A biosignature is a phenomenon that can be explained by biological processes where all possible abiotic causes of this phenomenon have been eliminated. This term is mainly used in the field of astrobiology in the search for past or present extraterrestrial life, from planets and moons in the Solar System to exoplanets. Candidate biosignatures strongly indicate some of the earliest known life forms, aid studies of the origin of life on Earth as well as the possibility of life on Mars, Venus and elsewhere in the universe.

History

The term "biosignature" and its definition have evolved over time. In the 1960s, the phrase "life detection" was used as seen in two Nature papers "A physical basis for life detection experiments," by James. E. Lovelock (1965) and "Signs of Life: Criterion-system of exobiology," by Joshua Lederberg (1965). In 1973, Joon H. Rho used the term "biomarker" in his paper, "A search for porphyrin biomarkers in nonesuch shale and extraterrestrial samples" to describe a fossil organic compound that can be traced back to a specific organism. In medicine, biomarker (medicine) has a different definition. In 1995, the term biosignature was first used by the NASA Exobiology Program office (now the NASA Astrobiology Program) in "An Exobiological Strategy for Mars Exploration." The term has since become widely used in astrobiology.

The definition of "biosignature" continued to be refined. In 2003, it was described as an object, substance, and/or pattern that unequivocally was originated through a biological process. By 2018, the definition had broadened to a substance or phenomenon that presents evidence of life. In 2023, the astrobiology community further refined the concept, agreeing that a biosignature is a phenomenon that can only be explained by biological processes, with all plausible abiotic explanations having been considered and eliminated.

  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.

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.thumb|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.]]

Reliability

A biosignature must be able to dominate over all other processes that produce similar physical, spectral, and chemical features. Many forms of life are known to mimic geochemical reactions. One of the theories on the origin of life involves molecules developing the ability to catalyse geochemical reactions to exploit the energy being released by them. These are some of the earliest known metabolisms (see methanogenesis). 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.

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. Finding and distinguishing a biosignature from its abiotic mechanisms is one of the major challenges of confirming the viability of a biosignature.

False negatives

False negative biosignatures occur when life is present, but environmental processes and/or measurement limitations may obscure or suppress features that would otherwise indicate biological activity.

For example, the particular fatty acids measured in a sample can indicate which types of bacteria and archaea live in that environment. Another example is the long-chain fatty alcohols with more than 23 atoms that are produced by planktonic bacteria. When used in this sense, geochemists often prefer the term biomarker. Another example is the presence of straight-chain lipids in the form of alkanes, alcohols, and fatty acids with 20–36 carbon atoms in soils or sediments. Peat deposits are an indication of originating from the epicuticular wax of higher plants.

Life processes may produce a range of biosignatures such as nucleic acids, lipids, proteins, amino acids, kerogen-like material and various morphological features that are detectable in rocks and sediments. Microbes often interact with geochemical processes, leaving features in the rock record indicative of biosignatures. For example, bacterial micrometer-sized pores in carbonate rocks resemble inclusions under transmitted light, but have distinct sizes, shapes, and patterns (swirling or dendritic) and are distributed differently from common fluid inclusions. A potential biosignature is a phenomenon that may have been produced by life, but for which alternate abiotic origins may also be possible.

Morphology

Another possible biosignature might be morphology since the shape and size of certain objects may potentially indicate the presence of past or present life. Morphology has sparked debate as it is inconclusive and has resulted in disputed claims of early life on Earth.

Stromatolites are difficult to identify chemically and are sometimes claimed based on morphology alone. However geological processes may produce false positive candidates. One case is a 3.7 Ga structure in West Greenland which could be explained by tectonic processes.

Chemistry

No single compound will prove life once existed. Rather, it will be distinctive patterns present in any organic compounds showing a process of selection. For example, membrane lipids left behind by degraded cells will be concentrated, have a limited size range, and comprise an even number of carbons. Similarly, life only uses left-handed amino acids.

thumb|upright|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. Each biomarker is quite distinctive when compared to its counterparts, as the time required for organic matter to convert to crude oil is characteristic. Most biomarkers also usually have high molecular mass.

Some examples of biomarkers found in petroleum are pristane, triterpanes, steranes, phytane and porphyrin. Such petroleum biomarkers are produced via chemical synthesis using biochemical compounds as their main constituents. For instance, triterpenes are derived from biochemical compounds found on land angiosperm plants. The abundance of petroleum biomarkers in small amounts in its reservoir or source rock make it necessary to use sensitive and differential approaches to analyze the presence of those compounds. The techniques typically used include gas chromatography and mass spectrometry.

Petroleum biomarkers are highly important in petroleum inspection as they help indicate the depositional territories and determine the geological properties of oils. For instance, they provide more details concerning their maturity and the source material. In addition to that they can also be good parameters of age, hence they are technically referred to as "chemical fossils". The ratio of pristane to phytane (pr:ph) is the geochemical factor that allows petroleum biomarkers to be successful indicators of their depositional environments.

Geologists and geochemists use biomarker traces found in crude oils and their related source rock to unravel the stratigraphic origin and migration patterns of presently existing petroleum deposits. The dispersion of biomarker molecules is also quite distinctive for each type of oil and its source; hence, they display unique fingerprints. Another factor that makes petroleum biomarkers more preferable than their counterparts is that they have a high tolerance to environmental weathering and corrosion. Such biomarkers are very advantageous and often used in the detection of oil spillage in the major waterways. 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.

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. 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. 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.

Some scientists have reported methods of detecting hydrogen and methane in extraterrestrial atmospheres. Habitability indicators and biosignatures must be interpreted within a planetary and environmental context. Two of the top 14,000 proposed atmospheric biosignatures are dimethyl sulfide and chloromethane ().

thumb|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.

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. 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. Depending on the host star, a disequilibrium in the methane abundance on another planet may indicate a biosignature. In this context, a strong, unutilized chemical disequilibrium can function as an antibiosignature, by implying that biological activity is unlikely. 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. As with most scientific discoveries, discovery of a biosignature will require evidence building up until no other explanation exists.

thumb|A Life Detection Ladder summarizing measurable traits associated with living systems, ordered by how strongly they may indicate extant life. The framework, inspired by lessons from Viking and later missions, links biological features with the types of measurements that could support a confident life-detection claim.

Possible examples of a biosignature include complex organic molecules or structures whose formation is virtually unachievable in the absence of life: and in 1976 by the Viking biological experiments. Significant levels of oxygen were detected in Gale Crater by Curiosity Rover in 2019 with seasonal variability that has not fully been explained. Studies indicate that the Martian atmosphere was once oxygen-rich. Today they are no longer considered valid biosignatures and are proposed to be the result of photodissociation of carbon dioxide.

Methane in the Martian Atmosphere

Martian methane 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.

Since 2004 there have been several detection claims 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. However 2019 measurements put an upper bound on the overall methane abundance at 0.05 p.p.b.v and later in sedimentary rocks by Curiosity likely from perchlorate reactions with organic matter.

Some discoveries have been found in areas confirmed previously be wet, adding weight to their significance. In 2018 at Gale Crater, Curiosity discovered Thiophene () and polymers (Polythiophene). Natural sulfur reduction has been proposed as a possible abiotic source. Dimethyl sulfide () was also detected. In Cheyava Falls discovered by Perseverance in July 2024, organic matter was detected. Also found were millimeter-sized splotches resembling "leopard spots" containing iron and phosphate, elements often associated with microbial life. Molecules detected included Trimethylbenzene, Tetramethylbenzene, Methyl benzoate, Dihydronaphthalene, Naphthalene, Benzothiophene and Methylnaphthalene. 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.

Mars 2020

thumb|Cheyava Falls rock

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

In 2024, Perseverance found a rock, called Cheyava Falls, during its exploration of the Jezero Crater. The rover's instruments detected organic compounds within the rock. On Earth, vivianite is frequently found in sediments, peat bogs, and around decaying organic matter. Similarly, certain forms of microbial life on Earth can produce greigite.

If confirmed, this biosignature would mean that there were a microbial life on Mars around 3.5 billion years ago. According to geologist Michael Tice: Ammonia is essential to life and is both a metabolic input and output, as such it has been explored as having strong potential as a biosignature. Pioneer Venus also detected substantial quantities of the gas. Of particular interest is that unlike the Martian atmosphere where conditions would suit ammonia's presence only transient trace amounts have been detected, on Venus with conditions less conducive to its presence it appears to somehow be replenished. A 2021 paper claimed that it could be a byproduct of life that is in turn providing a stable habitable environment for life to continue in the upper atmosphere. Another has proposed that lightning could be producing it though whether Venus has lightning at all has been extensively debated.

Ozone in the Venusian Atmosphere

Ozone () was first detected at concentrations of up to 1 ppm in the night side upper atmosphere by Venus Express in 2011. As a byproduct of living organisms this was once regarded as a candidate biosignature. Known since the 1970s to exists in trace amounts in the Martian atmosphere, Venus in comparison possesses a significant layer similar to but substantially less concentrated than Earth's. Photochemical processes, specifically dissociation of carbon dioxide (CO2) by sunlight, is now offered an explanation for its presence.

Phosphine in the Venusian Atmosphere

Phosphine () was first detected in 2020 by the James Clerk Maxwell Telescope and the Atacama Large Millimeter/submillimeter Array in trace amounts in the upper cloud deck. There was no known abiotic source for the quantities detected. Subsequent analysis and investigation between 2020 and 2015 indicated possible false detection, or a much lower concentration of 1 ppb. Titan is the largest moon of Saturn and is widely believed to have a large subsurface ocean consisting of a salty brine. 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. Mission design concepts include:

  • Enceladus Life Finder (ELF)
  • Enceladus Life Signatures and Habitability
  • Enceladus Organic Analyzer
  • Enceladus Explorer (En-Ex)
  • Explorer of Enceladus and Titan (E<sup>2</sup>T)
  • Journey to Enceladus and Titan (JET)
  • Life Investigation For Enceladus (LIFE)
  • Testing the Habitability of Enceladus's Ocean (THEO)

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.

Meteorites

Martian Meteorites

ALH84001

right|thumb|Some researchers suggested that these microscopic structures on the Martian [[ALH84001 meteorite could be fossilized bacteria.]]

Microscopic magnetite crystals in the Martian meteorite ALH84001