; Lovelock, 1965). For example, O2 and CH4 readily react to form CO2 and water, and the co-occurrence of O2 and CH4 in Earth's atmosphere points to a large input of these gases from biogenic sources that outcompete abiotic sinks (Krissansen-Totton et al., 2016, 2018). Atmospheric chemical disequilibrium is an important biosignature because it is a generalized signature that makes no assumptions regarding metabolism(s) or biogenic gas(es) at play (see Chapter 9.4.1).
During the Archean Earth era, when Earth's atmosphere was essentially anoxic, the atmosphere contained incompatible species like CH4, CO2, N2, and water due to microbially driven disequilibrium (see Chapter 5.2.1). This type of anoxic disequilibrium could be detectable for exoplanets, since CO2 has numerous distinguishable absorption features, and high abundances of CH4 are remotely detectable (Krissansen-Totton et al., 2018). In contrast, more recent studies have shown that, before the origin of life, Earth may also have had a prebiotic disequilibrium signature due to coexistence of H2 and CO2 or CO and water (Wogan and Catling, 2020). The rise of microbial chemotrophic life likely destroyed this particular disequilibrium signature via metabolic reactions. In this context, the nondetection of a strong disequilibrium would have constituted a false negative or an incorrect assumption that life was not present on Earth. Therefore, to distinguish a biosignature from abiotic processes, it is critical to understand the activation energy for reactions that would resolve an existing disequilibrium (Wogan and Catling, 2020).
8.3.2. Biological pigments
Biological pigments are light-reactive compounds that can be broadly categorized based on their functional roles, including (but not limited to) photosynthetic, photoprotective, biocontrol, and light-harvesting (Giovannetti et al., 2013; Schwieterman et al., 2015). Due to their ability to absorb particular wavelengths of light, various types of pigments and their by-products can be characterized using remote spectroscopic techniques (see Chapter 8.4.3). Spectral absorption features from pigments on Earth are routinely detected from satellite imagery (Fig. 8.6; Richardson and Ledrew, 2006; Schwieterman et al., 2015; Salvatore et al., 2020). Numerous types of pigments are associated with life on Earth (Fig. 8.7), including those that span large spatial scales such as forests or microorganisms in oceans, lakes, and streams across Earth (Richardson and Ledrew, 2006; Kiang et al., 2007a; Salvatore, 2015; Schwieterman et al., 2015; Power et al., 2020).
8.3.2.1. Photosynthetic pigments
Photosynthetic pigments, such as chlorophyll, help cells convert CO2 into sugars by using energy from light (see Chapter 6.2.1.1). Chlorophyll can be detected remotely due to its large spectral absorption feature around 680 nm. This feature, specific to photosynthetic life, contributes to the formation of what is known as the vegetation red edge (VRE). The VRE in spectroscopy refers to a sharp increase in reflectance around the 700–800 nm range (Fig. 8.6). This large slope in the spectra is diagnostic of the transition from pigment light absorption within cells in the visible wavelength range (400–700 nm) to the scattering of light due to the cellular structure of an organism in the near-infrared wavelength range (700–1000 nm; Seager et al., 2005; Kiang et al., 2007a, 2007b). Biological pigments and the VRE could also represent biosignatures for exoplanets (Kiang et al., 2007a, 2007b; Hegde et al., 2015; Schwieterman et al., 2015). However, these studies assume that a distribution of life on an exoplanet is similar to that of Earth's throughout ancient and modern history.
Photosynthetic pigments can also contribute to surface biosignatures in the form of circular polarizing light (Sparks et al., 2009; Patty et al., 2018). Pigments or pigment complexes of photosynthetic organisms contain chiral centers (see Chapter 2.2.3.5 and Chapter 8.1.1.3). These are generally optically active and can preferentially interact with, or absorb, left- or right-polarized light (Patty et al., 2019), potentially resulting in reflectance signatures that could be observed remotely (Chapter 8.4.3.1). In particular, anoxygenic and oxygenic phototrophs are distributed globally on Earth and could have contributed to surface polarization biosignatures for up to 80% of Earth's history (Sparks et al., 2021). Evolutionary changes through time (e.g., changes in the abundance and location of biological colonies) or even relatively rapid changes (e.g., seasonal tree changes) can act as temporal biosignatures.
8.3.2.2. Protective pigments
Photoprotective pigments protect organisms from UV radiation that can lead to DNA damage, such as cyanobacterial scytonemin that absorbs UVA radiation (Giovannetti et al., 2013). As solar irradiance increases, the concentration of scytonemin in the cells of cyanobacteria increases to protect against DNA damage and can be detected in biocrusts or dried cyanobacterial cultures for extended periods of time (Proteau et al., 1993; Giovannetti et al., 2013). Other non-photosynthetic pigments, such as flavonoids, anthocyanins, betacyanins, and certain carotenoids belonging to various types of archaea and bacteria, have been shown to dominate spectral reflectance signatures of Earth's surface seen from orbit (Edreva, 2005; Schwieterman et al., 2015).
8.3.2.3. Effects of star types on biological pigments
A caveat when using pigments as surface biosignatures for planets outside the Solar System is that it is possible that the spectral features of pigments will vary depending on the function of the pigments and/or the host star of the planet with the evolved pigments (Seager et al., 2005; Kiang et al., 2007a, 2007b). Photosynthesis is inherently linked to the wavelengths and intensity of the light produced by the host star, as well as the atmosphere of the planet considered. This information could be used to predict certain spectral features associated with extraterrestrial photosynthetic organisms. However, spectral features of non-photosynthetic pigments may not be as easily predictable. Quantifying and establishing spectral databases of various pigments for organisms on Earth can help establish comparative examples and enhance techniques necessary to detect these potential biosignatures on exoplanets (Hegde et al., 2015).
8.3.3. Technosignatures
The term technosignatures refers to evidence of technology that alters its environment or produces detectable signatures in ways that could indicate the existence (present or past) of intelligent life-forms. Technosignatures can take different forms, including unusual atmospheric and thermal conditions, surface and orbital conditions that could signal the intervention of an intelligent civilization (Tarter, 2006), and the possible presence of “megastructures” built to collect a star's energy and the detection of interstellar crafts. Remote technosignatures could also include light emissions from a distant planet that could be interpreted as purposeful. Radio signals have been of particular interest in the search for extraterrestrial intelligent life because they can travel relatively unimpeded through interstellar space compared with other wavelengths due to lessened attenuation from clouds of interstellar gas and dust. Artificial light sources such as pulsed lasers or nightside illumination could also be detectable (Schneider et al., 2010).
Technosignatures also include industrial signatures such as radioisotope anomalies, the presence of synthetic chemicals (e.g., chlorofluorocarbons [CFCs] and perfluorocarbons [PFCs]), or other persistent pollutants in the exoplanet's or exomoon's atmosphere. The presence of rare elements in higher abundance than expected based on planetary formation models (e.g., plutonium-239, uranium-233, or technetium) that would require mining, disposing, or synthesizing has been proposed as a possible indicator of a technologically advanced civilization (Whitmire and Wright, 1980). Similarly, glints of sunlight in interplanetary space could also suggest the presence of artificial satellites or other artifacts that could be attributed to intelligent extraterrestrial civilization (Lacki, 2019).
It is important to note that the paths of biological, cultural, and technological evolution of an extraterrestrial civilization could be vastly different than the ones taken on Earth. Another important consideration concerns the length of existence of civilizations (Frank and Sullivan, 2016; Cai et al., 2021), as it is possible that civilizations could be long-lived and thus planets hosting these civilizations could have experienced long-term coevolution of technology with planetary physical and biogeochemical cycles. Conversely, short-lived “bursts” of civilizations would lead to the host planets bearing signatures of short-term, intense, and unsustainable coevolution with technology. The technosignatures for these two scenarios could be vastly different, and a wide range of technosignature possibilities should be considered. Studying and understanding the evolution of intelligence and technology on Earth may help constrain the potential that intelligent life could develop elsewhere.
8.4. Remote Biosignature Detection
Remote detection techniques, most often reflectance, emission, and transmission spectroscopy (see Chapter 8.2.2.3), can be used to search for biosignatures on bodies that cannot be visited robotically. This section discusses various techniques that can be used to detect and, to some extent, characterize target exoplanets, and the types of spectroscopy that can be used to determine the composition of atmospheres, elucidate the surface signatures of life such as biological pigments, and even detect signs of extraterrestrial intelligence.
8.4.1. Exoplanet detection
Detecting exoplanets is the first step toward detecting life. Additional information about several techniques that are used in combination to detect and investigate planets beyond the Solar System are provided in the supplementary information. The next generation of high-resolution telescopes (e.g., James Webb Space Telescope [JWST] and the Wide Field Infrared Survey Telescope [WFIRST]) will further characterize the potential habitability of exoplanets and search for biosignatures (see Chapter 8.4.3).
8.4.2. Terrestrial exoplanet detections to date
Currently, there is only one known example of a planet with abundant life: Earth. Since the first exoplanet was discovered in 1992 (Wolszczan and Frail, 1992), thousands of exoplanets have been detected using the techniques described in Chapter 8.4.1 (see the NASA Exoplanet Archive2 for a current list). However, many of these can be excluded from biosignature searches because they are presumably uninhabitable or unlikely to exhibit detectable biosignatures. Additionally, exoplanet detection techniques typically have biases toward particular planetary and orbital characteristics, thus skewing the population of detected planets away from a representative sample. Understanding and accounting for detection biases is an essential component of exoplanet surveys.
Searches for habitable exoplanets tend to focus first on Earth-like planets, meaning approximately Earth-sized, predominantly rocky (i.e., silicate) composition, and at a distance from the parent star that allows liquid water to exist on the surface (see Chapter 7.1). The Kepler mission team announced the discovery of the first rocky planet (i.e., mass and radius are consistent with a largely silicate composition), Kepler-10b, but it is not considered to be in the conventional habitable zone (Chapter 7.1.4.1 in the supplementary material for Chapter 7). Subsequently, the mission found approximately eight roughly Earth-sized planets orbiting in their star's habitable zone. Most notable was Kepler-452b, which is ∼50% larger than Earth and has been referred to as “Earth's cousin” (Hsu et al., 2019). However, nearly all are too far away for direct follow-up observations or radial velocity measurements. Including detections from ground-based observatories, scientists have identified nearly 20 rocky exoplanets in the habitable zones of their stars (see the NASA Exoplanet Archive).
Estimates suggest 1 in 3 Sun-like stars are expected to contain Earth-like planets (Howell, 2020) with a wide range in possible frequencies (e.g., 0.07–3.77; see analysis and review by Bryson et al., 2021). The large uncertainty stems from several unconstrained parameters such as (i) the effects of the host star type on planet abundances (Silburt et al., 2015; Hsu et al., 2019; Zink and Hansen, 2019), (ii) ongoing revisions to definitions of the habitable zone (Chapter 7.1.4.1 in the supplementary material for Chapter 7), (iii) the range of planet sizes that can allow for global surface habitability (Zink and Hansen, 2019), and (iv) the techniques used to account for biases in the observational data (Weiss and Petigura, 2020). Missions such as the Transiting Exoplanet Survey Satellite (TESS) continue to provide additional data to bolster this statistical approach (Ricker et al., 2014).
8.4.3. Remote biosignature detection methods
Telescopes are being built or have been proposed such as the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT) that will have enhanced resolution and sensitivity capable of potentially resolving various spectral features of exoplanets (Johns, 2006; Gilmozzi and Spyromilio, 2007; Sanders, 2013). The 2020 Decadal Survey for Astronomy and Astrophysics (National Academies of Sciences, Engineering, and Medicine, 2021) also recommended the Habitable Worlds Observatory, inspired by the Habitable Exoplanet Observatory (HabEx; Gaudi et al., 2020) and the Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR; LUVOIR Mission Concept Study Team, 2019); LUVOIR and HabEx were designed to use an internal coronagraph or an external starshade (Fig. 8.8). The next generation of space-based telescope will provide a compelling opportunity to search for remote biosignatures of life.
8.4.3.1. Reflectance spectroscopy
Reflectance spectroscopy, a remote detection technique, has been utilized in terrestrial ecology, forestry, and land management for decades. It is used to identify spectral features, primarily in the shortwave, such as those produced by biological pigments and the VRE (see Chapter 8.3.2.1).
Direct (or high contrast) imaging can help indicate the presence of opacity sources on exoplanets (e.g., atmospheric gases, aerosols, and clouds; see Chapter 8.4.1.5 in the supplementary material). Spectral characterization of this type is typically considered at shorter wavelengths, since the ability to resolve two light sources (i.e., the star and the planet) decreases with wavelength. Stellar types F, G, and K (see Chapter 3.1.2.2) are prime targets for these studies since the habitable zone orbital distance creates a large enough separation for planets to be directly imaged (see Chapter 7.1.4.1 in the supplementary material for Chapter 7).
Modeling studies that incorporate systematic knowledge of planetary geochemical, physical, and atmospheric systems are integral to exoplanet investigations. Spectral, photochemical, and planetary evolution models can provide a benchmark for observations (Livengood et al., 2011; Robinson et al., 2011) and can be used to corroborate the detectability of biosignatures on other planets (Schwieterman et al., 2018). System-level approaches and network studies also serve as a means to understand the interaction between life and its planet's atmosphere, which can be used to understand equilibrium stages of systems and determine whether life or lifelike processes can be inferred (Solé and Munteanu, 2004; Walker et al., 2018).
From Earth orbit, a range of metrics, called spectral indices, are used to analyze the strength of specific features within a reflectance spectrum (e.g., pigments or the VRE). Spectral indices help identify and distinguish various types of microbial communities or detect specific events (Tucker, 1979; Lee and Carder, 2004). Some examples include the normalized pigment to chlorophyll index (NPCI) and the cyanobacteria index (CI) (Wynne et al., 2010; Kudela et al., 2015). Additionally, the normalized difference vegetation index (NDVI) is often used to determine how much photosynthetic activity is occurring across a given landscape, including by microbial communities in Antarctica (O'Reilly et al., 1998; Power et al., 2020; Salvatore et al., 2020). Spectropolarimetry can be used to detect biological pigments based on how the chiral centers interact with polarized light (see Chapter 8.3.2.1). This method has also been used to detect other terran biosignatures using Earthshine (the spectral signature of Earth's reflection on the Moon; Sterzik et al., 2012), and it has been proposed as a tool for detecting chirality on remote exoplanets (Sparks et al., 2021).
8.4.3.2. Transit transmission spectroscopy
Transit transmission spectroscopy is another method used to detect and characterize potential biosignatures on exoplanets. During a primary transit event (Chapter 8.4.1.5 in the supplementary material), as the orbiting planet passes in front of its host star, light from the star is filtered through the planet's atmosphere, and absorptions in the transmission spectra due atmospheric chemical species can be used to search for life. Further, as a planetary body passes behind its stellar host (known as a secondary transit event or secondary eclipse), emission spectroscopy can be used to determine the planet's temperature. The James Webb Space Telescope (JWST) will use transit techniques to search for biosignature gases predominantly on exoplanets orbiting M dwarf stars due to the higher sensitivity for planets relatively close in size to their host star (Lustig-Yaeger et al., 2019).
8.4.3.3. Radio astronomy and technosignature detection
Atmospheric and infrared emission technosignatures could theoretically be detected by spectroscopic techniques. Because technosignatures must be discriminated from naturally occurring signals and sources of terrestrial interference, radio signals are often chosen because they will be Doppler shifted due to the relative motion of the extraterrestrial transmitter and a fixed receiver on Earth (Price et al., 2020). However, there is still a need to discriminate between naturally occurring radio signals (e.g., pulsars and radio galaxies) and intentional ones. Since the 1960s, concerted efforts to search for intelligent life have been conducted through searching for radio signals with the use of radio telescopes (Drake, 1961; Sheikh et al., 2020). An overview of the search for extraterrestrial intelligence (SETI) that has been conducted from the 1960s to the early 2000s is outlined in Tarter (2001).
In 2016, the Breakthrough Listen (BL) initiative was formed. BL is a 10-year plan involving two radio telescopes to search for signals of intelligent life between 1 and 10 GHz across various stellar targets (Worden et al., 2017). The BL initiative's first comprehensive study involved studying a sample set of 1327 stars at frequencies ranging between 1.10 and 3.45 GHz (Price et al., 2020). Although no confirmed technosignatures have been found to date, comprehensive searches for signals and robust data processing techniques are key to continuing the search for life beyond Earth.
8.4.4. Case studies
Several notable instances of potential remote life detections and lessons learned from Earth-based observations are described further in the supplementary material.
8.5. Definitiveness of Life Detection and the Grayness of Life
Life is an ever-evolving process, a spectrum with life at one end and nonlife at the other, with a “gray” area in between. Life, at its inception, was a transition from geochemistry to biochemistry (see Chapter 4). It transitioned from being a product of its environment to an independent entity that could shape its environment to perpetuate itself. This transition was likely gradual, and it is difficult to define at what stage a system is considered living or distinguishable from its environment (Smith et al., 2021). Thus, defining life and biosignatures is difficult (see Chapter 2.4). In addition to a spectrum of different stages of “aliveness” (Sutherland, 2016), numerous ambiguous entities are often debated as being alive or not (e.g., viruses; Bartlett and Wong, 2020).
This section discusses the concept of definitiveness of detection and the grayness of life, in an effort to better understand potential pitfalls in life-detection missions and develop universal life-detection strategies that account for the full spectrum of life.
8.5.1. Definitiveness of detection
Whether considering “gray” cases of life or less ambiguous living entities, environmental and evolutionary context may make it difficult to ascertain if a detection can conclusively be labeled as life based solely on initial evidence gathered. For definitive detection of life, efforts may need to rely on multiple lines of evidence; no single measurement may fully confirm biogenicity, and all other abiotic explanations must be exhaustively rejected (Catling et al., 2018; Neveu et al., 2018).
Researchers have suggested a Bayesian approach to quantifying the likelihood of the presence of life based on data gathered (Catling et al., 2018; Walker et al., 2018). Rather than viewing the detection of life as a binary question (i.e., life or nonlife), a Bayesian approach would evaluate the statistical probability that life was observed based on multiple lines of evidence. This approach takes into account certain assumptions about (i) the probability of a particular biosignature having been produced by life, (ii) the probability of that biosignature having been produced by abiotic processes, and (iii) the “prior probability of the living process” (see Walker et al., 2018). This type of probabilistic approach can also be used to (i) determine which methods would complement each other, (ii) establish a scale of statistical robustness when multiple measurements are considered together, (iii) help impart more certainty on the nature of the signals perceived, (iv) help quantify the likelihood of life being present in the sample (National Academies of Sciences, Engineering, and Medicine, 2019), and (v) interpret more challenging samples (e.g., where evidence has been degraded, or in “gray” cases of life). Discussion of the need for a framework for analysis of potential biosignature signals and reporting these results has been particularly active in recent years (e.g., Green et al., 2021; Meadows et al., 2022), due in part to claims of detection of phosphine in the atmosphere of Venus (see Chapter 8.3.1.4) and the launching of JWST (see Chapter 8.4.3.2).
8.5.2. Viruses
Viruses are often discussed in the context of whether they are living entities or not (see Chapter 2.3.2). A more detailed discussion on viruses as ambiguous biosignatures can be found in the supplementary material.
8.5.3. Artificial life and cellular automata
The study of life is not limited to natural biochemical systems. Indeed, a whole field of research aims to study the nature of living systems and their fundamental principles by building them de novo using computational, robotic, or chemical tools (artificial life, often abbreviated as ALife). Discussions around ALife can be found in the supplementary material.
8.6. Conclusion
The search for life in the universe continues to be a challenging pursuit. As the understanding of life on Earth and the types of biosignatures that could be detected beyond Earth are broadened, we must consider lessons learned from previous planetary exploration missions, laboratory and field studies, and our theoretical understanding of the chemical and physical space life occupies. We must also keep our minds open to the possibilities of the existence of life and biosignatures that are unknown to us (see Chapter 9). Because the detection of life beyond Earth could have profound global, scientific, social, and cultural implications, we acknowledge the difficulties in declaring a discovery of life beyond our planet. A positive life-detection declaration must be a “last resort” hypothesis, one only made when all other possible options have been ruled out. Because of these challenges, international collaborations are crucial in establishing rigorous guidelines for life detection as well as when seeking partnerships with a diverse set of expertise outside of traditional space and biological sciences (see Chapter 11 on international astrobiology societies and institutions). Fostering international and interdisciplinary collaborations will enhance our scientific, social, and cultural motivations to search for life in the Solar System and beyond.
Abbreviations Used
- BL
- Breakthrough Listen
- CE
- capillary electrophoresis
- EPS
- extracellular polymeric substances
- GC
- gas chromatography
- HabEx
- Habitable Exoplanet Observatory
- JWST
- James Webb Space Telescope
- LC
- liquid chromatography
- LIBS
- laser-induced breakdown spectroscopy
- LUVOIR
- Large Ultraviolet/Optical/Infrared Surveyor
- MISS
- microbially induced sedimentary structures
- TRL
- Technology Readiness Level
- VISNIR
- visible near-infrared
- VRE
- vegetation red edge
Acknowledgments
The authors would like to acknowledge Paul Mahaffy and Sarah S. Johnson for providing valuable feedback that helped improve the quality of the manuscript. The authors acknowledge lead Primer 3.0 editors, M. Schaible, N. Szeinbaum, and G. Tan, for providing feedback and logistics support. S. Borges was funded by the National Science Foundation Office of Polar Programs award 1745053 (Antarctic Organisms & Ecosystems) and the National Science Foundation Graduate Research Fellowship when writing the manuscript. T. Caro was supported by the Interdisciplinary Quantitative Biology (IQ Biology) PhD Program at the BioFrontiers Institute, University of Colorado Boulder. L. Chou was supported by an appointment to the NASA Postdoctoral Program at the NASA Goddard Space Flight Center, administered by USRA and ORAU through a contract with NASA, by the NASA-funded Laboratory for Agnostic Biosignatures Project (NASA Grant “80NSSC18K114”), the Mars Science Laboratory Mission, and by the NASA award number 80GSFC21M0002. N. Grefenstette was supported by the NASA-funded Laboratory for Agnostic Biosignatures Project and the Santa Fe Institute. The authors would like to thank M. Styczinski for contributing to the in situ chemical and thermodynamic equilibrium biosignatures, Venus, and the remote biosignatures sections. G. Trubl's work, conducted by the Lawrence Livermore National Laboratory (LLNL), was supported by the DOE Office of Science, Office of Biological and Environmental Research Genomic Science program award SCW1632, LLNL LDRD 21-LW-060, and under the auspices of the US Department of Energy under contract DE-AC52-07NA27344. A. Young was supported by the NASA Exobiology Research Program (grant NO. 80NSSC19K0473).
Footnotes
1
For more NASA documentation on the TRL scale, see https://www.nasa.gov/pdf/458490main_TRL_Definitions.pdf
2
More information is available at https://exoplanetarchive.ipac.caltech.edu
Supplementary Material
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History
Published online: 18 March 2024
Published in print: March 2024
Accepted: 14 November 2023
Received: 15 July 2021
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Author Contributions
All authors provided editing feedback for the entire manuscript.
L. Chou co-managed the team, co-edited and co-developed the structure of the chapter, and co-authored the chapter introduction, section introductions, and conclusions. L. Chou authored the chemical biosignatures section (molecular biosignatures, isotopic biosignatures, structural and compositional biases, and minerals as biosignatures), mass spectrometry detection instrument, electrochemistry detection, the Allan Hills meteorite case study, the Galileo spacecraft case study of Earth, and the sample return section.
N. Grefenstette co-managed the team, co-edited and co-developed the structure of the chapter, and co-authored the chapter introduction, section introductions, and conclusions. N. Grefenstette authored the chemical and thermodynamic disequilibrium as a biosignature, the definitiveness of detection and the grayness of life (grayness of life, artificial life and cellular automata, and definitiveness of detection) sections.
S. Borges authored the physical biosignatures section, spectroscopy techniques for in situ biosignatures detection, biological pigments as remote biosignatures and co-authored the remote detection instruments section.
T. Caro authored the isotope labeling for in situ detection sections and the Viking mission case study and co-authored the gas chromatography section and the sample return section.
E. Catalano authored the technosignatures section and the WOW! signal case study and co-authored the technosignature detection methods: radio astronomy, and helped on some sections related to biosignatures.
C.E. Harman authored the exoplanet detection section and co-authored the remote detection instrument section.
J. McKaig authored the microscopy detection method section and the nucleic acid sequencing detection section.
C. Raj authored the separation techniques for in situ biosignature detection methods.
G. Trubl authored the virus section and the definitiveness of life detection section and co-authored the sample return section.
A. Young authored the gases as a biosignature section, the terrestrial planet detection section and co-authored the technosignature detection methods: radio astronomy section.
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