Biosignature
A biosignature is a phenomenon that can be explained by biological processes where all possible abiotic causes of this phenomenon have been eliminated. This term is mainly used in the field of astrobiology in the search for past or present extraterrestrial life, from planets and moons in the Solar System to exoplanets. Candidate biosignatures strongly indicate some of the earliest known life forms, aid studies of the origin of life on Earth as well as the possibility of life on Mars, Venus and elsewhere in the universe.
History
The term "biosignature" and its definition have evolved over time. In the 1960s, the phrase "life detection" was used as seen in two Nature papers "A physical basis for life detection experiments," by James. E. Lovelock and "Signs of Life: Criterion-system of exobiology," by Joshua Lederberg. In 1973, Joon H. Rho used the term "biomarker" in his paper, "A search for porphyrin biomarkers in nonesuch shale and extraterrestrial samples" to describe a fossil organic compound that can be traced back to a specific organism. In medicine, biomarker has a different definition. In 1995, the term biosignature was first used by the NASA Exobiology Program office in "An Exobiological Strategy for Mars Exploration." The term has since become widely used in astrobiology.The definition of "biosignature" continued to be refined. In 2003, it was described as an object, substance, and/or pattern that unequivocally was originated through a biological process. By 2018, the definition had broadened to a substance or phenomenon that presents evidence of life. In 2023, the astrobiology community further refined the concept, agreeing that a biosignature is a phenomenon that can only be explained by biological processes, with all plausible abiotic explanations having been considered and eliminated.
Types
Biosignatures can be grouped into ten broad categories:- Isotope patterns: Isotopic evidence or patterns that require biological processes.
- Chemistry: Chemical features that require biological activity.
- Organic matter: Organics formed by biological processes.
- Minerals: Minerals or biomineral-phases whose composition and/or morphology indicate biological activity.
- Microscopic structures and textures: Biologically-formed cements, microtextures, microfossils, and films.
- Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, biofilms, or fossils of larger organisms.
- Temporal variability: Variations in time of atmospheric gases, reflectivity, or macroscopic appearance that indicates life's presence.
- Surface reflectance features: Large-scale reflectance features due to biological pigments.
- Atmospheric gases: Gases formed by metabolic processes, which may be present on a planet-wide scale.
- Technosignatures: Signatures that indicate a technologically advanced civilization.
Viability
Reliability
A biosignature must be able to dominate over all other processes that produce similar physical, spectral, and chemical features. Many forms of life are known to mimic geochemical reactions. One of the theories on the origin of life involves molecules developing the ability to catalyse geochemical reactions to exploit the energy being released by them. These are some of the earliest known metabolisms. In such case, scientists might search for a disequilibrium in the geochemical cycle, which would point to a reaction happening more or less often than it should. A disequilibrium such as this could be interpreted as an indication of life. However when looking at disequilibria, it is important to consider the context of the environment, because not all atmospheric disequilibria has biotic causes. For example, prebiotic environments can have chemical disequilibria due to volcanic activity.Survivability
A biosignature must be able to last for long enough so that a probe, telescope, or human can be able to detect it. A consequence of a biological organism's use of metabolic reactions for energy is the production of metabolic waste. In addition, the structure of an organism can be preserved as a fossil and we know that some fossils on Earth are as old as 3.5 billion years. These byproducts can make excellent biosignatures since they provide direct evidence for life. However, in order to be a viable biosignature, a byproduct must subsequently remain intact so that scientists may discover it.Detectability
A biosignature must be detectable with current technology in order to be considered viable in scientific investigations. Although this may seem straightforward, there are many scenarios in which life may be present on a planet yet remain undetectable due to observational or technological limitations.False positives
Every possible biosignature is associated with its own set of unique false positive mechanisms, in which abiotic processes can mimic the detectable feature of biological activity. An important example is using oxygen as a biosignature. On Earth, most oxygen is produced by photosynthesis and is subsequently used by other life forms. Oxygen is also readily detectable in spectra, with multiple bands across a relatively wide wavelength range, therefore, it makes a very good biosignature. Finding oxygen alone in a planet's atmosphere is not enough to confirm a biosignature because of the false-positive mechanisms associated with it. One possibility is that oxygen can build up abiotically via photolysis if there is a low inventory of non-condensable gasses or if the planet loses a lot of water. Finding and distinguishing a biosignature from its abiotic mechanisms is one of the major challenges of confirming the viability of a biosignature.False negatives
biosignatures occur when life is present, but environmental processes and/or measurement limitations may obscure or suppress features that would otherwise indicate biological activity. This is another challenge that is a significant focus of ongoing research, especially in preparation for future telescope observations designed to observe exoplanetary atmospheres.Human limitations
Observational or technological limitations may also limit the detectability of a potential biosignature. Telescope resolution maybe insufficient to resolve spectral features needed to distinguish between biological signals and false positives. In addition, observatories and telescopes are designed by multidisciplinary teams, resulting in instrumentation that reflects compromises among a variety of scientific priorities. As a result, optimizing instruments for biosignature detection may requires trade-offs with capabilities aimed at other science goals.General examples
Geomicrobiology
The ancient record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life and how these signatures are preserved over geologic time. Some related disciplines such as geochemistry, geobiology, and geomicrobiology often use biosignatures to determine if living organisms are or were present in a sample. These possible biosignatures include: microfossils and stromatolites; molecular structures and isotopic compositions of carbon, nitrogen and hydrogen in organic matter; multiple sulfur and oxygen isotope ratios of minerals; and abundance relationships and isotopic compositions of redox-sensitive metals.For example, the particular fatty acids measured in a sample can indicate which types of bacteria and archaea live in that environment. Another example is the long-chain fatty alcohols with more than 23 atoms that are produced by planktonic bacteria. When used in this sense, geochemists often prefer the term biomarker. Another example is the presence of straight-chain lipids in the form of alkanes, alcohols, and fatty acids with 20-36 carbon atoms in soils or sediments. Peat deposits are an indication of originating from the epicuticular wax of higher plants.
Life processes may produce a range of biosignatures such as nucleic acids, lipids, proteins, amino acids, kerogen-like material and various morphological features that are detectable in rocks and sediments. Microbes often interact with geochemical processes, leaving features in the rock record indicative of biosignatures. For example, bacterial micrometer-sized pores in carbonate rocks resemble inclusions under transmitted light, but have distinct sizes, shapes, and patterns and are distributed differently from common fluid inclusions. A potential biosignature is a phenomenon that may have been produced by life, but for which alternate abiotic origins may also be possible.
Morphology
Another possible biosignature might be morphology since the shape and size of certain objects may potentially indicate the presence of past or present life. Morphology has sparked debate as it is inconclusive and has resulted in disputed claims of early life on Earth.Stromatolites are difficult to identify chemically and are sometimes claimed based on morphology alone. However geological processes may produce false positive candidates. One case is a 3.7 Ga structure in West Greenland which could be explained by tectonic processes.
Chemistry
No single compound will prove life once existed. Rather, it will be distinctive patterns present in any organic compounds showing a process of selection. For example, membrane lipids left behind by degraded cells will be concentrated, have a limited size range, and comprise an even number of carbons. Similarly, life only uses left-handed amino acids. Biosignatures need not be chemical, however, and can also be suggested by a distinctive magnetic biosignature.Chemical biosignatures include any suite of complex organic compounds composed of carbon, hydrogen, and other elements or heteroatoms such as oxygen, nitrogen, and sulfur, which are found in crude oils, bitumen, petroleum source rock and eventually show simplification in molecular structure from the parent organic molecules found in all living organisms. They are complex carbon-based molecules derived from formerly living organisms. Each biomarker is quite distinctive when compared to its counterparts, as the time required for organic matter to convert to crude oil is characteristic. Most biomarkers also usually have high molecular mass.
Some examples of biomarkers found in petroleum are pristane, triterpanes, steranes, phytane and porphyrin. Such petroleum biomarkers are produced via chemical synthesis using biochemical compounds as their main constituents. For instance, triterpenes are derived from biochemical compounds found on land angiosperm plants. The abundance of petroleum biomarkers in small amounts in its reservoir or source rock make it necessary to use sensitive and differential approaches to analyze the presence of those compounds. The techniques typically used include gas chromatography and mass spectrometry.
Petroleum biomarkers are highly important in petroleum inspection as they help indicate the depositional territories and determine the geological properties of oils. For instance, they provide more details concerning their maturity and the source material. In addition to that they can also be good parameters of age, hence they are technically referred to as "chemical fossils". The ratio of pristane to phytane is the geochemical factor that allows petroleum biomarkers to be successful indicators of their depositional environments.
Geologists and geochemists use biomarker traces found in crude oils and their related source rock to unravel the stratigraphic origin and migration patterns of presently existing petroleum deposits. The dispersion of biomarker molecules is also quite distinctive for each type of oil and its source; hence, they display unique fingerprints. Another factor that makes petroleum biomarkers more preferable than their counterparts is that they have a high tolerance to environmental weathering and corrosion. Such biomarkers are very advantageous and often used in the detection of oil spillage in the major waterways. The same biomarkers can also be used to identify contamination in lubricant oils. However, biomarker analysis of untreated rock cuttings can be expected to produce misleading results. This is due to potential hydrocarbon contamination and biodegradation in the rock samples.