Viking lander biological experiments

File:Viking Biological package.I.png

In 1976 two identical Viking program landers each carried four types of biological experiments to the surface of Mars. The first successful Mars landers, Viking 1 and Viking 2, then carried out experiments to look for biosignatures of microbial life on Mars. The landers each used a robotic arm to pick up and place soil samples into sealed test containers on the craft.

The two landers carried out the same tests at two places on Mars' surface, Viking 1 near the equator and Viking 2 further north.{{cite book |first=Paul |last=Chambers |name-list-style=vanc |title=Life on Mars; The Complete Story |place=London |publisher=Blandford |date=1999 |isbn=978-0-7137-2747-0 |url=https://archive.org/details/lifeonmarscomple00cham }}

The experiments

File:NASM-A19790215000-NASM2016-02690.jpg

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).{{cite web |url=https://history.nasa.gov/SP-4212/ch11-5.html |title=ch11-5 |publisher=NASA |access-date=2014-04-14 }}{{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 }}{{cite web |url=http://history.arc.nasa.gov/hist_pdfs/bio_klein.pdf |title=Harold P. Klein, NASA Ames Hall of Fame }}

= Gas chromatograph – mass spectrometer =

A 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.{{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 }}

The GCMS measured no significant amount of 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."{{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 }}

Experiments conducted in 2008 by the 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"."{{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 }}

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."{{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 }} Biemann has written a commentary critical of the Navarro-González and McKay paper,{{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 }} to which the latter have replied;{{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= }} 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,{{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 }} 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. Periodically, the instrument sampled the atmosphere of the incubation chamber and used a gas chromatograph to measure the concentrations of several gases, including oxygen, CO₂, 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".{{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}}

= Labeled release =

The labeled release (LR) experiment (PI: Gilbert Levin, Biospherics Inc.) gave the most promise for exobiologists. 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 ¹⁴C. The air above the soil was monitored for the evolution of radioactive ¹⁴CO₂ (or other carbon-based{{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}}) gas as evidence that microorganisms in the soil had 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. Sterilization control tests were subsequently carried out by heating various soil samples. Samples heated for 3 hours at 160 °C gave off no radioactive gas when nutrients were injected, and samples heated for 3 hours at 50 °C exhibited a substantial reduction in radioactive gas released following nutrient injection.{{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 }} A sample stored at 10 °C for several months was later tested showing significantly reduced radioactive gas release.{{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 }}

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."{{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 }} 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."{{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] }} 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 superoxides (salts containing O₂⁻). 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."

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.{{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 }}

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.{{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 }}{{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}} Schulze-Makuch revisited the idea in an article for Big Think in 2023.{{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}}{{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}}

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 1976 Viking Mission.{{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 }}{{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 |access-date=2013-07-16 }}

= Pyrolytic release =

The pyrolytic release (PR) experiment (PI: Norman Horowitz, Caltech) consisted of the use of light, water, and a carbon-containing atmosphere of carbon monoxide (CO) and carbon dioxide (CO₂), simulating that on Mars. The carbon-bearing gases were made with carbon-14 (¹⁴C), a heavy, radioactive isotope of carbon. If there were 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 °C (1200 °F), and collected the products in a device which counted radioactivity. If any of the ¹⁴C 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.{{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 }} 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." However, given the persistence of organic release at 90 °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.{{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}} 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."{{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 }}

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.{{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 }} 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.{{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}} 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.{{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 }}{{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 }}{{cite web |url=http://www.esa.int/SPECIALS/ExoMars/SEMK39JJX7F_0.html |title=ExoMars rover |publisher=ESA |access-date=2014-04-14 }}

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 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 (CO₂).

Norman Horowitz was the chief of the Jet Propulsion Laboratory bioscience section for the Mariner and 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.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}} However, he also considered that the conditions found on Mars were incompatible with carbon based life.

In August 2008, the Phoenix lander detected perchlorate, a strong oxidizer when heated above 200 °C. This was initially thought to be the cause of a false positive LR result.{{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 }}{{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 }} However, results of experiments published in December 2010{{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 }}{{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 }} 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.{{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 }}

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.{{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 }}{{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 }} He concluded that neither hydrogen peroxide nor superoxide is required to explain the results of the Viking biology experiments. 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 °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."{{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 }}

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 H₂O₂-H₂O solvent well exceeded the reducing power of the organic compounds of the organisms.{{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 }}

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. 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.{{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}}{{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}} 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.{{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 }}

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.{{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 }}{{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 }}{{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 }} In 2006, scientist Rafael Navarro demonstrated that the Viking biological experiments likely lacked sensitivity to detect trace amounts of organic compounds. In a paper published in December 2010, 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 biosignatures in the future.{{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 }} Results from the current Mars Science Laboratory mission and the under-development ExoMars program may help settle this controversy.

In 2006, Mario Crocco went as far as proposing the creation of a new nomenclatural rank that classified some Viking results as 'metabolic' and therefore representative of a new form of life.{{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 }} 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 °C and destroyed with long-term storage in the dark at 10 °C, as data suggest.{{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 }}{{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}}

Critiques

= Failure to examine the atmosphere =

James Lovelock argued that the Viking mission would have done better to examine the Martian atmosphere than look at the soil. He theorised that all life tends to expel waste gases into the atmosphere, and as such it would be possible to theorise the existence of life on a planet by detecting an atmosphere that was not in chemical equilibrium.{{cite web |url=http://archive.salon.com/people/feature/2000/08/17/lovelock/ |title=James Lovelock, Gaia's grand old man |last=Joseph |first=Lawrence E. | name-list-style = vanc |date=2000-08-17 |work=Salon |access-date=2009-02-10 |url-status=dead |archive-url=https://web.archive.org/web/20090408145043/http://archive.salon.com/people/feature/2000/08/17/lovelock/ |archive-date=2009-04-08 }} He concluded that there was enough information about Mars' atmosphere at that time to discount the possibility of life there. Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate. Although in 2013 the Curiosity rover failed to detect methane at its location in levels exceeding 1.3ppb.{{cite journal | vauthors = Webster CR, Mahaffy PR, Atreya SK, Flesch GJ, Farley KA | title = Low upper limit to methane abundance on Mars | journal = Science | volume = 342 | issue = 6156 | pages = 355–7 | date = October 2013 | pmid = 24051245 | doi = 10.1126/science.1242902 | bibcode = 2013Sci...342..355W | s2cid = 43194305 | url = https://authors.library.caltech.edu/42376/7/Webster.SM.pdf }} later in 2013 and in 2014, measurements by Curiosity did detect methane,NASA, [https://science.nasa.gov/science-news/science-at-nasa/2014/16dec_methanespike Curiosity Detects Methane Spike on Mars], Dec. 16, 2014 (accessed 25 Oct. 2016) suggesting a time-variable source. The ExoMars Trace Gas Orbiter, launched in March 2016, implements this approach and will focus on detection, characterization of spatial and temporal variation, and localization of sources for a broad suite of atmospheric trace gases on Mars and help determine if their formation is of biological or geological origin.{{cite news |first=Paul |last=Rincon | name-list-style = vanc |title=Agencies outline Mars initiative |date=2009-07-09 |publisher=BBC |url=http://news.bbc.co.uk/2/hi/science/nature/8130393.stm |work=BBC News |access-date=2009-07-26 }}{{cite news |title=NASA orbiter to hunt for source of Martian methane in 2016 |date=2009-03-06 |url=http://www.thaindian.com/newsportal/health/nasa-orbiter-to-hunt-for-source-of-martian-methane-in-2016_100163335.html |work=Thaindian News |access-date=2009-07-26 |archive-date=2018-10-05 |archive-url=https://web.archive.org/web/20181005142406/http://www.thaindian.com/newsportal/health/nasa-orbiter-to-hunt-for-source-of-martian-methane-in-2016_100163335.html |url-status=dead }} The Mars Orbiter Mission has also been attempting – since late 2014 – to detect and map methane on Mars' atmosphere.

Studies using the Trace Gas Orbiter and Curiosity have shown that methane is only detectable at night and therefore cannot be detected by the Trace Gas Orbiter which relies on sunlight for its measurements.{{Cite web |date=2021-07-03 |title=Mars methane mystery? Depends on the time of day |url=https://earthsky.org/space/mars-methane-mystery-depends-on-the-time-of-day/ |access-date=2024-12-12 |website=earthsky.org |language=en-US}}{{Cite journal |last1=Webster |first1=Christopher R. |last2=Mahaffy |first2=Paul R. |last3=Pla-Garcia |first3=Jorge |last4=Rafkin |first4=Scot C. R. |last5=Moores |first5=John E. |last6=Atreya |first6=Sushil K. |last7=Flesch |first7=Gregory J. |last8=Malespin |first8=Charles A. |last9=Teinturier |first9=Samuel M. |last10=Kalucha |first10=Hemani |last11=Smith |first11=Christina L. |last12=Viúdez-Moreiras |first12=Daniel |last13=Vasavada |first13=Ashwin R. |date=2021-06-01 |title=Day-night differences in Mars methane suggest nighttime containment at Gale crater |url=https://www.aanda.org/articles/aa/full_html/2021/06/aa40030-20/aa40030-20.html |journal=Astronomy & Astrophysics |language=en |volume=650 |pages=A166 |doi=10.1051/0004-6361/202040030 |issn=0004-6361|doi-access=free |bibcode=2021A&A...650A.166W }}

= Effects of landing rockets =

A press commentary argued that, if there was life at the Viking lander sites, it may have been killed by the exhaust from the landing rockets.{{cite web |title=Did probes find Martian life ... or kill it off? |url=http://www.nbcnews.com/id/16516952 |archive-url=https://web.archive.org/web/20131205002852/http://www.nbcnews.com/id/16516952/ |url-status=dead |archive-date=December 5, 2013 |last=Borenstein |first=Seth | name-list-style = vanc |publisher=Associated Press via NBC News |date=2007-01-07 |access-date=2007-05-31 }} That is not a problem for missions which land via an airbag-protected capsule, slowed by parachutes and retrorockets, and dropped from a height that allows rocket exhaust to avoid the surface. Mars Pathfinder's Sojourner rover and the Mars Exploration Rovers each used this landing technique successfully. The Phoenix Scout lander descended to the surface with retro-rockets, however, their fuel was hydrazine, and the end products of the plume (water, nitrogen, and ammonia) were not found to have affected the soils at the landing site.

References

External links

{{cite web | title = The Viking Lander Labeled Release Data Set | url = http://pds-geosciences.wustl.edu/missions/vlander/lr.html | publisher = U.S. National Aeronautics and Space Administration (NASA) }}
{{cite web | first = David R. | last = Williams | name-list-style = vanc | title = Viking Biology from the NASA NSSDC Master Catalog of Experiments | url = https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1975-075C&ex | work = National Space Science Data Center | publisher = U.S. National Aeronautics and Space Administration (NASA) }}
{{cite web | date = 12 March 2004 | title = The Viking Mission Labeled Release Experiment and the Search for Martian Life: Part I | first = Reginald | last = Smith | name-list-style = vanc | url = http://www.sas.org/E-Bulletin/2004-03-12/features/body.html | archive-url = https://web.archive.org/web/20090402162936/http://www.sas.org/E-Bulletin/2004-03-12/features/body.html | archive-date = 2 April 2009 | url-status = dead | work = Semester at Sea | publisher = Colorado State University }}
{{cite web | first = Nancy | last = Whitaker | name-list-style = vanc | title = The first in situ search for extraterrestrial life: Viking Mission to Mars | url = http://lhag.org/2009%20Presentations/Dec%202009/Exobiology%20for%20Astronomy%20club%20online.pdf | archive-url = https://web.archive.org/web/20130928100118/http://lhag.org/2009%20Presentations/Dec%202009/Exobiology%20for%20Astronomy%20club%20online.pdf | archive-date = 28 September 2013 | url-status = dead | work = SCLH Astronomy Club | date = December 2009 }}

Category:Astrobiology space missions

Category:Viking program

Category:Extraterrestrial life