A biosignature is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of biomass and wastes. Due to its unique characteristics, a biosignature can be interpreted as having been produced by living organisms; however, it is important that they not be considered definitive because there is no way of knowing in advance which ones are universal to life and which ones are unique to the peculiar circumstances of life on Earth.
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).
For example, the particular fatty acids measured in a sample can indicate which types of bacteria and archaea live in that environment. Another example are 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. An other example is the presence of straight-chain lipids in the form of alkanes, alcohols an 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 size, 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.
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 nonbiological (abiotic) processes producing it. An example of such a biosignature might be complex organic molecules and/or structures whose formation is virtually unachievable in the absence of life. For example, some categories of biosignatures can include the following: cellular and extracellular morphologies, biogenic substance in rocks, bio-organic molecular structures, chirality, biogenic minerals, biogenic stable isotope patterns in minerals and organic compounds, atmospheric gases, and remotely detectable features on planetary surfaces, such as photosynthetic pigments, etc.
Biosignatures need not be chemical, however, and can also be suggested by a distinctive magnetic biosignature. 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 were the longest-debated of several potential biosignatures in that specimen because it was believed until recently that only bacteria could create crystals of their specific shape. However, anomalous features discovered that are "possible biosignatures" for life forms would be investigated as well. Such features constitute a working hypothesis, not a confirmation of detection of life. 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. For example, 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. 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.
Scientific observations include the possible identification of biosignatures through indirect observation. For example, electromagnetic information through infrared radiation telescopes, radio-telescopes, space telescopes, etc. From this discipline, the hypothetical electromagnetic radio signatures that SETI scans for would be a biosignature, since a message from intelligent aliens would certainly demonstrate the existence of extraterrestrial life.
On Mars, surface oxidants and UV radiation will have altered or destroyed organic molecules at or near the surface. One issue that may add ambiguity in such a search is the fact that, throughout Martian history, abiogenic organic-rich chondritic meteorites have undoubtedly rained upon the Martian surface. At the same time, strong oxidants in Martian soil along with exposure to ionizing radiation might alter or destroy molecular signatures from meteorites or organisms. An alternative approach would be to seek concentrations of buried crystalline minerals, such as clays and evaporites, which may protect organic matter from the destructive effects of ionizing radiation and strong oxidants. The search for Martian biosignatures has become more promising due to the discovery that surface and near-surface aqueous environments existed on Mars at the same time when biological organic matter was being preserved in ancient aqueous sediments on Earth.
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. The presence of methane in the atmosphere of Mars indicates that there must be an active source on the planet, as it is an unstable gas. Furthermore, current photochemical models cannot explain the presence of methane in the atmosphere of Mars and its reported rapid variations in space and time. Neither its fast appearance nor disappearance can be explained yet. 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. In June, 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres. The planned ExoMars Trace Gas Orbiter to be launched in 2016 to Mars, will study atmospheric trace gases and will attempt to characterize potential biochemical and geochemical processes at work.
The Viking missions to Mars
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'.
Mars Science Laboratory
The Curiosity rover from the Mars Science Laboratory mission, is currently assessing the potential past and present habitability of the Martian environment and is attempting to detect biosignatures on the surface of Mars. 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. Of these, biogenic organic molecules and biogenic atmospheric gases are considered the most definitive and most readily detectable by MSL. The Curiosity rover targets outcrops to maximize the probability of detecting 'fossilized’ organic matter preserved in sedimentary deposits.
On January 24, 2014, NASA reported that current studies by the Curiosity and Opportunity rovers on the planet Mars will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.
The 2016 Trace Gas Orbiter (TGO) will be a Mars telecommunications orbiter and atmospheric gas analyzer mission. It will deliver the ExoMars EDM lander and then proceed to map the sources of methane on Mars and other gases, and in doing so, help select the landing site for the ExoMars rover to be launched on 2018. The primary objective of the 2018 ExoMars 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).
- 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.A.: the Mars Exploration Program Analysis Group (MEPAG) - NASA. p. 72. Retrieved 2009-07-22.
- "Biosignature - definition". Science Dictionary. 2011. Retrieved 2011-01-12.
- Summons, Roger E.; Jan P. Amend; David Bish; Roger Buick; George D. Cody; David J. Des Marais (23 February 2011). "Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group". The Astrobiology Journal 11 (2). Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. PMID 21417945. Retrieved 2013-06-22.
- Carol Cleland; Gamelyn Dykstra; Ben Pageler (2003). "Philosophical Issues in Astrobiology". NASA Astrobiology Institute. Retrieved 2011-04-15.
- "SIGNATURES OF LIFE FROM EARTH AND BEYOND". Penn State Astrobiology Research Center (PSARC). Penn State. 2009. Retrieved 2011-01-14.
- "Reading Archaean Biosignatures". NASA. July 30, 2008. Retrieved 2011-01-14.
- Fatty alcohols
- Beegle, Luther W.; et al (August 2007). "A Concept for NASA's Mars 2016 Astrobiology Field Laboratory". Astrobiology 7 (4): 545–577. Bibcode:2007AsBio...7..545B. doi:10.1089/ast.2007.0153. PMID 17723090. Retrieved 2009-07-20.
- Bosak, Tanja Bosak; Virginia Souza-Egipsy, Frank A. Corsetti and Dianne K. Newman (May 18, 2004). "Micrometer-scale porosity as a biosignature in carbonate crusts". Geology 32 (9): 781–784. Bibcode:2004Geo....32..781B. doi:10.1130/G20681.1. Retrieved 2011-01-14.
- Crenson, Matt (2006-08-06). "After 10 years, few believe life on Mars". Associated Press (on usatoday.com). Retrieved 2009-12-06.
- McKay, David S.; et al. (1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science 273 (5277): 924–930. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069.
- Rothschild, Lynn (September 2003). "Understand the evolutionary mechanisms and environmental limits of life". NASA. Retrieved 2009-07-13.
- Wall, Mike (13 December 2011). "Mars Life Hunt Could Look for Magnetic Clues". Space.com. Retrieved 2011-12-15.
- Gardner, James N. (February 28, 2006). "The Physical Constants as Biosignature: An anthropic retrodiction of the Selfish Biocosm Hypothesis". Kurzweil. Retrieved 2011-01-14.
- "Astrobiology". Biology Cabinet. September 26, 2006. Retrieved 2011-01-17.
- "Artificial Life Shares Biosignature With Terrestrial Cousins". The Physics arXiv Blog. MIT. 10 January 2011. Retrieved 2011-01-14.
- Mars Trace Gas Mission (September 10, 2009)
- Remote Sensing Tutorial, Section 19-13a - Missions to Mars during the Third Millennium, Nicholas M. Short, Sr., et al., NASA
- Oze, Christopher; Jones, Camille; Goldsmith, Jonas I.; Rosenbauer, Robert J. (June 7, 2012). "Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces". PNAS 109 (25): 9750–9754. Bibcode:2012PNAS..109.9750O. doi:10.1073/pnas.1205223109. PMC 3382529. PMID 22679287. Retrieved June 27, 2012.
- Staff (June 25, 2012). "Mars Life Could Leave Traces in Red Planet's Air: Study". Space.com. Retrieved June 27, 2012.
- Brogi, Matteo; Snellen, Ignas A. G.; de Krok, Remco J.; Albrecht, Simon; Birkby, Jayne; de Mooij, Ernest J. W. (June 28, 2012). "The signature of orbital motion from the dayside of the planet t Boötis b". Nature 486 (7404): 502–504. arXiv:1206.6109. Bibcode:2012Natur.486..502B. doi:10.1038/nature11161. Retrieved June 28, 2012.
- Mann, Adam (June 27, 2012). "New View of Exoplanets Will Aid Search for E.T.". Wired (magazine). Retrieved June 28, 2012.
- Mark Allen; = Olivier Witasse (June 16, 2011), "2016 ESA/NASA ExoMars Trace Gas Orbiter", MEPAG June 2011, Jet Propulsion Laboratory, retrieved 2011-06-29 (PDF)
- Levin, G and P. Straaf. 1976. Viking Labeled Release Biology Experiment: Interim Results. Science: vol: 194. pp: 1322-1329.
- Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0.
- Klein, Harold P.; Levin, Gilbert V. (1976-10-01). "The Viking Biological Investigation: Preliminary Results". Science 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090. Retrieved 2008-08-15.
- ExoMars rover
- Grotzinger, John P. (January 24, 2014). "Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science 343 (6169): 386–387. doi:10.1126/science.1249944. Retrieved January 24, 2014.
- Various (January 24, 2014). "Special Issue - Table of Contents - Exploring Martian Habitability". Science 343 (6169): 345–452. Retrieved January 24, 2014.
- Various (January 24, 2014). "Special Collection - Curiosity - Exploring Martian Habitability". Science. Retrieved January 24, 2014.
- Grotzinger, J.P. et al. (January 24, 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science 343 (6169). doi:10.1126/science.1242777. Retrieved January 24, 2014.
- "ExoMars program gathers strength". The Voice of Russia. Jul 15, 2012. Retrieved 2012-07-15.
- "Mars Science Laboratory: Mission". NASA/JPL. Retrieved 2010-03-12.