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Biosignature

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(Redirected from Biosignatures)
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 its physical or chemical structures, its use of free energy, and the production of biomass and wastes.

The field of astrobiology uses biosignatures as evidence for the search for past or present extraterrestrial life.

Types

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Biosignatures can be grouped into ten broad categories:[4]

  1. Isotope patterns: Isotopic evidence or patterns that require biological processes.
  2. Chemistry: Chemical features that require biological activity.
  3. Organic matter: Organics formed by biological processes.
  4. Minerals: Minerals or biomineral-phases whose composition and/or morphology indicate biological activity (e.g., biomagnetite).
  5. Microscopic structures and textures: Biologically-formed cements, microtextures, microfossils, and films.
  6. Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, biofilms (e.g., stromatolites), or fossils of larger organisms.
  7. Temporal variability: Variations in time of atmospheric gases, reflectivity, or macroscopic appearance that indicates life's presence.
  8. Surface reflectance features: Large-scale reflectance features due to biological pigments.
  9. Atmospheric gases: Gases formed by metabolic processes, which may be present on a planet-wide scale.
  10. Technosignatures: Signatures that indicate a technologically advanced civilization.[5]

Viability

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Determining whether an observed feature is a true biosignature is complex. There are three criteria that a potential biosignature must meet to be considered viable for further research: 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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A biosignature must be able to dominate over all other processes that may produce similar physical, spectral, and chemical features. When investigating a potential biosignature, scientists must carefully consider all other possible origins of the biosignature in question. 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 (see methanogenesis).[10][11] 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.[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 most latest 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.

False positives

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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, with multiple bands across a relatively wide wavelength range, therefore, it makes a very good biosignature. 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. 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.[14][15][16] Finding and distinguishing a biosignature from its potential false-positive mechanisms is one of the most complicated parts of testing for viability because it relies on human ingenuity to break an abiotic-biological degeneracy, if nature allows.

False negatives

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Opposite to false positives, false negative biosignatures arise in a scenario where life may be present on another planet, but some processes on that planet make potential biosignatures undetectable.[17] This is an ongoing problem and area of research in preparation for future telescopes that will be capable of observing exoplanetary atmospheres.

Human limitations

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There are many ways in which humans may limit the viability of a potential biosignature. The resolution of a telescope becomes important when vetting certain false-positive mechanisms, and many current telescopes do not have the capabilities to observe at the resolution needed to investigate some of these. In addition, probes and telescopes are worked on by huge collaborations of scientists with varying interests. As a result, new probes and telescopes carry a variety of instruments that are a compromise to everyone's unique inputs. For a different type of scientist to detect something unrelated to biosignatures, a sacrifice may have to be made in the capability of an instrument to search for biosignatures.[18]

General examples

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Geomicrobiology

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

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: (a) microfossils and stromatolites; (b) molecular structures (biomarkers) and isotopic compositions of carbon, nitrogen and hydrogen in organic matter; (c) multiple sulfur and oxygen isotope ratios of minerals; and (d) abundance relationships and isotopic compositions of redox-sensitive metals (e.g., Fe, Mo, Cr, and rare earth elements).[19][20]

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.[21] 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.[22] 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 (swirling or dendritic) and are distributed differently from common fluid inclusions.[23] A potential biosignature is a phenomenon that may have been produced by life, but for which alternate abiotic origins may also be possible.

Morphology

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Some researchers suggested that these microscopic structures on the Martian ALH84001 meteorite could be fossilized bacteria.[24][25]

Another possible biosignature might be morphology since the shape and size of certain objects may potentially indicate the presence of past or present life. For example, microscopic magnetite crystals in the Martian meteorite ALH84001[25][26][27] are one of the longest-debated of several potential biosignatures in that specimen.[28] The possible biomineral studied in the Martian ALH84001 meteorite includes putative microbial fossils, tiny rock-like structures whose shape was a potential biosignature because it resembled known bacteria. Most scientists ultimately concluded that these were far too small to be fossilized cells.[29] 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".[30][31][32] Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.[30]

Chemistry

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No single compound will prove life once existed. Rather, it will be distinctive patterns present in any organic compounds showing a process of selection.[33] 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.[33] Biosignatures need not be chemical, however, and can also be suggested by a distinctive magnetic biosignature.[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, 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.[35] Each biomarker is quite distinctive when compared to its counterparts, as the time required for organic matter to convert to crude oil is characteristic.[36] Most biomarkers also usually have high molecular mass.[37]

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.[38] 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.[39]

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.[40] In addition to that they can also be good parameters of age, hence they are technically referred to as "chemical fossils".[41] The ratio of pristane to phytane (pr:ph) is the geochemical factor that allows petroleum biomarkers to be successful indicators of their depositional environments.[42]

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.[43] 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.[44] Such biomarkers are very advantageous and often used in the detection of oil spillage in the major waterways.[35] The same biomarkers can also be used to identify contamination in lubricant oils.[45] 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.[46]

Atmospheric

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The atmospheric properties of exoplanets are of particular importance, as atmospheres provide the most likely observables for the near future, including habitability indicators and biosignatures.[47] 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][48][49] For example, large amounts of oxygen and small amounts of methane are generated by life on Earth.

An exoplanet's color—or reflectance spectrum—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.[50][51][52][53][54] Scientists use the Earth as an example of this when looked at from far away (see Pale Blue Dot) as a comparison to worlds observed outside of our solar system.[55] Ultraviolet radiation on life forms could also induce biofluorescence in visible wavelengths that may be detected by the new generation of space observatories under development.[56][57]

Some scientists have reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.[58][59] Habitability indicators and biosignatures must be interpreted within a planetary and environmental context.[4] For example, the presence of oxygen and methane together could indicate the kind of extreme thermochemical disequilibrium generated by life.[60] Two of the top 14,000 proposed atmospheric biosignatures are dimethyl sulfide and chloromethane (CH
3Cl).[49] An alternative biosignature is the combination of methane and carbon dioxide.[61][62]

The detection of phosphine in the atmosphere of Venus is being investigated as a possible biosignature.

Atmospheric disequilibrium

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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.[63]

A disequilibrium in the abundance of gas species in an atmosphere can be interpreted as a biosignature. Life has greatly altered the atmosphere on Earth in a way that would be unlikely for any other processes to replicate. Therefore, a departure from equilibrium is evidence for a biosignature.[64][65][66][67] For example, the abundance of methane in the Earth's atmosphere is orders of magnitude above the equilibrium value due to the constant methane flux that life on the surface emits.[66][68] Depending on the host star, a disequilibrium in the methane abundance on another planet may indicate a biosignature.[69]

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 from 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.[70] This must involve some sort of chemical disequilibrium, which can be exploited for metabolism.[71][64][65] 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 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 the resources available into energy. The evidence that nothing on an earth is taking advantage of the "free lunch" available due to an observed redox imbalance is called antibiosignatures.[72]

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.[73] Benner and others proposed methods for concentrating and analyzing these polyelectrolyte genetic biopolymers on Mars,[74] Enceladus,[75] and Europa.[76]

Specific examples

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See also: Life on Mars § Possible biosignatures

Methane on Mars

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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.[64][65]

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.[77][78][79][80][81][82] 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.)[83] to as much as 45 ± 10 p.p.b.v.[79]

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.[84] 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.[72] Neither its fast appearance nor disappearance can be explained yet.[85] 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.[86]

Martian atmosphere

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The Martian atmosphere contains high abundances of photochemically produced CO and H2, 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.[87][88] Scientists have used this concept as an argument against life on Mars.[89]

Missions inside the Solar System

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Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life. The usefulness of a biosignature is determined not only by the probability of life creating it but also by the improbability of non-biological (abiotic) processes producing it.[90] Concluding that evidence of an extraterrestrial life form (past or present) has been discovered requires proving that a possible biosignature was produced by the activities or remains of life.[1] As with most scientific discoveries, discovery of a biosignature will require evidence building up until no other explanation exists.

Possible examples of a biosignature include complex organic molecules or structures whose formation is virtually unachievable in the absence of life:[90]

  1. Cellular and extracellular morphologies
  2. Biomolecules in rocks
  3. Bio-organic molecular structures
  4. Chirality
  5. Biogenic minerals
  6. Biogenic isotope patterns in minerals and organic compounds
  7. Atmospheric gases
  8. Photosynthetic pigments

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][91][92][93][94]

Mars Science Laboratory

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Main article: Timeline of Mars Science Laboratory

The Curiosity rover from the Mars Science Laboratory mission, with its Curiosity rover 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] Considering the MSL instrument payload package, the following classes of biosignatures are within the MSL detection window: organism morphologies (cells, body fossils, casts), biofabrics (including microbial mats), diagnostic organic molecules, isotopic signatures, evidence of biomineralization and bioalteration, spatial patterns in chemistry, and biogenic gases.[3] The Curiosity rover targets outcrops to maximize the probability of detecting 'fossilized' organic matter preserved in sedimentary deposits.

ExoMars Orbiter

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The 2016 ExoMars Trace Gas Orbiter (TGO) is a Mars telecommunications orbiter and atmospheric gas analyzer mission. It delivered the Schiaparelli EDM lander and then began to settle into its science orbit to map the sources of methane on Mars and other gases, and in doing so, will help select the landing site for the Rosalind Franklin rover to be launched in 2022.[95] 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.[94][96]

Mars 2020 Rover

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The Mars 2020 rover, which launched in 2020, is intended to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability, the possibility of past life on Mars, and potential for preservation of biosignatures within accessible geological materials.[97][98] In addition, it will cache the most interesting samples for possible future transport to Earth.

Titan Dragonfly

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NASA's Dragonfly[99] lander/aircraft concept is proposed to launch in 2025 and would seek evidence of biosignatures on the organic-rich surface and atmosphere of Titan, as well as study its possible prebiotic primordial soup.[100][101] Titan is the largest moon of Saturn and is widely believed to have a large subsurface ocean consisting of a salty brine.[102][103] In addition, scientists believe that Titan may have the conditions necessary to promote prebiotic chemistry, making it a prime candidate for biosignature discovery.[104][105][106]

See also: Life on Titan

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.[107] The mission launched in October 2024 and is set to reach Europa in April 2030, where it 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:

  • Voyager 1 (1979): The first close-up photos of Europa are taken. Scientists propose that a subsurface ocean could cause the tectonic-like marks on the surface.[108]
  • Galileo (1997): The magnetometer aboard this probe detected a subtle change in the magnetic field near Europa. This was later interpreted as a disruption in the expected magnetic field due to the current induction in a conducting layer on Europa. The composition of this conducting layer is consistent with a salty subsurface ocean.[109]
  • Hubble Space Telescope (2012): An image was taken of Europa which showed evidence for a plume of water vapor coming off the surface.[110][111]

The Europa Clipper probe includes instruments to help confirm the existence and composition of a subsurface ocean and thick icy layer. In addition, the instruments will be used to map and study surface features that may indicate tectonic activity due to a subsurface ocean.[112]

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, the prospects of biosignature discovery there are exciting enough to warrant several mission concepts that may be funded in the future. 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[113][114] and were later determined to contain salt as well as organic compounds.[115][116] 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.[117][118][119] Mission design concepts include:

All of these concept missions have similar science goals: To assess the habitability of Enceladus and search for biosignatures, in line with the strategic map for exploring the ocean-world Enceladus.[130]

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.[131][132] 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).[133] 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 will not be compatible with some of the more important atmospheric biosignature gas bands like oxygen, it will still be able to detect some evidence for oxygen false positive mechanisms.[134]

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.[135] These telescopes will be capable of distinguishing some of the more difficult false positive mechanisms such as the abiotic buildup of oxygen via photolysis. 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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  1. ^ a b c Steele; Beaty; et al. (September 26, 2006). "Final report of the MEPAG Astrobiology Field Laboratory Science Steering Group (AFL-SSG)" (.doc). The Astrobiology Field Laboratory. U.S.: the Mars Exploration Program Analysis Group (MEPAG) - NASA. p. 72.
  2. ^ "Biosignature - definition". Science Dictionary. 2011. Archived from the original on 2010-03-16. Retrieved 2011-01-12.
  3. ^ a b c Summons RE, Amend JP, Bish D, Buick R, Cody GD, Des Marais DJ, et al. (March 2011). "Preservation of martian organic and environmental records: final report of the Mars biosignature working group" (PDF). Astrobiology. 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. hdl:1721.1/66519. PMID 21417945. S2CID 9963677. Archived from the original (PDF) on 2019-11-28. Retrieved 2013-06-22.
  4. ^ a b NASA Astrobiology Strategy 2015 Archived 2016-12-22 at the Wayback Machine.(PDF), NASA.
  5. ^ Frank, Adam (31 December 2020). "A new frontier is opening in the search for extraterrestrial life - The reason we haven't found life elsewhere in the universe is simple: We haven't looked until now". The Washington Post. Retrieved 1 January 2021.
  6. ^ Domagal-Goldman SD, Meadows VS, Claire MW, Kasting JF (June 2011). "Using biogenic sulfur gases as remotely detectable biosignatures on anoxic planets". Astrobiology. 11 (5): 419–41. Bibcode:2011AsBio..11..419D. doi:10.1089/ast.2010.0509. PMC 3133782. PMID 21663401.
  7. ^ Seager S, Schrenk M, Bains W (January 2012). "An astrophysical view of Earth-based metabolic biosignature gases". Astrobiology. 12 (1): 61–82. Bibcode:2012AsBio..12...61S. doi:10.1089/ast.2010.0489. hdl:1721.1/73073. PMID 22269061. S2CID 18142901.
  8. ^ Meadows VS (October 2017). "2 as a Biosignature in Exoplanetary Atmospheres". Astrobiology. 17 (10): 1022–1052. doi:10.1089/ast.2016.1578. PMC 5655594. PMID 28443722.
  9. ^ a b Meadows VS, Reinhard CT, Arney GN, Parenteau MN, Schwieterman EW, Domagal-Goldman SD, et al. (June 2018). "Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment". Astrobiology. 18 (6): 630–662. arXiv:1705.07560. Bibcode:2018AsBio..18..630M. doi:10.1089/ast.2017.1727. PMC 6014580. PMID 29746149.
  10. ^ Ver Eecke HC, Butterfield DA, Huber JA, Lilley MD, Olson EJ, Roe KK, et al. (August 2012). "Hydrogen-limited growth of hyperthermophilic methanogens at deep-sea hydrothermal vents". Proceedings of the National Academy of Sciences of the United States of America. 109 (34): 13674–9. Bibcode:2012PNAS..10913674V. doi:10.1073/pnas.1206632109. PMC 3427048. PMID 22869718.
  11. ^ a b Szostak J (May 2018). "How Did Life Begin?". Nature. 557 (7704): S13–S15. Bibcode:2018Natur.557S..13S. doi:10.1038/d41586-018-05098-w. PMID 29743709.
  12. ^ University of New South Wales (May 9, 2017). "Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks". Phys.org. Retrieved 2019-06-12.
  13. ^ Ward, Colin R.; Walter, Malcolm R.; Campbell, Kathleen A.; Kranendonk, Martin J. Van; Djokic, Tara (2017-05-09). "Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits". Nature Communications. 8: 15263. Bibcode:2017NatCo...815263D. doi:10.1038/ncomms15263. ISSN 2041-1723. PMC 5436104. PMID 28486437.
  14. ^ Luger R, Barnes R (February 2015). "Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs". Astrobiology. 15 (2): 119–43. arXiv:1411.7412. Bibcode:2015AsBio..15..119L. doi:10.1089/ast.2014.1231. PMC 4323125. PMID 25629240.
  15. ^ Wordsworth, Robin; Pierrehumbert, Raymond (1 April 2014). "Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets". The Astrophysical Journal. 785 (2): L20. arXiv:1403.2713. Bibcode:2014ApJ...785L..20W. doi:10.1088/2041-8205/785/2/L20. S2CID 17414970.
  16. ^ a b Lisse, Carey (2020). "A Geologically Robust Procedure for Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen Is a Modern Earth-like Biosignature". Astrophysical Journal Letters. 898 (577): L17. arXiv:2006.07403. Bibcode:2020ApJ...898L..17L. doi:10.3847/2041-8213/ab9b91. S2CID 219687224.
  17. ^ Reinhard, Christopher T.; Olson, Stephanie L.; Schwieterman, Edward W.; Lyons, Timothy W. (April 2017). "False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth". Astrobiology. 17 (4): 287–297. arXiv:1702.01137. Bibcode:2017AsBio..17..287R. doi:10.1089/ast.2016.1598. PMC 5399744. PMID 28418704.
  18. ^ Board, Space Studies (2010-08-13). New Worlds, New Horizons in Astronomy and Astrophysics. National Academies Press. ISBN 978-0-309-15799-5.
  19. ^ "SIGNATURES OF LIFE FROM EARTH AND BEYOND". Penn State Astrobiology Research Center (PSARC). Penn State. 2009. Archived from the original on 2018-10-23. Retrieved 2011-01-14.
  20. ^ Tenenbaum, David (July 30, 2008). "Reading Archaean Biosignatures". NASA. Archived from the original on November 29, 2014. Retrieved 2014-11-23.
  21. ^ "Fatty alcohols". Archived from the original on 2012-06-25. Retrieved 2006-04-01.
  22. ^ a b Beegle LW, Wilson MG, Abilleira F, Jordan JF, Wilson GR (August 2007). "A concept for NASA's Mars 2016 astrobiology field laboratory". Astrobiology. 7 (4): 545–77. Bibcode:2007AsBio...7..545B. doi:10.1089/ast.2007.0153. PMID 17723090. S2CID 7127896.
  23. ^ Bosak, Tanja; Souza-Egipsy, Virginia; Corsetti, Frank A.; Newman, Dianne K. (2004). "Micrometer-scale porosity as a biosignature in carbonate crusts". Geology. 32 (9): 781. Bibcode:2004Geo....32..781B. doi:10.1130/G20681.1.
  24. ^ Crenson M (2006-08-06). "After 10 years, few believe life on Mars". Associated Press (on usatoday.com). Retrieved 2009-12-06.
  25. ^ a b McKay DS, Gibson EK, Thomas-Keprta KL, Vali H, Romanek CS, Clemett SJ, et al. (August 1996). "Search for past life on Mars: possible relic biogenic activity in martian meteorite ALH84001". Science. 273 (5277): 924–30. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069. S2CID 40690489.
  26. ^ Friedmann EI, Wierzchos J, Ascaso C, Winklhofer M (February 2001). "Chains of magnetite crystals in the meteorite ALH84001: evidence of biological origin". Proceedings of the National Academy of Sciences of the United States of America. 98 (5): 2176–81. doi:10.1073/pnas.051514698. PMC 30112. PMID 11226212.
  27. ^ Thomas-Keprta KL, Clemett SJ, Bazylinski DA, Kirschvink JL, McKay DS, Wentworth SJ, et al. (February 2001). "Truncated hexa-octahedral magnetite crystals in ALH84001: presumptive biosignatures". Proceedings of the National Academy of Sciences of the United States of America. 98 (5): 2164–9. doi:10.1073/pnas.051500898. PMC 30110. PMID 11226210.
  28. ^ Choi CQ (August 2016). "Mars Life? 20 Years Later, Debate Over Meteorite Continues". Space.com. Retrieved 2019-06-07.
  29. ^ McSween HY (2019), "The Search for Biosignatures in Martian Meteorite Allan Hills 84001", in Cavalazzi B, Westall F (eds.), Biosignatures for Astrobiology, Advances in Astrobiology and Biogeophysics, Springer International Publishing, pp. 167–182, doi:10.1007/978-3-319-96175-0_8, ISBN 978-3-319-96175-0, S2CID 186696892
  30. ^ a b Garcia-Ruiz JM (December 30, 1999). "Morphological behavior of inorganic precipitation systems". In Hoover RB (ed.). Instruments, Methods, and Missions for Astrobiology II. Vol. SPIE Proceedings 3755. p. 74. doi:10.1117/12.375088. S2CID 84764520. It is concluded that "morphology cannot be used unambiguously as a tool for primitive life detection".
  31. ^ Agresti; House; Jögi; Kudryavstev; McKeegan; Runnegar; Schopf; Wdowiak (3 December 2008). "Detection and geochemical characterization of Earth's earliest life". NASA Astrobiology Institute. NASA. Archived from the original on 23 January 2013. Retrieved 2013-01-15.
  32. ^ Schopf JW, Kudryavtsev AB, Czaja AD, Tripathi AB (28 April 2007). "Evidence of Archean life: Stromatolites and microfossils" (PDF). Precambrian Research. 158 (3–4): 141–155. Bibcode:2007PreR..158..141S. doi:10.1016/j.precamres.2007.04.009. Archived from the original (PDF) on 2012-12-24. Retrieved 2013-01-15.
  33. ^ a b Cousins, Claire (5 January 2018). "Rover could discover life on Mars – here's what it would take to prove it". PhysOrg.
  34. ^ Wall, Mike (13 December 2011). "Mars Life Hunt Could Look for Magnetic Clues". Space.com. Retrieved 2011-12-15.
  35. ^ a b Wang, Z.; Stout, S.; Fingas, M. Environmental Forensics, 2006 7, 105-146.
  36. ^ Stevens, Douglas; Hsu, Chang Samuel; Shi, Quan (2013). "Petroleum biomarkers analyzed by atmospheric gas chromatography-tandem mass spectroscopy" (PDF). Waters.
  37. ^ Osadetz, K.G; Pasadakis, N.; Obermajer, M. (2002). "Definition and characterization of petroleum compositional families using principal component analysis of gasoline and saturate fraction composition ratios" (PDF). Energy and Resources. 1: 3–14.
  38. ^ Hsu, Chang S.; Walters, Clifford; Peters, Kenneth E. (2003). Analytical advances for hydrocarbon research. pp. 223–245.
  39. ^ Niessen, Wilfried M.A. (2001). Current Practice of gas chromatography-mass spectrometry (1 ed.). pp. 55–94.
  40. ^ Chosson, P; Lanau, C; Connan, J; Dessort, D (1991). "Biodegradation of refractory hydrocarbon biomarkers from petroleum under laboratory conditions". Nature. 351 (6328): 640–642. Bibcode:1991Natur.351..640C. doi:10.1038/351640a0. PMID 2052089. S2CID 4305795.
  41. ^ Wang, Zhendi; Stout, Scott A. (2007). Oil spill environmental forensics: fingerprinting and source identification. pp. 1–53.
  42. ^ Roushdy, M.I.; El Nady, M.M.; Mostafa, Y.M.; El Gendy, N.Sh.; Ali, H.R. (2010). "Biomarkers characteristics of crude oils from some oilfields in the gulf of suez, egypt". Journal of American Science. 6 (11). S2CID 55952894.
  43. ^ Head, Ian M.; Jones, Martin; Larter, Steve R. (2003). "Biological activity in the deep subsurface and the origin of heavy oil" (PDF). Nature. 426 (6964): 344–352. Bibcode:2003Natur.426..344H. doi:10.1038/nature02134. PMID 14628064. S2CID 4372154.
  44. ^ Ashton, Buffy M.; East, Rebecca S.; Walsh, Maud M.; Miles, Scott; Obeton, Edward B. (2000). "Studying and Verifying the Use of Chemical Biomarkers for Identifying and Quantitating Oil Residues in the Environment". Journal of Ocean and Climate Systems: 1–54. S2CID 201925529.
  45. ^ Bieger, Tilman; Hellou, Jocelyne; Abrajano Jr., Teofilou A. (1996). "Petroleum biomarkers as tracers of lubricating oil contamination". Marine Pollution Bulletin. 32 (2): 270–274. Bibcode:1996MarPB..32..270B. doi:10.1016/0025-326X(95)00151-C.
  46. ^ Ratnayake, Amila Sandaruwan; Sampei, Yoshikazu (2019-06-01). "Organic geochemical evaluation of contamination tracers in deepwater well rock cuttings from the Mannar Basin, Sri Lanka". Journal of Petroleum Exploration and Production Technology. 9 (2): 989–996. Bibcode:2019JPEPT...9..989R. doi:10.1007/s13202-018-0575-8. ISSN 2190-0566.
  47. ^ Gertner, Jon (15 September 2022). "The Search for Intelligent Life Is About to Get a Lot More Interesting - There are an estimated 100 billion galaxies in the universe, home to an unimaginable abundance of planets. And now, there are new ways to spot signs of life on them". The New York Times. Retrieved 15 September 2022.
  48. ^ "Artificial Life Shares Biosignature With Terrestrial Cousins". The Physics arXiv Blog. MIT. 10 January 2011. Archived from the original on 2018-10-23. Retrieved 2011-01-14.
  49. ^ a b Seager S, Bains W, Petkowski JJ (June 2016). "Toward a List of Molecules as Potential Biosignature Gases for the Search for Life on Exoplanets and Applications to Terrestrial Biochemistry" (PDF). Astrobiology. 16 (6): 465–85. Bibcode:2016AsBio..16..465S. doi:10.1089/ast.2015.1404. hdl:1721.1/109943. PMID 27096351. S2CID 4350250.
  50. ^ DasSarma, Shiladitya; Schwieterman, Edward W. (2018). "Early evolution of purple retinal pigments on Earth and implications for exoplanet biosignatures". International Journal of Astrobiology. 20 (3): 1–10. arXiv:1810.05150. Bibcode:2018arXiv181005150D. doi:10.1017/S1473550418000423. ISSN 1473-5504. S2CID 119341330.
  51. ^ Berdyugina SV, Kuhn J, Harrington D, Santl-Temkiv T, Messersmith EJ (January 2016). "Remote sensing of life: polarimetric signatures of photosynthetic pigments as sensitive biomarkers". International Journal of Astrobiology. 15 (1): 45–56. Bibcode:2016IJAsB..15...45B. doi:10.1017/S1473550415000129.
  52. ^ Hegde S, Paulino-Lima IG, Kent R, Kaltenegger L, Rothschild L (March 2015). "Surface biosignatures of exo-earths: remote detection of extraterrestrial life". Proceedings of the National Academy of Sciences of the United States of America. 112 (13): 3886–91. Bibcode:2015PNAS..112.3886H. doi:10.1073/pnas.1421237112. PMC 4386386. PMID 25775594.
  53. ^ Cofield C (30 March 2015). "Catalog of Earth Microbes Could Help Find Alien Life". Space.com. Retrieved 2015-05-11.
  54. ^ Claudi, R.; Erculiani, M. S.; Galletta, G.; Billi, D.; Pace, E.; Schierano, D.; Giro, E.; D'Alessandro, M. (20 May 2015). "Simulating super earth atmospheres in the laboratory". International Journal of Astrobiology. 15 (1): 35–44. doi:10.1017/S1473550415000117. S2CID 125008098.
  55. ^ Krissansen-Totton J, Schwieterman EW, Charnay B, Arney G, Robinson TD, Meadows V, Catling DC (January 2016). "Is the Pale Blue Dot unique? Optimized photometric bands for identifying Earth-like exoplanets". The Astrophysical Journal. 817 (1): 31. arXiv:1512.00502. Bibcode:2016ApJ...817...31K. doi:10.3847/0004-637X/817/1/31. S2CID 119211858.
  56. ^ Cornell University (13 August 2019). "Fluorescent glow may reveal hidden life in the cosmos". EurekAlert!. Retrieved 13 August 2019.
  57. ^ O'Malley-James, Jack T; Kaltenegger, Lisa (2019). "Biofluorescent Worlds – II. Biological fluorescence induced by stellar UV flares, a new temporal biosignature". Monthly Notices of the Royal Astronomical Society. 488 (4): 4530–4545. arXiv:1608.06930. Bibcode:2019MNRAS.488.4530O. doi:10.1093/mnras/stz1842. S2CID 118394043.
  58. ^ Brogi M, Snellen IA, de Kok RJ, Albrecht S, Birkby J, de Mooij EJ (June 2012). "The signature of orbital motion from the dayside of the planet τ Boötis b". Nature. 486 (7404): 502–4. arXiv:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161. PMID 22739313. S2CID 4368217.
  59. ^ Mann, Adam (June 27, 2012). "New View of Exoplanets Will Aid Search for E.T." Wired. Retrieved June 28, 2012.
  60. ^ Where are they? (PDF) Mario Livio and Joseph Silk. Physics Today, March 2017.
  61. ^ Wall, Mike (24 January 2018). "Alien Life Hunt: Oxygen Isn't the Only Possible Sign of Life". Space.com. Retrieved 24 January 2018.
  62. ^ Krissansen-Totton J, Olson S, Catlig DC (24 January 2018). "Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life". Science Advances. 4 (1, eaao5747): eaao5747. arXiv:1801.08211. Bibcode:2018SciA....4.5747K. doi:10.1126/sciadv.aao5747. PMC 5787383. PMID 29387792.
  63. ^ Arney, Giada N. (March 2019). "The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets". The Astrophysical Journal. 873 (1): L7. arXiv:2001.10458. Bibcode:2019ApJ...873L...7A. doi:10.3847/2041-8213/ab0651. ISSN 2041-8205. S2CID 127742050.
  64. ^ a b c Lovelock JE (August 1965). "A physical basis for life detection experiments". Nature. 207 (997): 568–70. Bibcode:1965Natur.207..568L. doi:10.1038/207568a0. PMID 5883628. S2CID 33821197.
  65. ^ a b c Hitchcock DR, Lovelock JE (1967-01-01). "Life detection by atmospheric analysis". Icarus. 7 (1): 149–159. Bibcode:1967Icar....7..149H. doi:10.1016/0019-1035(67)90059-0. ISSN 0019-1035.
  66. ^ a b Krissansen-Totton J, Bergsman DS, Catling DC (January 2016). "On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres". Astrobiology. 16 (1): 39–67. arXiv:1503.08249. Bibcode:2016AsBio..16...39K. doi:10.1089/ast.2015.1327. PMID 26789355. S2CID 26959254.
  67. ^ Lovelock James Ephraim; Kaplan I. R.; Pirie Norman Wingate (1975-05-06). "Thermodynamics and the recognition of alien biospheres". Proceedings of the Royal Society of London. Series B. Biological Sciences. 189 (1095): 167–181. Bibcode:1975RSPSB.189..167L. doi:10.1098/rspb.1975.0051. S2CID 129105448.
  68. ^ Krissansen-Totton J, Arney GN, Catling DC (April 2018). "Constraining the climate and ocean pH of the early Earth with a geological carbon cycle model". Proceedings of the National Academy of Sciences of the United States of America. 115 (16): 4105–4110. arXiv:1804.00763. Bibcode:2018PNAS..115.4105K. doi:10.1073/pnas.1721296115. PMC 5910859. PMID 29610313.
  69. ^ Arney, Giada N. (March 2019). "The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets". The Astrophysical Journal. 873 (1): L7. arXiv:2001.10458. Bibcode:2019ApJ...873L...7A. doi:10.3847/2041-8213/ab0651. ISSN 2041-8205.
  70. ^ Benner SA (December 2010). "Defining life". Astrobiology. 10 (10): 1021–30. Bibcode:2010AsBio..10.1021B. doi:10.1089/ast.2010.0524. PMC 3005285. PMID 21162682.
  71. ^ National Academies Of Sciences Engineering; Division on Engineering Physical Sciences; Space Studies Board; Committee on Astrobiology Science Strategy for the Search for Life in the Universe (2019). Read "An Astrobiology Strategy for the Search for Life in the Universe" at NAP.edu. doi:10.17226/25252. ISBN 978-0-309-48416-9. PMID 30986006. S2CID 243600456.
  72. ^ a b Zahnle K, Freedman RS, Catling DC (2011-04-01). "Is there methane on Mars?". Icarus. 212 (2): 493–503. Bibcode:2011Icar..212..493Z. doi:10.1016/j.icarus.2010.11.027. ISSN 0019-1035.
  73. ^ Benner, Steven A.; Hutter, Daniel (2002-02-01). "Phosphates, DNA, and the Search for Nonterrean Life: A Second Generation Model for Genetic Molecules". Bioorganic Chemistry. 30 (1): 62–80. doi:10.1006/bioo.2001.1232. PMID 11955003.
  74. ^ Špaček, Jan; Benner, Steven A. (2022-10-01). "Agnostic Life Finder (ALF) for Large-Scale Screening of Martian Life During In Situ Refueling". Astrobiology. 22 (10): 1255–1263. Bibcode:2022AsBio..22.1255S. doi:10.1089/ast.2021.0070. ISSN 1531-1074. PMID 35796703.
  75. ^ Benner, Steven A. (2017). "Detecting Darwinism from Molecules in the Enceladus Plumes, Jupiter's Moons, and Other Planetary Water Lagoons". Astrobiology. 17 (9): 840–851. Bibcode:2017AsBio..17..840B. doi:10.1089/ast.2016.1611. ISSN 1531-1074. PMC 5610385. PMID 28665680.
  76. ^ Sutton, Mark A.; Burton, Aaron S.; Zaikova, Elena; Sutton, Ryan E.; Brinckerhoff, William B.; Bevilacqua, Julie G.; Weng, Margaret M.; Mumma, Michael J.; Johnson, Sarah Stewart (2019-03-29). "Radiation Tolerance of Nanopore Sequencing Technology for Life Detection on Mars and Europa". Scientific Reports. 9 (1): 5370. Bibcode:2019NatSR...9.5370S. doi:10.1038/s41598-019-41488-4. ISSN 2045-2322. PMC 6441015. PMID 30926841.
  77. ^ Krasnopolsky VA, Maillard JP, Owen TC (2004-12-01). "Detection of methane in the martian atmosphere: evidence for life?". Icarus. 172 (2): 537–547. Bibcode:2004Icar..172..537K. doi:10.1016/j.icarus.2004.07.004. ISSN 0019-1035.
  78. ^ Formisano V, Atreya S, Encrenaz T, Ignatiev N, Giuranna M (December 2004). "Detection of methane in the atmosphere of Mars". Science. 306 (5702): 1758–61. Bibcode:2004Sci...306.1758F. doi:10.1126/science.1101732. PMID 15514118. S2CID 13533388.
  79. ^ a b Mumma MJ, Villanueva GL, Novak RE, Hewagama T, Bonev BP, Disanti MA, et al. (February 2009). "Strong release of methane on Mars in northern summer 2003". Science. 323 (5917): 1041–5. Bibcode:2009Sci...323.1041M. doi:10.1126/science.1165243. PMID 19150811. S2CID 25083438.
  80. ^ Krasnopolsky VA (2012-01-01). "Search for methane and upper limits to ethane and SO2 on Mars". Icarus. 217 (1): 144–152. Bibcode:2012Icar..217..144K. doi:10.1016/j.icarus.2011.10.019. ISSN 0019-1035.
  81. ^ Webster CR, Mahaffy PR, Atreya SK, Flesch GJ, Mischna MA, Meslin PY, et al. (January 2015). "Mars atmosphere. Mars methane detection and variability at Gale crater" (PDF). Science. 347 (6220): 415–7. Bibcode:2015Sci...347..415W. doi:10.1126/science.1261713. PMID 25515120. S2CID 20304810.
  82. ^ Amoroso M, Merritt D, Parra JM, Cardesín-Moinelo A, Aoki S, Wolkenberg P, Alessandro Aronica, Formisano V, Oehler D (May 2019). "Independent confirmation of a methane spike on Mars and a source region east of Gale Crater". Nature Geoscience. 12 (5): 326–332. Bibcode:2019NatGe..12..326G. doi:10.1038/s41561-019-0331-9. ISSN 1752-0908. S2CID 134110253.
  83. ^ Webster CR, Mahaffy PR, Atreya SK, Moores JE, Flesch GJ, Malespin C, et al. (June 2018). "Background levels of methane in Mars' atmosphere show strong seasonal variations". Science. 360 (6393): 1093–1096. Bibcode:2018Sci...360.1093W. doi:10.1126/science.aaq0131. PMID 29880682.
  84. ^ Korablev O, Vandaele AC, Montmessin F, Fedorova AA, Trokhimovskiy A, Forget F, et al. (April 2019). "No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations" (PDF). Nature. 568 (7753): 517–520. Bibcode:2019Natur.568..517K. doi:10.1038/s41586-019-1096-4. PMID 30971829. S2CID 106411228.
  85. ^ Mars Trace Gas Mission Archived 2011-07-21 at the Wayback Machine (September 10, 2009).
  86. ^ Remote Sensing Tutorial, Section 19-13a Archived 2011-10-21 at the Wayback Machine - Missions to Mars during the Third Millennium, Nicholas M. Short Sr., et al., NASA.
  87. ^ Catling DC, Krissansen-Totton J, Kiang NY, Crisp D, Robinson TD, DasSarma S, et al. (June 2018). "Exoplanet Biosignatures: A Framework for Their Assessment". Astrobiology. 18 (6): 709–738. arXiv:1705.06381. Bibcode:2018AsBio..18..709C. doi:10.1089/ast.2017.1737. PMC 6049621. PMID 29676932.
  88. ^ Wang Y, Tian F, Li T, Hu Y (2016-03-01). "On the detection of carbon monoxide as an anti-biosignature in exoplanetary atmospheres". Icarus. 266: 15–23. Bibcode:2016Icar..266...15W. doi:10.1016/j.icarus.2015.11.010. ISSN 0019-1035.
  89. ^ Sholes SF, Krissansen-Totton J, Catling DC (May 2019). "2 as Potential Antibiosignatures". Astrobiology. 19 (5): 655–668. arXiv:1811.08501. Bibcode:2019AsBio..19..655S. doi:10.1089/ast.2018.1835. PMID 30950631. S2CID 96435170.
  90. ^ a b Rothschild, Lynn (September 2003). "Understand the evolutionary mechanisms and environmental limits of life". NASA. Archived from the original on 2011-01-26. Retrieved 2009-07-13.
  91. ^ Levin, G and P. Straaf. 1976. Viking Labeled Release Biology Experiment: Interim Results. Science: vol: 194. pp: 1322-1329.
  92. ^ Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0.
  93. ^ Klein HP, Horowitz NH, Levin GV, Oyama VI, Lederberg J, Rich A, et al. (October 1976). "The viking biological investigation: preliminary results". Science. 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090. S2CID 24957458.
  94. ^ a b "European Space Agency". www.esa.int.
  95. ^ Pavlishchev, Boris (Jul 15, 2012). "ExoMars program gathers strength". The Voice of Russia. Archived from the original on 2012-08-06. Retrieved 2012-07-15.
  96. ^ "Mars Science Laboratory: Mission". NASA/JPL. Archived from the original on 2006-03-05. Retrieved 2010-03-12.
  97. ^ Chang, Alicia (July 9, 2013). "Panel: Next Mars rover should gather rocks, soil". Associated Press. Retrieved July 12, 2013.
  98. ^ Schulte, Mitch (December 20, 2012). "Call for Letters of Application for Membership on the Science Definition Team for the 2020 Mars Science Rover" (PDF). NASA. NNH13ZDA003L.
  99. ^ "Dragonfly". dragonfly.jhuapl.edu. Retrieved 2019-06-07.
  100. ^ Dragonfly: Exploring Titan's Surface with a New Frontiers Relocatable Lander. American Astronomical Society, DPS meeting #49, id.219.02. October 2017.
  101. ^ Turtle P, Barnes JW, Trainer MG, Lorenz RD, MacKenzie SM, Hibbard KE, Adams D, Bedini P, Langelaan JW, Zacny K (2017). Dragonfly: Exploring titan's prebiotic organic chemistry and habitability (PDF). Lunar and Planetary Science Conference.
  102. ^ Fortes AD (2000-08-01). "Exobiological Implications of a Possible Ammonia–Water Ocean inside Titan". Icarus. 146 (2): 444–452. Bibcode:2000Icar..146..444F. doi:10.1006/icar.2000.6400. ISSN 0019-1035.
  103. ^ Grasset O, Sotin C, Deschamps F (2000-06-01). "On the internal structure and dynamics of Titan". Planetary and Space Science. 48 (7): 617–636. Bibcode:2000P&SS...48..617G. doi:10.1016/S0032-0633(00)00039-8. ISSN 0032-0633.
  104. ^ JPL/NASA (April 3, 2013). "NASA team investigates complex chemistry at Titan". Phys.org. Retrieved 2019-06-07.
  105. ^ Desai, Ravi (July 27, 2017). "Saturn's moon Titan may harbour simple life forms – and reveal how organisms first formed on Earth". The Conversation. Retrieved 2019-06-07.
  106. ^ Gudipati MS, Jacovi R, Couturier-Tamburelli I, Lignell A, Allen M (2013-04-03). "Photochemical activity of Titan's low-altitude condensed haze". Nature Communications. 4: 1648. Bibcode:2013NatCo...4.1648G. doi:10.1038/ncomms2649. PMID 23552063.
  107. ^ "Europa Clipper". www.jpl.nasa.gov. Retrieved 2019-06-07.
  108. ^ Smith BA, Soderblom LA, Johnson TV, Ingersoll AP, Collins SA, Shoemaker EM, et al. (June 1979). "The jupiter system through the eyes of voyager 1". Science. 204 (4396): 951–72. Bibcode:1979Sci...204..951S. doi:10.1126/science.204.4396.951. PMID 17800430. S2CID 33147728.
  109. ^ Kivelson MG, Khurana KK, Russell CT, Volwerk M, Walker RJ, Zimmer C (August 2000). "Galileo magnetometer measurements: a stronger case for a subsurface ocean at Europa". Science. 289 (5483): 1340–3. Bibcode:2000Sci...289.1340K. doi:10.1126/science.289.5483.1340. PMID 10958778. S2CID 44381312.
  110. ^ "Hubble discovers water vapour venting from Jupiter's moon Europa". www.spacetelescope.org. Retrieved 2019-06-07.
  111. ^ "Photo composite of suspected water plumes on Europa". www.spacetelescope.org. Retrieved 2019-06-07.
  112. ^ Phillips CB, Pappalardo RT (2014-05-20). "Europa Clipper Mission Concept: Exploring Jupiter's Ocean Moon". Eos, Transactions American Geophysical Union. 95 (20): 165–167. Bibcode:2014EOSTr..95..165P. doi:10.1002/2014EO200002.
  113. ^ Porco CC, Helfenstein P, Thomas PC, Ingersoll AP, Wisdom J, West R, et al. (March 2006). "Cassini observes the active south pole of Enceladus" (PDF). Science. 311 (5766): 1393–401. Bibcode:2006Sci...311.1393P. doi:10.1126/science.1123013. PMID 16527964. S2CID 6976648.
  114. ^ "Enceladus rains water onto Saturn". European Space Agency. 26 July 2011. Retrieved 2019-06-07.
  115. ^ Postberg F, Schmidt J, Hillier J, Kempf S, Srama R (June 2011). "A salt-water reservoir as the source of a compositionally stratified plume on Enceladus". Nature. 474 (7353): 620–2. Bibcode:2011Natur.474..620P. doi:10.1038/nature10175. PMID 21697830. S2CID 4400807.
  116. ^ "Cassini samples the icy spray of Enceladus' water plumes". European Space Agency. 22 June 2011. Retrieved 2019-06-07.
  117. ^ Witze, Alexandra (2014). "Icy Enceladus hides a watery ocean". Nature News. doi:10.1038/nature.2014.14985. S2CID 131145017.
  118. ^ Iess, L.; Stevenson, D. J.; Parisi, M.; Hemingway, D.; Jacobson, R.A.; Lunine, Jonathan I.; Nimmo, F.; Armstrong, J. W.; Asmar, S. W.; Ducci, M.; Tortora, P. (April 4, 2014). "The Gravity Field and Interior Structure of Enceladus" (PDF). Science. 344 (6179): 78–80. Bibcode:2014Sci...344...78I. doi:10.1126/science.1250551. PMID 24700854. S2CID 28990283.
  119. ^ Amos, Jonathan (2014-04-03). "Saturn moon hides 'great lake'". Retrieved 2019-06-07.
  120. ^ Reh, K.; Spilker, L.; Lunine, Jonathan I.; Waite Jr., Jack Hunter; Cable, M. L.; Postberg, Frank; Clark, K. (March 2016). "Enceladus Life Finder: The search for life in a habitable Moon". 2016 IEEE Aerospace Conference. pp. 1–8. doi:10.1109/AERO.2016.7500813. ISBN 978-1-4673-7676-1. S2CID 22950150.
  121. ^ Clark, Stephen (2015-04-06). "Diverse destinations considered for new interplanetary probe". Spaceflight Now. Retrieved 2019-06-07.
  122. ^ "Future Planetary Exploration: Proposed New Frontiers Missions". Future Planetary Exploration. 2017-08-04. Archived from the original on 2017-09-20. Retrieved 2019-06-07.
  123. ^ "EOA – Enceladus Organic Analyzer". Retrieved 2019-06-07.
  124. ^ Konstantinidis, Konstantinos; Flores Martinez, Claudio L.; Dachwald, Bernd; Ohndorf, Andreas; Dykta, Paul; Bowitz, Pascal; Rudolph, Martin; Digel, Ilya; Kowalski, Julia; Voigt, Konstantin; Förstner, Roger (January 2015). "A lander mission to probe subglacial water on Saturn׳s moon Enceladus for life". Acta Astronautica. 106: 63–89. Bibcode:2015AcAau.106...63K. doi:10.1016/j.actaastro.2014.09.012.
  125. ^ "E2T - Explorer of Enceladus and Titan". E2T - Explorer of Enceladus and Titan. Retrieved 2019-06-07.
  126. ^ Voosen, Paul (2017-01-04). "Updated: NASA taps missions to tiny metal world and Jupiter Trojans". Science | AAAS. Retrieved 2019-06-07.
  127. ^ Sotin C, Altwegg K, Brown RH, Hand K, Lunine JI, Soderblom J, Spencer J, Tortora P, JET Team (2011). JET: Journey to Enceladus and Titan. 42nd Lunar and Planetary Science Conference. p. 1326. Bibcode:2011LPI....42.1326S.
  128. ^ Tsou P, Brownlee DE, McKay CP, Anbar AD, Yano H, Altwegg K, et al. (August 2012). "LIFE: Life Investigation For Enceladus A Sample Return Mission Concept in Search for Evidence of Life". Astrobiology. 12 (8): 730–42. Bibcode:2012AsBio..12..730T. doi:10.1089/ast.2011.0813. PMID 22970863. S2CID 34375065.
  129. ^ MacKenzie SM, Caswell TE, Phillips-Lander CM, Stavros EN, Hofgartner JD, Sun VZ, Powell KE, Steuer CJ, O'Rourke JG, Dhaliwal JK, Leung CW (2016-09-15). "THEO concept mission: Testing the Habitability of Enceladus's Ocean". Advances in Space Research. 58 (6): 1117–1137. arXiv:1605.00579. Bibcode:2016AdSpR..58.1117M. doi:10.1016/j.asr.2016.05.037. ISSN 0273-1177. S2CID 119112894.
  130. ^ Sherwood B (2016-09-01). "Strategic map for exploring the ocean-world Enceladus". Acta Astronautica. Space Flight Safety. 126: 52–58. Bibcode:2016AcAau.126...52S. doi:10.1016/j.actaastro.2016.04.013. ISSN 0094-5765. S2CID 112827329.
  131. ^ Anglada-Escudé, Guillem; Amado, Pedro J.; Barnes, John; et al. (2016). "A terrestrial planet candidate in a temperate orbit around Proxima Centauri". Nature. 536 (7617): 437–440. arXiv:1609.03449. Bibcode:2016Natur.536..437A. doi:10.1038/nature19106. PMID 27558064. S2CID 4451513.
  132. ^ Meadows VS, Arney GN, Schwieterman EW, Lustig-Yaeger J, Lincowski AP, Robinson T, et al. (February 2018). "The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants". Astrobiology. 18 (2): 133–189. arXiv:1608.08620. Bibcode:2018AsBio..18..133M. doi:10.1089/ast.2016.1589. PMC 5820795. PMID 29431479.
  133. ^ "How Fast Can Juno Go?". Mission Juno. Retrieved 2019-06-08.
  134. ^ Lincowski AP, Meadows VS, Lustig-Yaeger J (2019-05-17). "The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST". The Astronomical Journal. 158 (1): 27. arXiv:1905.07070v1. Bibcode:2019AJ....158...27L. doi:10.3847/1538-3881/ab21e0. S2CID 158046684.
  135. ^ Crossfield IJ (2016-04-21). "Exoplanet Atmospheres and Giant Ground-Based Telescopes". arXiv:1604.06458v1 [astro-ph.IM].
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