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Biosignature

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For other uses, see Biomarker (disambiguation).

A biosignature (sometimes called chemical fossil or molecular fossil) is any substance, such as an element, isotope, molecule, or phenomenon, that provides scientific evidence of past or present life on a planet.[1][2][3] Measurable attributes of life include complex physical or chemical structures, use of free energy, and the production of biomass and wastes. Biosignatures are central to the field of astrobiology and the search for extraterrestrial intelligence.

Types

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In general, biosignatures can be grouped into ten broad categories:[4]

Viability

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Many possible explanations exist for most biosignatures, and understanding of chemical and geological processes on other planets is currently limited. In addition, human-made measurement tools can have errors, and indicators of biological processes can be extremely subtle, so an error can potentially falsely indicate a biosignature. Because of these factors, a biosignature can only be considered viable for further research if every other possibility has been exhausted. Scientists have therefore determined three general criteria that a potential biosignature must meet to be considered viable: reliability, survivability, and detectability.[6][7][8][9]

False positive mechanisms for oxygen on a variety of planet scenarios. The molecules in each large rectangle represent the main contributors to a spectrum of the planet's atmosphere. The molecules circled in yellow represent the molecules that would help confirm a false positive biosignature if they were detected. Furthermore, the molecules crossed out in red would help confirm a false positive biosignature if they were not detected. Cartoon adapted from Victoria Meadows' 2018 oxygen as a biosignature study.[9]

Reliability

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Many forms of life are known to mimic geochemical reactions. A biosignature must be able to provide a better explanation than any and all other processes that may produce similar physical, spectral, and chemical features.

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 (see methanogenesis).[10][11] To identify a case such as this, scientists might search for a disequilibrium in the geochemical cycle, which would point to a reaction happening more or less often than it should if only non-biological processes are considered.[11]

Survivability

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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.[12][13] 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

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A biosignature must be detectable with the current technology to be relevant in scientific investigation. This seems to be an obvious statement, however, there are many scenarios in which life may be present on a planet yet remain undetectable because of human-caused limitations.

Other considerations

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False positives and false negatives

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Every possible biosignature is associated with its own set of unique false positive mechanisms or non-biological processes that can mimic the detectable feature of a biosignature. An important example is using oxygen as a biosignature. On Earth, the majority of life is centred around oxygen. It is a byproduct of photosynthesis and is subsequently used by other life forms to breathe. Oxygen is also readily detectable in spectra. However, finding oxygen alone in a planet's atmosphere is not enough to confirm a biosignature because of the false-positive mechanisms associated with it. For example, photolysis can adequately explain the presence of oxygen on many Earth-like planets.[14][15][16]

Conversely, false negatives arise in a scenario where life may be present on another planet, but some processes on that planet make potential biosignatures undetectable.[17]

Human limitations

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A wide variety of humans make telescopes in different ways, which can limit their overall functionality regarding specific biosignatures, because of the need for compromise in certain areas.[18] Additionally, regardless of the complexity of the telescope, they are only capable of observing to the limits of human technology. Although some telescopes have observed objects near the end of the observable universe,[19] the resolution of these objects is generally insufficient for determining conditions to the degree of specificity that would be required to rule out all alternatives.

General examples

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Biosignatures on Earth

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Electron micrograph of microfossils from a sediment core obtained by the Deep Sea Drilling Program

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; the isotope ratios of minerals, particularly those of sulfur and oxygen; and the presence and composition of redox-sensitive metals.[20][21]

Other biosignatures present in the environment include lipids, nucleic and amino acids, and proteins,[22] which can indicate the type of bacteria that live in an environment through the presence of fatty acids or fatty alcohols.[23] Biomarkers of this sort can also indicate the presence of more advanced life, such as peat depots being indicative of the epicuticular wax produced by plants.

The fossil record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life and how these signatures are preserved over time. For example, bacterial micrometer-sized pores in carbonate rocks have distinct sizes, shapes, and patterns from common fluid inclusions, and are distributed differently.[24]

Morphology

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Some researchers suggested that these microscopic structures on the Martian ALH84001 meteorite could be fossilized bacteria.[25][26]

The shape and size of certain objects may also indicate the presence of past or present life. For example, microscopic magnetite crystals in the Martian meteorite ALH84001[26][27][28] are one of the longest-debated of several potential biosignatures in that specimen.[29] Most scientists ultimately concluded that these were far too small to be fossilized cells,[30] but a consensus that has emerged from these discussions, and is now seen as a critical requirement, is the demand for further lines of evidence in addition to any morphological data that supports such extraordinary claims.[1] Currently, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection".[31][32][33] Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.[31]

Chemistry

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Chemical biosignatures take the form of distinctive patterns present in any organic compounds showing a process of selection.[34] 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 as we know it only uses left-handed amino acids.[34]

Structures of prime examples of biomarkers (petroleum), from top to bottom: Pristane, Triterpane, Sterane, Phytane and Porphyrin

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, and petroleum source rock.[35] Most biomarkers also usually have high molecular mass.[36] While no singular chemical compound can prove the presence of life, some compounds, such as dimethyl sulfide and chloromethane,[37] are strong indicators of the possibility. The presence of phosphine on Venus is significant in the search for life on the planet, due to the absence of known abiotic processes that would produce phosphine.[38]

Petroleum biomarkers are often referred to as "chemical fossils",[39] as they help indicate the deposition and geological properties of oils,[40] such as through the ratio of pristane to phytane.[41] Such petroleum biomarkers are typically produced via chemical synthesis using biochemical compounds as their main constituents. For instance, triterpenes are derived from biochemical compounds found on land angiosperm plants.[42] However, biomarker analysis of untreated petroleum rock cuttings can be misleading, due to potential hydrocarbon contamination and biodegradation in the rock samples.[43]

Atmospheric

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Over billions of years, the processes of life on a planet would result in a mixture of chemicals unlike anything that could form in an ordinary chemical equilibrium.[16][44][37] For example, large amounts of oxygen and small amounts of methane are generated by life on Earth.[45] An alternative biosignature is the combination of methane and carbon dioxide.[46][47]

An exoplanet's color can also be used as a biosignature due to the effect of pigments that are uniquely biologic in origin, such as the pigments of phototrophic and photosynthetic life forms.[48][49][50][51][52] Scientists use the Earth as an example of this; when looked at from far away, Earth appears blue.[53] Ultraviolet radiation on life forms could also induce biofluorescence in visible wavelengths.[54][55]

Biogenic methane production is the main contributor to the methane flux coming from the surface of Earth. Methane has a photochemical sink in the atmosphere but will build up if the flux is high enough. If there is detectable methane in the atmosphere of another planet, especially with a host star of G or K type, this may be interpreted as a viable biosignature.[56]

Agnostic biosignatures

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Because the only form of known life is that on Earth, the search for biosignatures is heavily influenced by the products that life produces on Earth. However, life that is different than life on Earth may still produce biosignatures that are detectable by humans, even though nothing is known about their specific biology. This form of biosignature is called an "agnostic biosignature" because it is independent of the form of life that produces it. It is widely agreed that all life, no matter how different it is from life on Earth, needs a source of energy to thrive.[57] This must involve some sort of chemical disequilibrium, which can be exploited for metabolism.[58][59][60] Geological processes are independent of life, and if scientists can constrain the geology well enough on another planet, then they know what the particular geologic equilibrium for that planet should be. A deviation from geological equilibrium can be interpreted as an atmospheric disequilibrium, and therefore an agnostic biosignature.

Antibiosignatures

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In the same way that detecting a biosignature would be a significant discovery about a planet, finding evidence that life is not present can also be an important discovery about a planet. Life relies on redox imbalances to metabolize available resources into energy. The evidence that nothing on a world is taking advantage of an observed redox imbalance is called an antibiosignature.[61]

Polyelectrolytes

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Main article: Polyelectrolyte theory of the gene

The Polyelectrolyte theory of the gene is a proposed generic biosignature. In 2002, Steven A. Benner and Daniel Hutter proposed that for a linear genetic biopolymer dissolved in water, such as DNA, to undergo Darwinian evolution anywhere in the universe, it must be a polyelectrolyte, a polymer containing repeating ionic charges.[62] Benner and others proposed methods for concentrating and analyzing these polyelectrolyte genetic biopolymers on Mars,[63] Enceladus,[64] and Europa.[65]

Specific examples

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See also: Life on Titan

Methane on Mars

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See also: Life on Mars § Possible biosignatures
Main article: Methane on Mars
Methane (CH4) on Mars - potential sources and sinks.

The presence of methane in the atmosphere of Mars is an area of ongoing research and a highly contentious subject. Because of its tendency to be destroyed in the atmosphere by photochemistry, the presence of excess methane on a planet can indicate that there must be an active source. With life being the strongest source of methane on Earth, observing a disequilibrium in the methane abundance on another planet could be a viable biosignature.[59][60]

Since 2004, there have been several detections of methane in the Mars atmosphere by a variety of instruments onboard orbiters and ground-based landers on the Martian surface as well as Earth-based telescopes.[66][67][68][69][70][71] These missions reported values anywhere between a 'background level' ranging between 0.24 and 0.65 parts per billion by volume (p.p.b.v.)[72] to as much as 45 ± 10 p.p.b.v.[73]

However, recent measurements using the ACS and NOMAD instruments on board the ESA-Roscosmos ExoMars Trace Gas Orbiter have failed to detect any methane over a range of latitudes and longitudes on both Martian hemispheres. These highly sensitive instruments were able to put an upper bound on the overall methane abundance at 0.05 p.p.b.v.[74] This nondetection is a major contradiction to what was previously observed with less sensitive instruments and will remain a strong argument in the ongoing debate over the presence of methane in the Martian atmosphere.

Furthermore, current photochemical models cannot explain the presence of methane in the atmosphere of Mars and its reported rapid variations in space and time.[61] Neither its fast appearance nor disappearance can be explained yet.[75] To rule out a biogenic origin for the methane, a future probe or lander hosting a mass spectrometer will be needed, as the isotopic proportions of carbon-12 to carbon-14 in methane could distinguish between a biogenic and non-biogenic origin, similarly to the use of the δ13C standard for recognizing biogenic methane on Earth.[76]

Martian atmosphere

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The Martian atmosphere contains high abundances of photochemically produced carbon monoxide and hydrogen gas, which are reducing molecules. Mars' atmosphere is otherwise mostly oxidizing, leading to a source of untapped energy that life could exploit if it used by a metabolism compatible with one or both of these reducing molecules. Because these molecules can be observed, scientists use this as evidence for an antibiosignature.[77][78][79]

Phosphine on Venus

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See also: Life on Venus and Atmosphere of Venus

In 2020, the Atacama Large Millimeter Array (ALMA) telescope indicated the presence of phosphine gas in Venus' atmosphere.[80] This discovery has since been disputed, but pending its accuracy, the presence of phosphine gas is not adequately explained by any known chemical processes in Venus, and therefore may be indicative of life.

Missions inside the Solar System

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The Viking missions to Mars

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Main article: Viking biological experiments

The Viking missions to Mars in the 1970s conducted the first experiments which were explicitly designed to look for biosignatures on another planet. Each of the two Viking landers carried three life-detection experiments which looked for signs of metabolism; however, the results were declared inconclusive.[22][81][82][83][84]

Mars Science Laboratory

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Main article: Timeline of Mars Science Laboratory

The Curiosity rover from the Mars Science Laboratory mission is currently assessing the potential past and present habitability of the Martian environment and is attempting to detect biosignatures on the surface of Mars.[3] The rover targets outcrops to maximize the probability of detecting "fossilized" organic matter preserved in sedimentary deposits.

ExoMars Orbiter

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The Trace Gas Orbiter (TGO) is a Mars telecommunications orbiter and atmospheric gas analyzer mission. It delivered the Schiaparelli EDM lander in 2016 as the first half of the ExoMars NASA mission. The second half, Rosalind Franklin rover, was set to be launched in 2022,[85] but has been delayed to 2028[86] due to the COVID-19 pandemic and geopolitical instability between the USA and Russia following the Russian invasion of Ukraine in 2022. The primary objective of the Rosalind Franklin rover mission is the search for biosignatures on the surface and subsurface by using a drill able to collect samples down to a depth of 2 metres (6.6 ft), away from the destructive radiation that bathes the surface.[84][87]

Mars 2020 Rover

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The Mars 2020 Perseverance rover is intended to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, and assess its past habitability, the possibility of life in the past, and potential for preservation of biosignatures.[88][89] In addition, it will cache the most interesting samples for possible future transport to Earth.

Titan Dragonfly

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NASA's Dragonfly lander is proposed to launch in 2028[90] and would seek evidence of biosignatures on the organic-rich surface and atmosphere of Titan, as well as study its possible prebiotic primordial soup.[91][92] Titan is the largest moon of Saturn and is widely believed to have a large subsurface ocean consisting of a salty brine.[93][94] In addition, scientists believe that Titan may have the conditions necessary to promote prebiotic chemistry, making it a prime candidate for biosignature discovery.[95][96][97]

Europa Clipper

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Europa Clipper

NASA's Europa Clipper probe is designed as a flyby mission to Jupiter's smallest Galilean moon, Europa.[98] Set to launch in 2024, this probe will investigate the potential for habitability on Europa. Europa is one of the best candidates for biosignature discovery in the Solar System because of the scientific consensus that it retains a subsurface ocean, with two to three times the volume of water on Earth. Evidence for this subsurface ocean includes close-up photos taken by Voyager 1,[99] a magnetometer abnormality detected by Galileo,[100] and an image taken by the Hubble Space Telescope which showed evidence for a plume of water vapor coming off the surface.[101][102] The Europa Clipper will carry instruments to help confirm the existence and composition of this ocean.[103]

Enceladus

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An image of the plumes of water and ice coming from the surface of Enceladus. Future missions will investigate these geysers to determine the composition and look for signs of life.

Although there are no set plans to search for biosignatures on Saturn's sixth-largest moon, Enceladus, some missions have entered the planning stage. From 2012, to 2015, research was conducted for the Enceladus Explorer, and similar projects are underway at NASA[104] and the European Space Agency.[105] Similar to Jupiter's moon Europa, there is much evidence for a subsurface ocean to also exist on Enceladus. Plumes of water vapor were first observed in 2005 by the Cassini mission[106][107] and were later determined to contain salt as well as organic compounds.[108][109] In 2014, more evidence was presented using gravimetric measurements on Enceladus to conclude that there is in fact a large reservoir of water underneath an icy surface.[110][111][112] Mission design concepts include:

Searching outside of the Solar System

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At 4.2 light-years (1.3 parsecs, 40 trillion km, or 25 trillion miles) away from Earth, the closest potentially habitable exoplanet is Proxima Centauri b, which was discovered in 2016.[122][123] This means it would take more than 18,100 years to get there if a vessel could consistently travel as fast as the Juno spacecraft (250,000 kilometers per hour or 150,000 miles per hour).[124] It is currently not feasible to send humans or even probes to search for biosignatures outside of the Solar System. The only way to search for biosignatures outside of the Solar System is by observing exoplanets with telescopes.

There have been no plausible or confirmed biosignature detections outside of the Solar System. Despite this, it is a rapidly growing field of research due to the prospects of the next generation of telescopes. The James Webb Space Telescope, which launched in December 2021, will be a promising next step in the search for biosignatures. Although its wavelength range and resolution are not compatible with some of the more important atmospheric biosignature gas bands like oxygen, it is still able able to detect some evidence for oxygen false positive mechanisms.[125]

The new generation of ground-based 30-meter class telescopes (Thirty Meter Telescope and Extremely Large Telescope) will have the ability to take high-resolution spectra of exoplanet atmospheres at a variety of wavelengths.[126] In addition, their large collecting area will enable high angular resolution, making direct imaging studies more feasible.

See also

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References

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