Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies Framework for Decision Making (1998) / Chapter Skim
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1 Introduction
1 Introduction
Pages 8-20
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From page 8... ...
, which assessed the potential for inclusion of a viable exogenous biological entity in a sample returned to Earth from Mars as well as the potential for large-scale effects if such an entity were inadvertently introduced into Earth's biosphere. The report addressed how to protect Earth from possible contamination by putative martian biota and provided justification for and recommendations on procedures for the quarantine of samples returned from Mars.
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Based on the six parameters identified as relevant, the task group formulated a series of questions as a basis for identification of whether or not samples require containment. These questions were used to assess the potential for a biological entity to be present in or on samples returned from planetary satellites, asteroids, comets, and cosmic dust.
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Every documented terrestrial cellular life form is a self-replicating entity that has genetic information in the form of nucleic acid polymers coding for proteins. Biologically active systems require at a minimum liquid water, carbon, nitrogen, phosphate, sulfur, various metals, and a source of energy either in the form of solar radiation or from chemosynthetic processes.
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They grow in temperate marine and terrestrial settings, within other microbial or multicellular organisms, in deep subsurface niches, and in extreme environments that would be lethal for other life forms. They often influence geochemical reactions within the biosphere and frequently play key roles in food webs and complex ecosystems.
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may well increase as further research is done Oxygen Aerobe Anaerobe Facultative anaerobe Capable of using oxygen as a terminal electron acceptor; can tolerate a level of oxygen equivalent to or higher than the 21 percent oxygen present in an air atmosphere and has a strictly respiratory-type metabolism Grows in the absence of oxygen; some anaerobes have a fermentative-type metabolism; others may carry out anaerobic respiration in which a terminal electron acceptor other than oxygen is used Can grow aerobically or anaerobically characteristic of a large number of genera of bacteria including coliforms such as Escherichia cold Microaerophile Capable of oxygen-dependent growth but only at low oxygen levels; cannot grow in the presence of a level of oxygen equivalent to that present in an air atmosphere (21 percent oxygen) pH Acidophile Alkal op hi l e Neutrophile Salinity Grows at pH values less than 2 Grows at pH values greater than 10 Grows best at pH values near 7 Halophile Requires salt for growth: extreme halophiles (all are archaea)
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Because the upper temperature limit for growth will depend on pressure, and the environments likely to be encountered in sample return missions will be at low rather than high pressure, the theoretical 160 °C upper temperature limit for growth is reasonable. Unless sequestered within a protective niche, microorganisms associated with small solar system bodies will also be exposed to UV and ionizing radiation.
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Terrestrial organisms can use the energy of sunlight or the free energy associated with chemical disequilibrium for growth. Solar power for driving biological processes will also be available in many extraterrestrial environments; however, such a source of energy would require surface liquid water and perhaps atmospheres capable of filtering out harmful radiation without blocking beneficial wavelengths.
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Although metabolically inactive cells cannot correct defects caused by ionizing radiation, the molecular machinery required for such repair can sometimes be reactivated in response to environmental or physiological change. There are several reports, some controversial, of quiescent eukaryotic and prokaryotic organisms having survived for thousands of years.
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However, when organisms are metabolically inactive, they no longer have the capacity to repair the devastating effects of exposure to relatively low levels of radiation or damage from the formation of free radicals. QUESTIONS APPROPRIATE FOR ASSESSING THE BIOLOGICAL POTENTIAL OF SMALL BODIES As indicated by the task group's review of current data and understanding, life on Earth and the survival of metabolically active cells absolutely require the presence of liquid water, a source of energy that can be tapped by metabolic processes, temperatures that do not exceed cat 160 °C, carbonaceous material, and shielding from highintensity or long-term exposure to ionizing radiation or UV flux.
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6. Does the preponderance of scientific evidence indicate that there has been a natural influx to Earth, e.g., via meteorites, of material equivalent to a sample returned from the target body?
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Yes No Special Containment Required Beyond What Is Needed for Scientific Purposes '1 No or Uncertain Does the preponderance of scientific evidence indicate that there was never sufficient organic matter (or CO2 or carbonates Assad an appropriate source of reducing equivalents) in or on the target body to support life?
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1996. Radioresistance of Deinococcus radiodurans: Functions necessary to survive ionizing radiation are also necessary to survive prolonged desiccation.
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1997. Microbial life in deep granitic rock.
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