Editing Viking lander biological experiments (section)

=== Labeled release ===
The labeled release ('''LR''') experiment (PI: [[Gilbert Levin]], Biospherics Inc.) gave the most promise for [[exobiologist]]s. In the LR experiment, a sample of Martian soil was inoculated with a drop of very dilute aqueous nutrient solution. The nutrients (7 molecular compounds that were [[Miller-Urey]] products) were tagged with radioactive <sup>14</sup>C.  The air above the soil was monitored for the evolution of radioactive <sup>14</sup>CO<sub>2</sub> (or other carbon-based<ref>{{Cite journal|last1=Levin|first1=Gilbert V.|last2=Straat|first2=Patricia Ann|date=October 2016|title=The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment|journal=Astrobiology|volume=16|issue=10|pages=798–810|doi=10.1089/ast.2015.1464|issn=1557-8070|pmc=6445182|pmid=27626510|bibcode=2016AsBio..16..798L}}</ref>) gas as evidence that microorganisms in the soil had [[metabolism|metabolized]] one or more of the nutrients. Such a result was to be followed with the control part of the experiment as described for the PR below. The result was quite a surprise, considering the negative results of the first two tests, with a steady stream of radioactive gases being given off by the soil immediately following the first injection. The experiment was done by both Viking probes, the first using a sample from the surface exposed to sunlight and the second probe taking the sample from underneath a rock; both initial injections came back positive.<ref name="Chambers" /> Sterilization control tests were subsequently carried out by heating various soil samples. Samples heated for 3 hours at 160&nbsp;°C gave off no radioactive gas when nutrients were injected, and samples heated for 3 hours at 50&nbsp;°C exhibited a substantial reduction in radioactive gas released following nutrient injection.<ref>{{cite journal |last1=Levin |first1=Gilbert |last2=Straat |first2=Patricia |title=Viking Labeled Release Biology Experiment: Interim Results |journal=Science |date=17 December 1976 |volume=194 |issue=4271 |pages=1322–1329 |doi=10.1126/science.194.4271.1322 |pmid=17797094 |bibcode=1976Sci...194.1322L |s2cid=24206165 |url=https://www.science.org/doi/10.1126/science.194.4271.1322 |access-date=27 September 2020|url-access=subscription }}</ref> A sample stored at 10&nbsp;°C for several months was later tested showing significantly reduced radioactive gas release.<ref>{{cite journal |last1=Levin |first1=Gilbert V. |last2=Straat |first2=Patricia Ann |title=Completion of the Viking labeled release experiment on Mars |journal=Journal of Molecular Evolution |date=1 March 1979 |volume=14 |issue=1 |pages=167–183 |doi=10.1007/BF01732376 |pmid=522152 |bibcode=1979JMolE..14..167L |s2cid=20915236 |url=https://link.springer.com/article/10.1007%2FBF01732376 |access-date=27 September 2020|url-access=subscription }}</ref>

A [[CNN]] article from 2000 noted that "Though most of his peers concluded otherwise, Levin still holds that the robot tests he coordinated on the 1976 Viking lander indicated the presence of living organisms on Mars."<ref name="cnn">{{cite news |first=Richard |last=Stenger | name-list-style = vanc |url=http://edition.cnn.com/2000/TECH/space/11/07/mars.sample/ |title=Mars sample return plan carries microbial risk, group warns |work=CNN |date=2000-11-07 }}</ref> A 2006 [[astrobiology]] textbook noted that "With unsterilized Terrestrial samples, though, the addition of more nutrients after the initial incubation would then produce still more radioactive gas as the dormant bacteria sprang into action to consume the new dose of food. This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas."<ref name="Plaxco2006">{{cite book |first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross | name-list-style = vanc |title=Astrobiology: A Brief Introduction |url=https://archive.org/details/astrobiologybrie0000plax |url-access=registration |date=2006 |publisher=JHU Press |isbn=978-0-8018-8366-8 |page=[https://archive.org/details/astrobiologybrie0000plax/page/223 223] }}</ref> The 2011 edition of the same textbook noted that "Albet Yen of the Jet Propulsion Laboratory has shown that, under extremely cold and dry conditions and in a carbon dioxide atmosphere, ultraviolet light (remember: Mars lacks an ozone layer, so the surface is bathed in ultraviolet) can cause carbon dioxide to react with soils to produce various oxidizers, including highly reactive [[superoxide]]s (salts containing O<sub>2</sub><sup>&minus;</sup>). When mixed with small organic molecules, superoxidizers readily oxidize them to carbon dioxide, which may account for the LR result. Superoxide chemistry can also account for the puzzling results seen when more nutrients were added to the soil in the LR experiment; because life multiplies, the amount of gas should have increased when a second or third batch of nutrients was added, but if the effect was due to a chemical being consumed in the first reaction, no new gas would be expected. Lastly, many superoxides are relatively unstable and are destroyed at elevated temperatures, also accounting for the "sterilization" seen in the LR experiment."<ref name="PlaxcoGross2011_2"/>

In a 2002 paper, Joseph Miller speculates that recorded delays in the system's chemical reactions point to biological activity similar to the [[circadian rhythm]] previously observed in terrestrial [[cyanobacteria]].<ref>{{cite journal | vauthors = Miller JD, Straat PA, Levin GV | editor-first1 = Richard B. | editor-first2 = Gilbert V. | editor-first3 = Roland R. | editor-first4 = Alexei Y. | editor-last1 = Hoover | editor-last2 = Levin | editor-last3 = Paepe | editor-last4 = Rozanov | title = Periodic analysis of the Viking lander Labeled Release experiment. | journal = Instruments, Methods, and Missions for Astrobiology IV | date = February 2002 | volume = 4495 | pages = 96–108 | url = http://www.gillevin.com/Mars/Reprint119-Miller-Straat-Levin-FINAL_files/Reprint119-Miller-Straat-Levin-FINAL.htm | doi = 10.1117/12.454748 | bibcode = 2002SPIE.4495...96M | s2cid = 96639386 | quote = One speculation is that the function represents metabolism during a period of slow growth or cell division to an asymptotic level of cellular confluence, perhaps similar to terrestrial biofilms in the steady state. | access-date = 2015-03-22 | archive-date = 2020-11-09 | archive-url = https://web.archive.org/web/20201109015837/http://www.gillevin.com/Mars/Reprint119-Miller-Straat-Levin-FINAL_files/Reprint119-Miller-Straat-Levin-FINAL.htm | url-status = dead | url-access = subscription }}</ref>

A 2007 paper by [[Dirk Schulze-Makuch]] and Joop M. Houtkooper argues that the experiment may have killed potential microbes by supplying them with an excessive amount of water.<ref>{{Cite journal |last1=Houtkooper |first1=Joop M. |last2=Schulze-Makuch |first2=Dirk |date=22 May 2007 |title=A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/possible-biogenic-origin-for-hydrogen-peroxide-on-mars-the-viking-results-reinterpreted/CFC8FE788E40D8A57388D2B35E224742 |journal=International Journal of Astrobiology |language=en |volume=6 |issue=2 |pages=147–152 |doi=10.1017/S1473550407003746 |issn=1475-3006|arxiv=physics/0610093 |bibcode=2007IJAsB...6..147H }}</ref><ref>{{Cite web |last=Than |first=Ker |date=2007-08-23 |title=Claim of Martian Life Called 'Bogus' |url=https://www.space.com/4267-claim-martian-life-called-bogus.html |access-date=2024-12-14 |website=Space.com |language=en}}</ref> Schulze-Makuch revisited the idea in an article for [[Big Think]] in 2023.<ref>{{Cite web |date=2023-06-27 |title=We might have accidentally killed the only life we ever found on Mars nearly 50 years ago |url=https://bigthink.com/hard-science/accidentally-killed-life-mars/ |access-date=2024-12-14 |website=Big Think |language=en-US}}</ref><ref>{{Cite web |date=2023-07-06 |title=Did we find life on Mars … and then wipe it out? |url=https://earthsky.org/space/life-on-mars-viking-1-and-2-astrobiology/ |access-date=2024-12-14 |website=earthsky.org |language=en-US}}</ref>

On 12 April 2012, an international team including Levin and [[Patricia Ann Straat]] published a [[peer reviewed]] paper suggesting the detection of "extant microbial life on Mars", based on mathematical speculation through [[cluster analysis]] of the Labeled Release experiments of the [[Viking program|1976 Viking Mission]].<ref name="Bianciardi-2012">{{cite journal |last1=Bianciardi |first1=Giorgio |last2=Miller |first2=Joseph D. |last3=Straat |first3=Patricia Ann |last4=Levin |first4=Gilbert V. | name-list-style = vanc |title=Complexity Analysis of the Viking Labeled Release Experiments |journal=IJASS |date=March 2012 |volume=13 |issue=1 |pages=14–26 |bibcode=2012IJASS..13...14B |doi=10.5139/IJASS.2012.13.1.14 |doi-access=free }}</ref><ref name="NGS-20120413">{{cite news |last=Than |first=Ker | name-list-style = vanc |title=Life on Mars Found by NASA's Viking Mission? |url=http://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/ |archive-url=https://web.archive.org/web/20120415072431/http://news.nationalgeographic.com/news/2012/04/120413-nasa-viking-program-mars-life-space-science/ |url-status=dead |archive-date=April 15, 2012 |date=2012-04-13 |work=[[National Geographic (magazine)|National Geographic]] |access-date=2013-07-16 }}</ref>