Biosignature

Source: Wikipedia, the free encyclopedia.
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

origin of life on Earth as well as the possibility of life on Mars, Venus
and elsewhere in the universe.

Types

Biosignatures can be grouped into ten broad categories:[5]

  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.[6]

Viability

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.[7][8][9][10]

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.[10]

Reliability

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).[11][12] 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.[12]

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.[13][14] 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 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

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.[15][16][17] 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

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.[18] This is an ongoing problem and area of research in preparation for future telescopes that will be capable of observing exoplanetary atmospheres.

Human limitations

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.[19]

General examples

Geomicrobiology

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).[20][21]

For example, the particular

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

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.[24] A potential biosignature is a phenomenon that may have been produced by life, but for which alternate abiotic
origins may also be possible.

Morphology

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.[30] 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

microbial life.[34]

No single compound will prove life once existed. Rather, it will be distinctive patterns present in any organic compounds showing a process of selection.

magnetic biosignature.[36]

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

Chemical biosignatures include any suite of complex

living organisms.[37] Each biomarker is quite distinctive when compared to its counterparts, as the time required for organic matter to convert to crude oil is characteristic.[38] Most biomarkers also usually have high molecular mass.[39]

Some examples of biomarkers found in petroleum are

angiosperm plants.[40] 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.[41]

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.[42] In addition to that they can also be good parameters of age, hence they are technically referred to as "chemical fossils".[43] The ratio of pristane to phytane (pr:ph) is the geochemical factor that allows petroleum biomarkers to be successful indicators of their depositional environments.[44]

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.[45] 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.[46] Such biomarkers are very advantageous and often used in the detection of oil spillage in the major waterways.[37] The same biomarkers can also be used to identify contamination in lubricant oils.[47] 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.[48]

Atmospheric

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.[49] 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.[17][50][51] 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.[52][53][54][55][56] 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.[57] 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.[58][59]

Some scientists have reported methods of detecting hydrogen and methane in

extraterrestrial atmospheres.[60][61] Habitability indicators and biosignatures must be interpreted within a planetary and environmental context.[5] For example, the presence of oxygen and methane together could indicate the kind of extreme thermochemical disequilibrium generated by life.[62] Two of the top 14,000 proposed atmospheric biosignatures are dimethyl sulfide and chloromethane (CH
3Cl).[51] An alternative biosignature is the combination of methane and carbon dioxide.[63][64]

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.[65]

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.[66][67][68][69] 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.[68][70] Depending on the host star, a disequilibrium in the methane abundance on another planet may indicate a biosignature.[71]

Methane on Mars

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. With life being the strongest source of methane on Earth, continued observation of such a disequilibrium could be a viable biosignature.[66][67] Current photochemical models cannot explain the reported rapid variations in space and time.[72] Neither its fast appearance nor disappearance have been explained.[73] Because of its tendency to be destroyed in the atmosphere by photochemistry, excess methane could indicate that there must be an active source.[citation needed]

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.[74][75][76][77][78][79][excessive citations] 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.)[80] to as much as 45 ± 10 p.p.b.v.[76] However 2019 measurements put an upper bound on the overall methane abundance at 0.05 p.p.b.v[81] contradicting previous observations.

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.[82]

Phosphine on Venus

The atmosphere of Venus appears darker and lined with shadows. The shadows trace the prevailing wind direction.
Cloud structure of the Venusian atmosphere (made visible through ultraviolet imaging) which contains phosphine, considered a candidate biosignature.

The detection of phosphine (PH3) in the atmosphere of Venus is being investigated as a possible biosignature. Phosphine, first detected in 2020 in trace amounts, has no known abiotic source for the quantities detected.[83][84] Subsequent analysis and investigation between 2020 and 2015 indicated possible false detection,[85][86] or a much lower concentration of 1 ppb.[87][88][89][90] However in September 2024, the preliminary analysis of the JCMT-Venus data confirmed a concentration of 300 ppb at altitude 55 km. Further data processing is still needed to measure phosphine concentration deeper in the Venusian cloud deck.[91]

Other planets

On September 12, 2023, NASA announced that their investigation into exoplanet K2-18b revealed the possible presence of dimethyl sulfide, noting "On Earth, this is only produced by life."[92]

Agnostic biosignatures

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.[93] This must involve some sort of chemical disequilibrium, which can be exploited for metabolism.[94][66][67] 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

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]

CO and H2 in Martian atmosphere

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.[95][96] Scientists have used this concept as an argument against life on Mars.[97]

Polyelectrolytes

Main article: Polyelectrolyte theory of the gene

The

ionic charges.[98] Benner and others proposed methods for concentrating and analyzing these polyelectrolyte genetic biopolymers on Mars,[99] Enceladus,[100] and Europa.[101]

Missions inside the Solar System

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.[102] 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:[102]

  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

Main article:
Viking biological experiments

The

life-detection experiments which looked for signs of metabolism; however, the results were declared inconclusive.[23][103][104][105][106][excessive citations
]

Mars Science Laboratory

Main article: Timeline of Mars Science Laboratory

The Curiosity rover from the

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

The 2016 ExoMars

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.[107] 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.[106][108]

Mars 2020 Rover

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.[109][110] In addition, it will cache the most interesting samples for possible future transport to Earth.

Titan Dragonfly

NASA's

prebiotic chemistry, making it a prime candidate for biosignature discovery.[116][117][118]

See also: Life on Titan

Europa Clipper

Europa Clipper

NASA's Europa Clipper probe is designed as a flyby mission to Jupiter's smallest Galilean moon, Europa.[119] 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.[120]
  • 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.[121]
  • Hubble Space Telescope (2012): An image was taken of Europa which showed evidence for a plume of water vapor coming off the surface.[122][123]

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.[124]

Enceladus

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[125][126] and were later determined to contain salt as well as organic compounds.[127][128] 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.[129][130][131] 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.[142]

Searching outside of the Solar System

At 4.2

km, or 25 trillion miles) away from Earth, the closest potentially habitable exoplanet is Proxima Centauri b, which was discovered in 2016.[143][144] 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).[145] 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.[146]

The new generation of ground-based 30-meter class telescopes (

direct imaging
studies more feasible.

See also

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