Editing Viking lander biological experiments (section)

== The experiments ==
[[File:NASM-A19790215000-NASM2016-02690.jpg|thumb|Viking lander]]

The four experiments below are presented in the order in which they were carried out by the two Viking landers. The biology team leader for the Viking program was [[Harold P. Klein]] (NASA Ames).<ref>{{cite web |url=https://history.nasa.gov/SP-4212/ch11-5.html |title=ch11-5 |publisher=NASA |access-date=2014-04-14 }}</ref><ref>{{Cite journal |first=Sara |last=Acevedo | name-list-style = vanc |title=In Memoriam Dr. Harold P. Klein (1921 - 2001) |journal=Origins of Life and Evolution of the Biosphere |volume=31 |issue=6 |pages=549–551 |date=2001-12-01 |doi=10.1023/A:1013387122386 |bibcode=2001OLEB...31..549A |s2cid=39294965 }}</ref><ref>{{cite web |url=http://history.arc.nasa.gov/hist_pdfs/bio_klein.pdf |title=Harold P. Klein, NASA Ames Hall of Fame }}</ref>

=== Gas chromatograph – mass spectrometer ===
A [[Gas chromatography–mass spectrometry|gas chromatograph – mass spectrometer]] ('''GCMS''') is a device that separates vapor components chemically via a [[gas chromatograph]] and then feeds the result into a [[mass spectrometer]], which measures the [[molecular weight]] of each chemical. As a result, it can separate, identify, and quantify a large number of different chemicals. The GCMS (PI: [[Klaus Biemann]], MIT) was used to analyze the components of untreated Martian soil, and particularly those components that are released as the soil is heated to different temperatures. It could measure molecules present at a level of a few parts per billion.<ref>{{cite book |title=Mars |work=Space Science Series |publisher=University of Arizona Press |date=1992-10-01 |first1=Hugh H. |last1=Kieffer |first2=Bruce M. |last2=Jakosky |first3=Conway W. |last3=Snyder |first4=Mildred |last4=Matthews |name-list-style=vanc |isbn=978-0-8165-1257-7 |url-access=registration |url=https://archive.org/details/mars0000unse }}</ref>

The GCMS measured no significant amount of [[natural organic matter|organic molecules]] in the Martian soil. In fact, Martian soils were found to contain less carbon than lifeless lunar soils returned by the [[Apollo program]]. This result was difficult to explain if Martian bacterial metabolism was responsible for the positive results seen by the Labeled Release experiment (see below). A 2011 [[astrobiology]] textbook notes that this was the decisive factor due to which "For most of the Viking scientists, the final conclusion was that the ''Viking'' missions failed to detect life in the Martian soil."<ref name="PlaxcoGross2011">{{cite book |first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross | name-list-style = vanc |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=x83omgI5pGQC&pg=PA282 |date=2011 |publisher=JHU Press |isbn=978-1-4214-0194-2 |pages=282–283 |edition=2nd }}</ref>

Experiments conducted in 2008 by the [[Phoenix (spacecraft)|''Phoenix'' lander]] discovered the presence of [[perchlorate]] in Martian soil. The 2011 astrobiology textbook discusses the importance of this finding with respect to the results obtained by ''Viking'' as "while perchlorate is too poor an oxidizer to reproduce the LR results (under the conditions of that experiment perchlorate does not oxidize organics), it does oxidize, and thus destroy, organics at the higher temperatures used in the Viking GCMS experiment. NASA astrobiologist [[Christopher McKay]] has estimated, in fact, that if ''Phoenix''-like levels of perchlorates were present in the Viking samples, the organic content of the Martian soil could have been as high as 0.1% and still would have produced the (false) negative result that the GCMS returned. Thus, while conventional wisdom regarding the ''Viking'' biology experiments still points to "no evidence of life", recent years have seen at least a small shift toward "inconclusive evidence"."<ref name="PlaxcoGross2011_2">{{cite book |first1=Kevin W. |last1=Plaxco |first2=Michael |last2=Gross | name-list-style = vanc |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=x83omgI5pGQC&pg=PA285 |date=2011-08-12 |publisher=JHU Press |isbn=978-1-4214-0194-2 |pages=285–286 |access-date=2013-07-16 }}</ref>

According to a 2010 NASA press release: "The only organic chemicals identified when the Viking landers heated samples of Martian soil were [[chloromethane]] and [[dichloromethane]]—chlorine compounds interpreted at the time as likely contaminants from cleaning fluids." According to a paper authored by a team led by [[Rafael Navarro-González]] of the [[National Autonomous University of Mexico]], "those chemicals are exactly what [their] new study found when a little perchlorate—the surprise finding from Phoenix—was added to desert soil from Chile containing organics and analyzed in the manner of the Viking tests." However, the 2010 NASA press release also noted that: "One reason the [[chlorinated organics]] found by Viking were interpreted as contaminants from Earth was that the ratio of two isotopes of chlorine in them matched the three-to-one ratio for those isotopes on Earth. The ratio for them on Mars has not been clearly determined yet. If it is found to be much different than Earth's, that would support the 1970s interpretation."<ref name="Webster">{{cite web |last1=Webster |first1=Guy |last2=Hoover |first2=Rachel |last3=Marlaire |first3=Ruth |last4=Frias |first4=Gabriela |name-list-style=vanc |date=2010-09-03 |url=http://www.jpl.nasa.gov/news/news.cfm?release=2010-286 |title=Missing Piece Inspires New Look at Mars Puzzle |publisher=NASA Jet Propulsion Laboratory |access-date=2010-10-24 |archive-date=2010-11-03 |archive-url=https://web.archive.org/web/20101103012112/http://www.jpl.nasa.gov/news/news.cfm?release=2010-286 |url-status=dead }}</ref> Biemann has written a commentary critical of the Navarro-González and McKay paper,<ref name = "Biemann_2011">{{cite journal | vauthors = Biemann K, Bada JL |title=Comment on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars" by Rafael Navarro-González et al. |doi=10.1029/2011JE003869 |journal=Journal of Geophysical Research |date=2011 |volume=116 |issue=E12 |pages=E12001 |bibcode=2011JGRE..11612001B |doi-access=free }}</ref> to which the latter have replied;<ref>{{cite journal | doi = 10.1029/2011JE003880 | bibcode=2011JGRE..11612002N | volume=116 | issue=E12 | pages=E12002 | title=Reply to comment by Biemann and Bada on "Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars" | journal=Journal of Geophysical Research| last1=Navarro-González | first1=Rafael | last2=McKay | first2=Christopher P. | name-list-style = vanc | year=2011 | doi-access= }}</ref> the exchange was published in December 2011. In 2021 the chlorine isotope ratio on Mars was measured by the [[Trace Gas Orbiter]] and found to be almost indistinguishable from the terrestrial ratio,<ref>{{cite journal| last1=Trokhimovskiy |first1=A. |last2=Fedorova |first2=A.A. |last3=Olsen |first3=K.S. |last4=Alday |first4=J. |last5=Korablev |first5=O. |last6=Montmessin |first6=F. |last7=Lefèvre |first7=F. |last8=Patrakeev |first8=A. |last9=Belyaev |first9=D. |last10=Shakun |first10=A.V. |title=Isotopes of chlorine from HCl in the Martian atmosphere |journal=Astronomy & Astrophysics |date=July 2021| volume=651 |issue=A32 |pages=A32 |doi= 10.1051/0004-6361/202140916 |bibcode=2021A&A...651A..32T |s2cid=236336984 |doi-access=free }}</ref>  leaving the interpretation of the GCMS results inconclusive.

=== Gas exchange ===
The gas exchange ('''GEX''') experiment (PI: [[Vance Oyama]], NASA Ames) looked for gases given off by an incubated soil sample by first replacing the Martian atmosphere with the inert gas [[helium]].  It applied a liquid complex of organic and inorganic nutrients and supplements to a soil sample, first with just nutrients added, then with water added too.<ref name="Chambers" /> Periodically, the instrument sampled the atmosphere of the incubation chamber and used a [[gas chromatograph]] to measure the concentrations of several gases, including [[oxygen]], CO<sub>2</sub>, [[nitrogen]], [[hydrogen]], and [[methane]].  The scientists hypothesized that metabolizing organisms would either consume or release at least one of the gases being measured.

In early November 1976, it was reported that "on Viking 2, the gas exchange experiment is producing analogous results to those from Viking 1. Again, oxygen disappeared once the nutrient solution came into contact with the soil. Again, carbon dioxide began to appear and still continues to evolve".<ref>{{Cite web | url=https://books.google.com/books?id=JqEhtUjqORIC&pg=PA272 |title = New Scientist| publisher = Reed Business Information|date = 1976-11-04|last1=Burgess|first1=Eric}}</ref>

=== 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>

=== 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>