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

In astrobiology[editar]

Some researchers suggested that these microscopic structures on the Martian ALH84001 meteorite could be fossilized bacteria.[2]​[3]​

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.[4]​ 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.[4]​

Biosignatures need not be chemical, however, and can also be suggested by a distinctive magnetic biosignature.[5]​ 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.[6]​ For example, the possible biomineral studied in the Martiam 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.[6]​

Scientific observations include the possible identification of biosignatures through indirect observation. For example, electromagnetic information through infrared radiation telescopes, radio-telescopes, space telescopes, etc.[7]​[8]​ 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.

Atmosphere

Over billions of years, the processes of life on a planet would create a fog of chemicals unlike anything that could form in an ordinary chemical equilibrium.[9]​ 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.[10]​ 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.[11]​

The Viking missions to Mars[editar]

Artículo principal: Viking biological experiments
Archivo:Sagan Viking.jpg
Carl Sagan with a model of the Viking lander

The Viking missions to Mars in the 1970s conducted the only experiments to date 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'.[12]​[13]​[14]​[15]​[16]​

Future missions such as Mars Science Laboratory and ExoMars will attempt to detect habitable environments on Mars as well as biosignatures.[16]​[17]​

See also[editar]

References[editar]

  1. ↑ Bosak, Tanja Bosak; Virginia Souza-Egipsy, Frank A. Corsetti and Dianne K. Newman (18 de mayo de 2004). «Micrometer-scale porosity as a biosignature in carbonate crusts». Geology 32 (9): 781-784. Bibcode:2004Geo....32..781B. doi:10.1130/G20681.1. Consultado el 14 de enero de 2011. 
  2. ↑ Crenson, Matt (6 de agosto de 2006). «After 10 years, few believe life on Mars». Associated Press (on usatoday.com). Consultado el 6 de diciembre de 2009. 
  3. ↑ 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. PMID 8688069. doi:10.1126/science.273.5277.924. 
  4. ↑ a b Rothschild, Lynn (September, 2003). «Understand the evolutionary mechanisms and environmental limits of life». NASA. Consultado el 13 de julio de 2009. 
  5. ↑ Wall, Mike (13 December 2011). «Mars Life Hunt Could Look for Magnetic Clues». Space.com. Consultado el 15 de diciembre de 2011. 
  6. ↑ a b Error en la cita: Etiqueta <ref> no válida; no se ha definido el contenido de las referencias llamadas SSG
  7. ↑ Gardner, James N. (February 28, 2006). «The Physical Constants as Biosignature: An anthropic retrodiction of the Selfish Biocosm Hypothesis». Kurzweil. Consultado el 14 de enero de 2011. 
  8. ↑ «Astrobiology». Biology Cabinet. September 26, 2006. Consultado el 17 de enero de 2011. 
  9. ↑ «Artificial Life Shares Biosignature With Terrestrial Cousins». The Physics arXiv Blog. MIT. 10 January 2011. Consultado el 14 de enero de 2011. 
  10. ↑ Mars Trace Gas Mission (September 10, 2009)
  11. ↑ Remote Sensing Tutorial, Section 19-13a - Missions to Mars during the Third Millennium, Nicholas M. Short, Sr., et al., NASA
  12. ↑ 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. PMID 17723090. doi:10.1089/ast.2007.0153. Consultado el 20 de julio de 2009. 
  13. ↑ Levin, G and P. Straaf. 1976. Viking Labeled Release Biology Experiment: Interim Results. Science: vol: 194. pp: 1322-1329.
  14. ↑ Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0. 
  15. ↑ Klein, Harold P.; Levin, Gilbert V. (1 de octubre de 1976). «The Viking Biological Investigation: Preliminary Results». Science 194 (4260): 99-105. Bibcode:1976Sci...194...99K. PMID 17793090. doi:10.1126/science.194.4260.99. Consultado el 15 de agosto de 2008. 
  16. ↑ a b ExoMars rover
  17. ↑ «Mars Science Laboratory: Mission». NASA/JPL. Consultado el 12 de marzo de 2010. 
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