Editing Viking lander biological experiments (section)

== Controversy ==
Before the discovery of the oxidizer [[perchlorate]] on Mars in 2008, some theories remained opposed to the general scientific conclusion. An investigator suggested that the biological explanation of the lack of detected organics by GC-MS could be that the oxidizing inventory of the H<sub>2</sub>O<sub>2</sub>-H<sub>2</sub>O solvent well exceeded the reducing power of the organic compounds of the organisms.<ref>{{cite journal | title = A Possible Biogenic Origin for Hydrogen Peroxide on Mars | journal = International Journal of Astrobiology | volume = 6 | issue = 2 | page = 147 | date = 2007-05-22 | first1 = Dirk | last1 = Schulze-Makuch | last2 = Houtkooper | first2 = Joop M.| name-list-style = vanc  | doi = 10.1017/S1473550407003746 | arxiv = physics/0610093 | bibcode = 2007IJAsB...6..147H | s2cid = 8091895 }}</ref>

It has also been argued that the Labeled Release (LR) experiment detected so few metabolising organisms in the Martian soil, that it would have been impossible for the gas chromatograph to detect them.<ref name="Chambers" /> This view has been put forward by the designer of the LR experiment, Gilbert Levin, who believes the positive LR results are diagnostic for life on Mars.<ref>{{Cite journal | doi = 10.1117/2.3201403.03| title = Gilbert Levin: Mars microbes -- proof from the Viking missions?| journal = SPIE Newsroom| year = 2014| last1 = Spie}}</ref><ref>{{Cite web|url=https://blogs.scientificamerican.com/observations/im-convinced-we-found-evidence-of-life-on-mars-in-the-1970s/|title=I'm Convinced We Found Evidence of Life on Mars in the 1970s|last=Levin|first=Gilbert V.|date=2019-10-10|website=Scientific American Blog Network|language=en|access-date=2020-01-13}}</ref> He and others have conducted ongoing experiments attempting to reproduce the Viking data, either with biological or non-biological materials on Earth.  While no experiment has ever precisely duplicated the Mars LR test and control results, experiments with [[hydrogen peroxide]]-saturated [[titanium dioxide]] have produced similar results.<ref name="Quinn">{{cite journal |last1=Quinn |first1=R.C. |last2=Zent |first2=A.P. | name-list-style = vanc |title=Peroxide-Modified Titanium Dioxide: a Chemical Analog of Putative Martian Soil Oxidants |journal=Journal Origins of Life and Evolution of Biospheres |date=1999 |volume=29 |issue=1 |pages=59–72 |doi=10.1023/A:1006506022182 |pmid=10077869 |bibcode=1999OLEB...29...59Q |s2cid=176902 }}</ref>

While the majority of astrobiologists still conclude that the Viking biological experiments were inconclusive or negative, Gilbert Levin is not alone in believing otherwise. The current claim for life on Mars is grounded on old evidence reinterpreted in the light of recent developments.<ref name="Levin">{{cite journal |last=Levin |first=Gilbert | name-list-style = vanc |title=Analysis of evidence of Mars life |journal=Electroneurobiología |date=2007 |volume=15 |issue=2 |pages=39–47 |issn=1850-1826 |arxiv=0705.3176 |bibcode=2007arXiv0705.3176L }}</ref><ref name="Navarro-González_2006">{{cite journal | vauthors = Navarro-González R, Navarro KF, de la Rosa J, Iñiguez E, Molina P, Miranda LD, Morales P, Cienfuegos E, Coll P, Raulin F, Amils R, McKay CP | display-authors = 6 | title = The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 44 | pages = 16089–94 | date = October 2006 | pmid = 17060639 | pmc = 1621051 | doi = 10.1073/pnas.0604210103 | bibcode = 2006PNAS..10316089N | doi-access = free }}</ref><ref name="Paepe">{{cite journal |title=The Red Soil on Mars as a proof for water and vegetation |journal=Geophysical Research Abstracts |date=2007 |first=Ronald |last=Paepe | name-list-style = vanc |volume=9 |issue=1794 |url=http://www.cosis.net/abstracts/EGU2007/01794/EGU2007-J-01794.pdf |access-date=2008-08-14 }}</ref> In 2006, scientist Rafael Navarro demonstrated that the Viking biological experiments likely lacked  sensitivity to detect trace amounts of organic compounds.<ref name="Navarro-González_2006" /> In a paper published in December 2010,<ref name="reanalysis" /> the scientists suggest that if organics were present, they would not have been detected because when the soil is heated to check for organics, perchlorate destroys them rapidly producing chloromethane and dichloromethane, which is what the Viking landers found. This team also notes that this is not a proof of life but it could make a difference in how scientists look for organic [[biosignature]]s in the future.<ref name="Webster" /><ref name="blocks">{{cite news |first=Mike |last=Wall | name-list-style = vanc |title=Life's Building Blocks May Have Been Found on Mars, Research Finds |date=2011-01-06 |url=http://www.space.com/scienceastronomy/mars-viking-detect-organic-molecules-110106.html |work=Space.com |access-date=2011-01-07 |url-status=dead |archive-url=https://web.archive.org/web/20110109010433/http://www.space.com/scienceastronomy/mars-viking-detect-organic-molecules-110106.html |archive-date=2011-01-09 }}</ref> Results from the current [[Mars Science Laboratory]] mission and the under-development [[ExoMars]] program may help settle this controversy.<ref name="blocks" />

In 2006, [[:es:Mario Crocco|Mario Crocco]] went as far as proposing the creation of a new [[Taxonomy (biology)|nomenclatural rank]] that classified some Viking results as '[[metabolism|metabolic]]' and therefore representative of a new form of life.<ref>{{cite web |url=http://contactincontext.org/Crocco_nomenclature_CIC.htm |title=Science works through Mars lander life controversy |publisher=Contactincontext.org |date=2007-03-22 |access-date=2014-04-14 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304034612/http://contactincontext.org/Crocco_nomenclature_CIC.htm |url-status=dead }}</ref> The taxonomy proposed by Crocco has not been accepted by the scientific community, and the validity of Crocco's interpretation hinged entirely on the absence of an oxidative agent in the Martian soil.

According to Gilbert Levin and Patricia Ann Straat, investigators of the LR experiment, no explanation involving inorganic chemistry as of 2016 is able to give satisfactory explanations of the complete data from the LR experiment, and specifically address the question of what active agent on the soil samples could be adversely affected by heating to approximately 50&nbsp;°C and destroyed with long-term storage in the dark at 10&nbsp;°C, as data suggest.<ref name="Levin 2016">{{cite journal | vauthors = Levin GV, Straat PA | 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 | date = October 2016 | pmid = 27626510 | pmc = 6445182 | doi = 10.1089/ast.2015.1464 | bibcode = 2016AsBio..16..798L }}</ref><ref>{{cite web |title=The Viking Lander Labeled Release Experiment Archive |url=http://pds-geosciences.wustl.edu/viking/vl1_vl2-m-lr-2-edr-v1/vl_9010/aareadme.htm |website=wustl.edu}}</ref>