Editing Viking lander biological experiments (section)

== Scientific conclusions ==
Organic compounds seem to be common, for example, on asteroids, meteorites, comets and the icy bodies orbiting the Sun, so detecting no trace of any organic compound on the surface of Mars came as a surprise. The GC-MS was definitely working, because the controls were effective and it was able to detect traces of chlorine, attributed to the cleaning solvents that had been used to sterilize it prior to launch.<ref name="Caplinger">{{cite news |first=Michael |last=Caplinger | name-list-style = vanc |title=Life on Mars |date=April 1995 |publisher=Malin Space Science Systems |url=http://www.msss.com/http/ps/life/life.html |archive-url=https://web.archive.org/web/20080527182311/http://www.msss.com/http/ps/life/life.html |archive-date=2008-05-27 |access-date=2008-10-13 }}</ref> A reanalysis of the GC-MS data was performed in 2018, suggesting that organic compounds may actually have been detected, corroborating with data from the ''Curiosity'' rover.<ref>{{cite journal |last1=Guzman |first1=Melissa |last2=McKay |first2=Christopher P. |last3=Quinn |first3=Richard C. |last4=Szopa |first4=Cyril |last5=Davila |first5=Alfonso F. |last6=Navarro-González |first6=Rafael |last7=Freissinet |first7=Caroline |title=Identification of Chlorobenzene in the Viking Gas Chromatograph-Mass Spectrometer Data Sets: Reanalysis of Viking Mission Data Consistent With Aromatic Organic Compounds on Mars |journal=Journal of Geophysical Research: Planets |date=July 2018 |volume=123 |issue=7 |pages=1674–1683 |doi=10.1029/2018JE005544 |bibcode=2018JGRE..123.1674G |s2cid=133854625 |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JE005544 |access-date=27 September 2020 |language=en |issn=2169-9100}}</ref> At the time, the total absence of organic material on the surface made the results of the biology experiments moot, since [[metabolism]] involving organic compounds were what those experiments were designed to detect. The general scientific community surmises that the Viking's biological tests remain inconclusive, and can be explained by purely chemical processes.<ref name="Chambers" /><ref name="The viking biological investigation">{{cite journal | vauthors = Klein HP, Horowitz NH, Levin GV, Oyama VI, Lederberg J, Rich A, Hubbard JS, Hobby GL, Straat PA, Berdahl BJ, Carle GC, Brown FS, Johnson RD | display-authors = 6 | title = The viking biological investigation: preliminary results | journal = Science | volume = 194 | issue = 4260 | pages = 99–105 | date = October 1976 | pmid = 17793090 | doi = 10.1126/science.194.4260.99 | bibcode = 1976Sci...194...99K | s2cid = 24957458 }}</ref><ref name="Beegle">{{cite journal | vauthors = Beegle LW, Wilson MG, Abilleira F, Jordan JF, Wilson GR | title = A concept for NASA's Mars 2016 astrobiology field laboratory | journal = Astrobiology | volume = 7 | issue = 4 | pages = 545–77 | date = August 2007 | pmid = 17723090 | doi = 10.1089/ast.2007.0153 | bibcode = 2007AsBio...7..545B }}</ref><ref>{{cite web |url=http://www.esa.int/SPECIALS/ExoMars/SEMK39JJX7F_0.html |title=ExoMars rover |publisher=ESA |access-date=2014-04-14 }}</ref>

Despite the positive result from the Labeled Release experiment, a general assessment is that the results seen in the four experiments are best explained by oxidative chemical reactions with the Martian soil. One of the current conclusions is that the Martian soil, being continuously exposed to [[ultraviolet|UV light]] from the Sun (Mars has no protective [[ozone layer]]), has built up a thin layer of a very strong [[oxidant]]. A sufficiently strong oxidizing molecule would react with the added water to produce oxygen and hydrogen, and with the nutrients to produce [[carbon dioxide]] (CO<sub>2</sub>).

[[Norman Horowitz]] was the chief of the [[Jet Propulsion Laboratory]] bioscience section for the [[Mariner program|Mariner]] and [[Viking program|Viking]] missions from 1965 to 1976.  Horowitz considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival of life on other planets.<ref name = Horowitz1986>Horowitz, N.H. (1986). ''Utopia and Back and the search for life in the solar system''. New York: W.H. Freeman and Company. {{ISBN|0-7167-1766-2}}</ref>  However, he also considered that the conditions found on Mars were incompatible with carbon based life.

In August 2008, the [[Phoenix (spacecraft)|''Phoenix'' lander]] detected [[perchlorate]], a strong oxidizer when heated above 200&nbsp;°C. This was initially thought to be the cause of a false positive LR result.<ref name="latimes.com">{{cite web |url=http://www.latimes.com/news/printedition/asection/la-sci-phoenix6-2008aug06,0,4986721.story |title=Perchlorate found in Martian soil |date=2008-08-06 |work=Los Angeles Times |last=Johnson |first=John | name-list-style = vanc }}</ref><ref name="sciencedaily.com">{{cite web |url=https://www.sciencedaily.com/releases/2008/08/080805192122.htm |publisher=Science Daily |date=2008-08-06 |title=Martian Life Or Not? NASA's ''Phoenix'' Team Analyzes Results }}</ref> However, results of experiments published in December 2010<ref name="reanalysis">{{cite journal |title=Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars |journal=Journal of Geophysical Research: Planets |volume=115 |issue=E12010 |pages=E12010 |date=2010-12-15 |first1=Rafael |last1=Navarro-Gonzáles |first2=Edgar |last2=Vargas |first3=José |last3=de la Rosa |first4=Alejandro C. |last4=Raga |first5=Christopher P. |last5=McKay |name-list-style=vanc |doi=10.1029/2010JE003599 |bibcode=2010JGRE..11512010N |doi-access=free }}</ref><ref>{{cite news |title=Correction to "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars" |doi=10.1029/2011JE003854 | journal=Journal of Geophysical Research | date=2011 |volume=116 |issue=E8 |first=Rafael |last=Navarro-González| name-list-style = vanc |bibcode=2011JGRE..116.8011N }}</ref> propose that organic compounds "could have been present" in the soil analyzed by both ''Viking 1'' and ''2'', since NASA's Phoenix lander in 2008 detected perchlorate, which can break down organic compounds. The study's authors found that perchlorate can destroy organics when heated and produce [[chloromethane]] and [[dichloromethane]] as byproduct, the identical chlorine compounds discovered by both Viking landers when they performed the same tests on Mars. Because perchlorate would have broken down any Martian organics, the question of whether Viking found organic compounds is still wide open, as alternative chemical and biological interpretations are possible.<ref>{{cite news |title=Did Viking Mars Landers Find Life's Building Blocks? Missing Piece Inspires New Look at Puzzle |date=2010-09-05 |url=https://www.sciencedaily.com/releases/2010/09/100904081050.htm |work=ScienceDaily |access-date=2010-09-23 }}</ref><ref name = "Biemann_2011" /><ref name="The viking biological investigation"/>

In 2013, astrobiologist Richard Quinn at the Ames Center conducted experiments in which amino acids reacting with hypochlorite, which is created when perchlorate is irradiated with gamma rays, seemed to reproduce the findings of the labeled-release experiment.<ref>{{cite journal | url = http://www.airspacemag.com/space/life-in-universe-special-what-is-life-180958432/#0u6MJfe0h6TiGK1b.99 | title = Would We Know Alien Life If We Saw It? | first = Trudy E. | last = Bell | name-list-style = vanc | journal = Air & Space Magazine | date =  April 2016 }}</ref><ref name="Quinn 2013">{{cite journal | vauthors = Quinn RC, Martucci HF, Miller SR, Bryson CE, Grunthaner FJ, Grunthaner PJ | title = Perchlorate radiolysis on Mars and the origin of martian soil reactivity | journal = Astrobiology | volume = 13 | issue = 6 | pages = 515–20 | date = June 2013 | pmid = 23746165 | pmc = 3691774 | doi = 10.1089/ast.2013.0999 | bibcode = 2013AsBio..13..515Q }}</ref> He concluded that neither hydrogen peroxide nor superoxide is required to explain the results of the Viking biology experiments.<ref name="Quinn 2013"/> A more detailed study was conducted in 2017 by a team of researchers including Quinn. While this study was not specifically designed to match the data from the LR experiment, it was found that hypochlorite could partially explain the control results, including the 160&nbsp;°C sterilization test. The authors stated "Further experiments are planned to characterize the thermal stability of hypochlorite and other oxychlorine species in the context of the LR experiments."<ref>{{cite journal |last1=Georgiou |first1=Christos D. |last2=Zisimopoulos |first2=Dimitrios |last3=Kalaitzopoulou |first3=Electra |last4=Quinn |first4=Richard C. |title=Radiation-Driven Formation of Reactive Oxygen Species in Oxychlorine-Containing Mars Surface Analogues |journal=Astrobiology |date=April 2017 |volume=17 |issue=4 |pages=319–336 |doi=10.1089/ast.2016.1539 |pmid=28418706 |bibcode=2017AsBio..17..319G |url=https://www.liebertpub.com/doi/abs/10.1089/ast.2016.1539 |access-date=27 September 2020|url-access=subscription }}</ref>