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However, in , after many years of further experiments on Earth, along with new discoveries on Mars which NASA has now declared "habitable" , and the discovery of microorganisms living under conditions on Earth as severe as those on Mars, he and his Viking Co-Experimenter, Dr. Patricia A.

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Straat, have argued that the Mars results are best explained by living organisms. Recently, Levin and Straat published a perspective piece in the journal Astrobiology in which they reconsider the results of the Viking LR experiment in light of recent findings on Mars and recent proposals for inorganic substances that may mimic the observed metabolism-like processes. They argue that none of the proposed abiotic substances sufficiently explains the Viking results, and that Martian microbes should still be considered as the best explanation of the results. In the LR experiment, both the Viking 1 and Viking 2 landers collected samples of Martian soil, injected them with a drop of dilute nutrient solution, and then monitored the air above the soil for signs of metabolic byproducts.

Since the nutrients were tagged with radioactive carbon, if microorganisms in the soil metabolized the nutrients, they would be expected to produce radioactive byproducts, such as radioactive carbon dioxide or methane. Before launching the Viking spacecraft, the researchers tested the experimental protocol on a wide variety of terrestrial soils from harsh environments, from Death Valley to Antarctica.

In each case, the experiments tested positive for life. In each case, the experiments now tested negative. To further confirm that the experimental procedure would not produce false positives, the researchers tested it on soils known to be sterile, such as those from the Moon and the Surtsey volcanic island near Iceland, which produced negative results as expected.

Mission to Mars

Once on Mars, the LR experiment was performed after the experiment searching for organic molecules came up empty-handed. So it came as a surprise when both Viking landers, located 4, miles apart, collected soil that tested positive for metabolism.

To rule out the possibility that the strong ultraviolet radiation on Mars might be causing the positive results, the landers collected soil buried underneath a rock, which again tested positive. Levin and Straat believe that these results provide some of the strongest evidence that the soil contained Martian life.

Viking Lander

Ever since the LR experiments, researchers have been searching for other kinds of nonbiological chemicals that might produce identical results. In their new paper, Levin and Straat review some of these proposals. One possible candidate is formate, which is a component of formic acid found naturally on Earth. A LR-type experiment found that formate in a soil sample from the Atacama Desert in South America produced a positive result, even though the soil contained virtually no microorganisms.

However, the study did not include a sterilization control, and it's likely that the formate concentration in the Atacama Desert is much higher than that on Mars. Another potential candidate is perchlorate or one of its breakdown products. In , the Phoenix mission to Mars detected perchlorates in the Martian soil. For these reasons, hypochlorite is arguably the best candidate yet to explain the LR results. So at this point, no nonbiological agent has satisfied all of the LR results. Today researchers know much more about Mars than they did 40 years ago. One of the biggest discoveries came in , when the Mars Science Laboratory Curiosity rover detected the presence of organic molecules on Mars for the first time.

Over the past two years, Curiosity's onboard Sample Analysis at Mars SAM laboratory has detected methane, chlorinated hydrocarbons, and other organic molecules. Researchers suspect that these organic substances may have formed on Mars or been carried there by meteorites. The discovery of organic matter on Mars raises the question of why the Viking experiment did not detect organic matter back in As Levin explains, there are multiple reasons that might explain why the Viking results were negative.

It required at least one million microbial cells to detect any organic matter. In addition, the instrument had frequently failed when tested on Earth. Later, it was claimed that perchlorate in the soil destroyed the organic matter. However, I view this cautiously as there is no evidence for perchlorate at the Viking sites.

In light of the recent findings, Levin and Straat believe that it's important to reconsider the LR results as having a biological origin. Other researchers who support this view have proposed that Martian life could take the form of methanogens microorganisms that produce methane as a byproduct , halophiles which can tolerate high salt concentrations as well as severe radiation and low oxygen concentrations , or some type of "cryptobiotic" microorganism that lies dormant until reactivated, such as by a nutrient solution like the one in the LR experiment.

Publishing a paper about life on Mars was very different than publishing more typical studies over the years, Levin's research has included low-calorie sweeteners, pharmaceutical drugs, safer pesticides, and wastewater treatment processes, among others. It took nearly 20 years for Levin and Straat to publish a peer-reviewed paper on their interpretation of the Viking LR results. Though these were invited papers, they were largely ignored by the bulk of astrobiologists in their publications.

Sherry Cady, the editor of Astrobiology. She invited me to submit a paper for peer review. I did and it was promptly bounced, not even sent out for review because of its life claim. It took years of countless renditions and compliance with or explanation away of a myriad of reviewers' comments, but we persisted until we disposed of every adverse comment. Thus, we think this publication is quite significant in that it was scrubbed so thoroughly that the points remaining are firmly established. You can say only that the steps are insufficient. But, to us, that seems a tenuous defense, since no one would refute these results had they been obtained on Earth.

For Levin and Straat, one of the most important reasons for considering the existence of life on Mars is a practical one that may affect future research. In the interest of their health and safety, biology should be held in the forefront of possible explanations for the LR results. Going forward, Levin and Straat propose that carefully designed experiments can help to answer the question of the existence of life on Mars.

In particular, LR-type experiments that test for chiral preference could tell whether the metabolizing substance is biological or chemical, since only biological agents can distinguish between left and right isomers. The scientists also emphasize the importance of the continued search for organic molecules, especially those with biological significance such as amino acids, simple carbohydrates, lipids, proteins, and DNA.

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Future experiments may also provide the possibility of examining Martian soil under a microscope. Despite the positive outlook, Levin and Straat note that all future experiments will have an unavoidable drawback: the potential for contamination by previous landers. In this regard, the Viking landers were unique in that they were the only pristine Martian life -detection experiment that we will ever have. Explore further. More from Astronomy and Astrophysics.

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Fortieth anniversary of Viking invasion of Mars

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Credit: Levin and Straat, , Biosystems. More information: Gilbert V. Levin and Patricia Ann Straat.

Viking 1 & 2

October , 16 10 : DOI: Citation : Did year-old Viking experiment discover life on Mars? This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Synthetic skin could aid wound healing 35 minutes ago. Relevant PhysicsForums posts How can an asteroid get caught at a Lagrange point without a "brake"?

Since the old spacecraft flew, Mars has been visited by many other spacecraft, including the Sojourner, Spirit, Opportunity and Curiosity rovers.

If we actually do manage to send humans to Mars in , as NASA hopes to, a pair of old Vikings will be there waiting for them. Contact us at editors time. Taken by the Viking 1 lander shortly after it touched down, this image is the first photograph ever taken from the surface of Mars, July 20, Contact us at editors time. Taken by the Viking 1 lander shortly after it touched down, this image is the first photograph ever taken from the surface of Mars, July 20, By Marisa Gertz July 18, The first panoramic view of Mars, taken minutes after Viking 1 landed on the surface of Mars, July 20,