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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Biosignature | 1/6 | https://en.wikipedia.org/wiki/Biosignature | reference | science, encyclopedia | 2026-05-05T13:15:42.723741+00:00 | kb-cron |
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.
== Types == Biosignatures can be grouped into ten broad categories:
Isotope patterns: Isotopic evidence or patterns that require biological processes. Chemistry: Chemical features that require biological activity. Organic matter: Organics formed by biological processes. Minerals: Minerals or biomineral-phases whose composition and/or morphology indicate biological activity (e.g., biomagnetite). Microscopic structures and textures: Biologically-formed cements, microtextures, microfossils, and films. Macroscopic physical structures and textures: Structures that indicate microbial ecosystems, biofilms (e.g., stromatolites), or fossils of larger organisms. Temporal variability: Variations in time of atmospheric gases, reflectivity, or macroscopic appearance that indicates life's presence. Surface reflectance features: Large-scale reflectance features due to biological pigments. Atmospheric gases: Gases formed by metabolic processes, which may be present on a planet-wide scale. Technosignatures: Signatures that indicate a technologically advanced civilization.
== Viability == 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.
=== 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. However when looking at disequilibria, it is important to consider the context of the environment, because not all atmospheric disequilibria has biotic causes. For example, prebiotic environments can have chemical disequilibria due to volcanic activity.
=== Survivability === A biosignature must be able to last for long enough so that a probe, telescope, or human can be able to detect it. A consequence of a biological organism's use of metabolic reactions for energy is the production of metabolic waste. In addition, the structure of an organism can be preserved as a fossil and we know that some fossils on Earth are as old as 3.5 billion years. These byproducts can make excellent biosignatures since they provide direct evidence for life. However, in order to be a viable biosignature, a byproduct must subsequently remain intact so that scientists may discover it.
=== Detectability === A biosignature must be detectable with current technology in order to be considered viable in scientific investigations. Although this may seem straightforward, there are many scenarios in which life may be present on a planet yet remain undetectable due to observational or technological limitations.
==== False positives ==== Every possible biosignature is associated with its own set of unique false positive mechanisms, in which abiotic processes can mimic the detectable feature of biological activity. An important example is using oxygen as a biosignature. On Earth, most oxygen is produced by photosynthesis and is subsequently used by other life forms. Oxygen is also readily detectable in spectra, with multiple bands across a relatively wide wavelength range, therefore, it makes a very good biosignature. Finding oxygen alone in a planet's atmosphere is not enough to confirm a biosignature because of the false-positive mechanisms associated with it. One possibility is that oxygen can build up abiotically via photolysis if there is a low inventory of non-condensable gasses or if the planet loses a lot of water. Finding and distinguishing a biosignature from its abiotic mechanisms is one of the major challenges of confirming the viability of a biosignature.
==== False negatives ==== 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. This is another challenge that is a significant focus of ongoing research, especially in preparation for future telescope observations designed to observe exoplanetary atmospheres.