Viking lander biological experiments: Difference between revisions
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In 2013, astrobiologist Richard Quinn at the Ames Center conducted experiments in which perchlorates irradiated with gamma rays seemed to reproduce the findings of the labeled-release experiment.<ref>[http://www.airspacemag.com/space/life-in-universe-special-what-is-life-180958432/#0u6MJfe0h6TiGK1b.99 Would We Know Alien Life If We Saw It?] Trudy E. Bell, ''Air & Space Magazine''. April 2016.</ref><ref name="Quinn 2013">{{cite journal |title=Perchlorate Radiolysis on Mars and the Origin of Martian Soil Reactivity |journal=Astrobiology |date=June 7, 2013 |last=Quinn |first=Richard C. |last2=Martucci |first2=Hana F.H. |last3=Miller |first3=Stephanie R. |last4=Bryson |first4=Charles E. |last5=Grunthaner |first5=Frank J. |volume=13 |issue=6 |pages=515–520 |doi=10.1089/ast.2013.0999 |url=http://online.liebertpub.com/doi/abs/10.1089/ast.2013.0999 |accessdate=2016-03-26 |bibcode=2013AsBio..13..515Q |pmid=23746165 |pmc=3691774}}</ref> He concluded that neither hydrogen peroxide nor superoxide is required to explain the results of the Viking biology experiments.<ref name="Quinn 2013"/>
==Did the experiments detect biology or reactive chemistry?==
[[File:Sagan large.jpg|thumb|400 px|Carl Sagan with a model of the Viking Lander in Death Valley California. Viking 1 and II were the first spacecraft to search for present day life on Mars.]]
The [[Viking program#Viking landers|Viking landers]] (operating on Mars from 1976-1982), are the only spacecraft so far to search directly for life on Mars. They landed in the equatorial regions of Mars. With our modern understanding of Mars, this would be a surprising location to find life, as the soil there is thought to be completely ice free to a depth of at least hundred meters, and possibly for a kilometer or more. It is not totally impossible though, as some scientists have suggested ways that life could exist even in such arid conditions, using the night time humidity of the atmosphere, and possibly in some way utilizing the frosts that form frequently in the mornings in equatorial regions.<ref name=sanddunesbioreactor/><ref name=LevinMarsLifeIdea>[http://www.astrobio.net/news-exclusive/the-viking-files/ The Viking Files] Astrobiology Magazine (NASA) - May 29, 2003, astrobio.net (summary of scientific research)</ref>
The Viking results were intriguing, and inconclusive.<ref name="Levin 1976 1322–1329">{{cite journal |doi=10.1126/science.194.4271.1322 |pages=1322–1329 |title=Viking Labeled Release Biology Experiment: Interim Results |date=1976 |last=Levin |first=G. V. |last2=Straat |first2=P. A. |journal=Science |volume=194 |issue=4271 |pmid=17797094 |bibcode=1976Sci...194.1322L }}</ref>
There has been much debate since then between a small number of scientists who think that the Viking missions did detect life,<ref name=VikingGasChromatograph/><ref name=JosephMiller/><ref name="Bianciardi-2012"/><ref name=Levin2016>Levin, G.V. and Straat, P.A., 2016. The case for extant life on Mars and its possible detection by the Viking labeled release experiment. Astrobiology, 16(10), pp.798-810.
{{quote|"'''''It is concluded that extant life is a strong possibility, that abiotic interpretations of the LR data are not conclusive, and that, even setting our conclusion aside, biology should still be considered as an explanation for the LR experiment. Because of possible contamination of Mars by terrestrial microbes after Viking, we note that the LR data are the only data we will ever have on biologically pristine martian samples'''''"}}
</ref> and the majority of scientists who think that it did not.<ref name="PlaxcoGross2011_2"/><ref name=Quine2013/>
The Viking lander had three main biological experiments, but only one of these experiments produced positive results.<ref>[http://www.astrobio.net/news-exclusive/the-viking-files/ The Viking Files, Astrobiology Magazine (NASA) - May 29, 2003]</ref>
* '''The Gas Chromatograph/Mass Spectrometer''' searched for organics, and found no trace of them.
* '''The Gas Exchange experiment''' searched for any gases that evolved from a sample of the Mars soil left in a nutrient solution in simulated martian atmosphere for twelve days. This experiment did detect gases, but so did the control, which repeated the experiment with a sample heated to sterilize it of any possible life. This suggests a chemical explanation.
* '''The labeled release experiment''' used nutrients tagged with <sup>14</sup>C. It then monitored the air above the experiment for radioactive <sup>14</sup>CO<sub>2</sub> gas as evidence that the nutrients had been taken up by micro-organisms. This experiment produced positive results. Also, in this case, the control experiments came out negative. Normally this would suggest a biological explanation. For this experiment the microbes don't have to grow, reproduce. They just have to metabolize the organics and produce the <sup>14</sup>CO<sub>2</sub> gas in the process.
The conclusion at the time, for most scientists, was that the Labeled release experiment had to have some non biological explanation involving the unusual chemistry on Mars. One idea put forward by Albert Yen of JPL was that first carbon dioxide could react with the soil to produce superoxides in the cold dry conditions with UV radiation, which could then react with the small organics of the LR experiment to produce carbon dioxide.<ref name="Plaxco2006">{{cite book |first=Kevin W. |last=Plaxco |first2=Michael |last2=Gross |title=Astrobiology: A Brief Introduction |url=https://books.google.com/books?id=OjYSAA5oHdcC&pg=PA223 |date=2006 |publisher=JHU Press |isbn=978-0-8018-8366-8 |page=223 }}</ref><ref name="PlaxcoGross2011_2"/> The other two experiments seemed to rule out any possibility of a biological explanation.
Some of the LR data remained hard to explain as chemistry and the experimenter's Principle Investigator Gilbert Levin maintained from the beginning that his experiment probably detected life.<ref name=LevinViking/> Here are some of the things that any theory has to explain, in addition to the non detection of organics by the other instruments:
* '''The LR response produced a lot of carbon dioxide rapidly''', which some criticized as "too much too soon" for the levels of life expected there.
* '''A second injection of nutrient actually lead to a 20% decrease''' in the previously evolved <sup>14</sup>CO<sub>2</sub>
* '''A sample maintained at 10 °C in darkness for two months''' at one site and three months at another had no response to the nutrient
* '''A sample heated to 46 °C''' produced 60% less gas
* '''A sample heated to 51 °C''' became erratic and produced 90% less gas
His comments on how this could be explained biologically are that first, the amount of <sup>14</sup>CO<sub>2</sub> released is comparable to a sample from Antarctica and less than is usually released in tests on Earth. The second injection seems to have just wetted the sample and lead to absorption of <sup>14</sup>CO<sub>2</sub> and his conclusion is that the life died during the experiment, which is not too surprising given that most microbes even on Earth can't be cultivated in the laboratory. The difference in effect between 46 °C and 51 °C he considers to be strongly suggestive of life and hard to explain chemically for such a small change. The results for the samples kept in darkness he also considers to be hard to explain without biology.<ref name=LevinViking>{{cite web|last1=Levin|first1=Gilbert|title=The Viking Labeled Release Experiment and Life on Mars|url=http://gillevin.com/Mars/Reprint_107-SPIE.pdf|website=gilbertlevin.com|date=1997}}</ref>
Most other scientists at the time continued to regard the experiments as inconclusive.<ref>{{cite journal |title=The Viking Biological Investigation: Preliminary Results |journal=Science |date=1976-10-01 |first=Harold P. |last=Klein |last2=Levin |first2=Gilbert V. |last3=Levin |first3=Gilbert V. |last4=Oyama |first4=Vance I. |last5=Lederberg |first5=Joshua |last6=Rich |first6=Alexander |last7=Hubbard |first7=Jerry S. |last8=Hobby |first8=George L. |last9=Straat |first9=Patricia A. |last10=Berdahl |first10=Bonnie J. |last11=Carle |first11=Glenn C. |last12=Brown |first12=Frederick S. |last13=Johnson |first13=Richard D. |volume=194 |issue=4260 |pages=99–105 |doi=10.1126/science.194.4260.99 |url=http://www.sciencemag.org/cgi/content/abstract/194/4260/99 |pmid=17793090 |bibcode=1976Sci...194...99K |accessdate=2008-08-15 }}</ref><ref name="Beegle">{{cite journal |title=A Concept for NASA's Mars 2016 Astrobiology Field Laboratory |journal=Astrobiology |date=August 2007 |last=Beegle |first=Luther W. |last2=Wilson |first2=Michael G.|volume=7 |issue=4 |pmid=17723090 |pages=545–577 |url=https://trs.jpl.nasa.gov/bitstream/handle/2014/41570/07-0985.pdf?sequence=1&isAllowed=y |doi=10.1089/ast.2007.0153 |bibcode=2007AsBio...7..545B |accessdate=2009-07-20 |last3=Abilleira |first3=Fernando |last4=Jordan |first4=James F. |last5=Wilson |first5=Gregory R. }}</ref><ref>{{cite web |url=http://www.esa.int/SPECIALS/ExoMars/SEMK39JJX7F_0.html |title=ExoMars rover |publisher=ESA |accessdate=2014-04-14 |deadurl=bot: unknown |archiveurl=https://web.archive.org/web/20080921032233/http://www.esa.int/SPECIALS/ExoMars/SEMK39JJX7F_0.html |archivedate=2008-09-21 |df= }}</ref>
Before the discovery of the oxidizer [[perchlorate]] on Mars in 2008, Joop Houtkooper and Dirk Schulze Makuch made another proposal in 2007 to explain the results, that life on Mars may be using a mixture of water and biogenically created [[hydrogen peroxide]] as its internal solvent. This possible form of life might explain some puzzling aspects of the Viking experiments. On cooling, the eutectic of 61.2% (by weight) mix of water and hydrogen peroxide has a freezing point of −56.5 °C, and also tends to [[super-cool]] rather than crystallize. It is also [[hygroscopic]], an advantage in a water-scarce environment.<ref name=hydrogen_peroxide_life>{{cite journal| url=http://www.cosis.net/abstracts/EPSC2007/00439/EPSC2007-J-00439.pdf|format=PDF| journal=EPSC Abstracts| volume= 2| id= EPSC2007-A-00439| date=2007| publisher=European Planetary Science Congress 2007| title=The H2O2-H2O Hypothesis: Extremophiles Adapted to Conditions on Mars?| first=Joop M.| last= Houtkooper|author2=Dirk Schulze-Makuch| bibcode=2007epsc.conf..558H| pages=558}}</ref><ref>{{cite web| url=http://www.planetary.org/blog/article/00001109/| title=Europlanet : Life's a bleach| date=2007-08-24| first=Doug| last= Ellison| publisher=Planetary.org}}</ref>. It would prefer regions with lower temperatures, and would avoid liquid water. Conditions at the poles would be optimal, but it could also survive in the equatorial regions visited by Viking<ref name=peroxide_life>{{cite journal | title = A Possible Biogenic Origin for Hydrogen Peroxide on Mars | journal = International Journal of Astrobiology | volume = 6 | issue = 2 | pages = 147 | date = 2007-05-22 | first = Joop M. | last = Houtkooper |author2=Dirk Schulze-Makuch | doi = 10.1017/S1473550407003746 | arxiv = physics/0610093 |bibcode = 2007IJAsB...6..147H }}</ref>
Upon heating, Houtkooper and Schulze Makuch's putative organisms might have auto-oxidized catastrophically, leaving the detected gases and very little solid residue. The release of oxygen in dry conditions could be the result of one of its metabolic pathways, decomposition of hydrogen peroxide to water and oxygen, and in wet conditions due to decomposition through hyperhydration. Organic synthesis occurred under dry and not under wet conditions, which again coudl be explained by this form of light. Some of the other data could be the result of microbes coping and failing to cope in the extremely unmartian conditions of high temperatures combined with saturation of water vapour.<ref name=hydrogen_peroxide_life/>
Houtkooper and Schulze Makuch reasoned at the time that no known Earth organisms can reproduce the Viking results, however no known Earth chemistry can either, and any explanation must be based on the differing geochemistry as well as the potential organisms.<ref name=hydrogen_peroxide_life/>
In 2006, [[:es:Mario Crocco|Mario Crocco]] proposed creation of a new [[Taxonomy (biology)|nomenclatural rank]] Gillevinia straata with this potential biochemistry based on the experimental results<ref>Mario Crocco. “Life's major domains and Mars' life nomenclature: first biological classification of a Martian organism and place of the Viking Mission's 1976 active agents in biological taxonomy and systematics.” Electroneurobiologia, vol. 15 (2), pp. 1-34, 2007.</ref>. Normally the organism's DNA has a major role in identification and naming. There is precedent for naming a new species for teaching purposes, raising visibility and encouraging further research. However this classification is seen by other biologists as premature, as it could lend inappropriate credibility to life detection, and the response could also be due to a mix of species<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 |accessdate=2014-04-14 }}</ref>.
Work since then has suggested a possible re-evaluation of those results.
First, some have suggested that the gas chromatograph may not have been sensitive enough to detect the organics.<ref name=VikingGasChromatograph>[http://www.space.com/3038-martian-life-evaded-detection-viking-landers.html Martian Life Could Have Evaded Detection by Viking Landers] Ker Than, Staff Writer | October 24, 2006 05:56pm, Space.com</ref><ref>{{cite journal | doi = 10.1073/pnas.0604210103 | pmid=17060639 | volume=103 | issue=44 | title=The limitations on organic detection in Mars-like soils by thermal volatilization-gas chromatography-MS and their implications for the Viking results | year=2006 | journal=Proceedings of the National Academy of Sciences | pages=16089–16094 | last1 = Navarro-Gonzalez | first1 = R.| bibcode=2006PNAS..10316089N | pmc=1621051 }}</ref> Though other scientists have suggested that they could have detected low levels of organics....<ref>{{cite journal | pmc= 1965509 | pmid=17548829 | doi=10.1073/pnas.0703732104 | volume=104 | issue=25 | title=On the ability of the Viking gas chromatograph-mass spectrometer to detect organic matter | date=June 2007 | journal=Proc. Natl. Acad. Sci. U.S.A. | pages=10310–3 | last1 = Biemann | first1 = K| bibcode=2007PNAS..10410310B }}</ref>
Then in 2002, Joseph Miller,<ref name=JosephMiller>[http://www.gillevin.com/Mars/Reprint119-Miller-Straat-Levin-FINAL_files/Reprint119-Miller-Straat-Levin-FINAL.htm Periodic Analysis of the Viking Lander Labeled Release Experiment], [http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=894168 Proc. SPIE 4495, Instruments, Methods, and Missions for Astrobiology IV, 96 (February 6, 2002)]; {{DOI|10.1117/12.454748}}{{quote|"Did Viking Lander biology experiments detect life on Mars? ... Recent observations of circadian rhythmicity in microorganisms and entrainment of terrestrial circadian rhythms by low amplitude temperature cycles argue that a Martian circadian rhythm in the LR experiment may constitute a biosignature."}}</ref> a specialist in [[circadian rhythm]]s thought he spotted these in the Viking data. He was able to get hold of the original Viking raw data (using printouts kept by Levin's co-researcher Pat Straat) and on re-analysis this seemed to confirm his conclusions.
* The data, though it follows temperature changes, is smoother than you'd expect from a purely chemical reaction response.
* It is also delayed by 2 hours. From analysis of the experiment he concluded that though a 20-minute delay could be explained using variability in CO2 solubility, 2 hours seems too much of a delay to explain that way.
* There are signs of a change of rhythm after the second nutrient injection.
* In an accidental experiment, one of the samples was kept for two months in cold and darkness before it was used. This showed no daily cycle. This is quite hard to explain on basis of chemistry.
Another paper published in 2012 uses cluster analysis [[cluster analysis]] and suggested once more that they may have detected biological activity.<ref name="Bianciardi-2012">{{cite journal |last=Bianciardi |first=Giorgio |last2=Miller |first2=Joseph D. |last3=Straat |first3=Patricia Ann |last4=Levin |first4=Gilbert V. |title=Complexity Analysis of the Viking Labeled Release Experiments |url=http://central.oak.go.kr/repository/journal/11315/HGJHC0_2012_v13n1_14.pdf|journal=IJASS |date=March 2012 |volume=13 |issue=1 |pages=14–26 |bibcode=2012IJASS..13...14B |doi=10.5139/IJASS.2012.13.1.14 |accessdate=2012-04-15|quote= These analyses support the interpretation that the Viking LR experiment did detect extant microbial life on Mars }}</ref><ref name="NGS-20120413">{{cite news |last=Than |first=Ker |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/ |date=2012-04-13 |work=[[National Geographic (magazine)|National Geographic]] |accessdate=2013-07-16 }}</ref>
There is progress in the chemical explanations too. In 1999, experiments with [[hydrogen peroxide]]-saturated [[titanium dioxide]] produced similar results.<ref name="Quinn">{{cite journal |last=Quinn |first=R.C. |last2=Zent |first2=A.P. |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 }}</ref>
On the other hand, a paper published in 2013 by Quinn has refined the chemical explanations suggested for the labeled release observations, using radiation damaged perchlorates. By simulating the radiation environment on Mars, he was able to duplicate radioactive <sup>14</sup>CO<sub>2</sub> emission from the sample.<ref name=Quine2013>[http://www.astrobio.net/news-exclusive/how-habitable-is-mars-a-new-view-of-the-viking-experiments/ How Habitable Is Mars? A New View of the Viking Experiments] By Elizabeth Howell -Astrobiology Magazine (NASA) Nov 21, 2013</ref>
In short, the findings are intriguing but there is no consensus yet on whether the correct interpretation is biological or chemical. Most scientists still favour the chemical explanation, though a few scientists have recently shown renewed interest in a possible biological explanation.
== Critiques ==
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