Editing Viking lander biological experiments (section)

=== Pyrolytic release ===
The pyrolytic release ('''PR''') experiment (PI: [[Norman Horowitz]], Caltech) consisted of the use of light, water, and a carbon-containing [[atmosphere of Mars|atmosphere]] of [[carbon monoxide]] (CO) and [[carbon dioxide]] (CO<sub>2</sub>), simulating that on Mars.  The carbon-bearing gases were made with [[carbon-14]] (<sup>14</sup>C), a  heavy, [[radioactive]] [[isotope]] of carbon.  If there were [[photosynthesis|photosynthetic]] organisms present, it was believed that they would incorporate some of the carbon as [[biomass]] through the process of [[carbon fixation]], just as plants and [[cyanobacteria]] on earth do.  After several days of incubation, the experiment removed the gases, baked the remaining soil at 650&nbsp;°C (1200&nbsp;°F), and collected the products in a device which counted radioactivity.  If any of the <sup>14</sup>C had been converted to biomass, it would be vaporized during heating and the radioactivity counter would detect it as evidence for life.  Should a positive response be obtained, a duplicate sample of the same soil would be heated to "sterilize" it.  It would then be tested as a control and should it still show activity similar to the first response, that was evidence that the activity was chemical in nature.  However, a nil, or greatly diminished response, was evidence for biology. This same control was to be used for any of the three life detection experiments that showed a positive initial result.<ref name="Sci194Interim">{{cite journal | vauthors = Horowitz NH, Hobby GL, Hubbard JS | title = The viking carbon assimilation experiments: interim report | journal = Science | volume = 194 | issue = 4271 | pages = 1321–2 | date = December 1976 | pmid = 17797093 | doi = 10.1126/science.194.4271.1321 | bibcode = 1976Sci...194.1321H | s2cid = 206569558 }}</ref> The initial assessment of results from the Viking 1 PR experiment was that "analysis of the results shows that a small but significant formation of organic matter occurred" and that the sterilized control showed no evidence of organics, showing that the "findings could be attributed to biological activity."<ref name="The viking biological investigation"/> However, given the persistence of organic release at 90&nbsp;°C, the inhibition of organics after injecting water vapor and, especially, the lack of detection of organics in the Martian soil by the GCMS experiment, the investigators concluded that a nonbiological explanation of the PR results was most likely.<ref>{{cite journal|title=Viking Biology Experiments: Lessons Learned and the Role of Ecology in Future Mars Life-Detection Experiments|journal=Space Science Reviews|last1=Schuerger|first1=Andrew|last2=Clark|first2=Benton|volume=135|date=March 2008|issue=1–4|pages=233–243|doi=10.1007/s11214-007-9194-2|bibcode=2008SSRv..135..233S |s2cid=189797714}}</ref><ref name="Sci194Interim" />  However, in subsequent years, as the GCMS results have come increasingly under scrutiny, the pyrolytic release experiment results have again come to be viewed as possibly consistent with biological activity, although "An explanation for the apparent small synthesis of organic matter in the pyrolytic release experiment remains obscure."<ref name="The Viking biological experiments on Mars">{{cite journal|last1=Klein|first1=Harold|title=The Viking biological experiments on Mars|journal=Icarus|date=June 1978|volume=34|issue=3|page=666|doi=10.1016/0019-1035(78)90053-2|bibcode=1978Icar...34..666K }}</ref>