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Do We Have Evidence for Life on Mars?

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Is there life on Mars? Engineer and former Viking program team leader Gilbert Levin and microbiologist Steve McDaniel debate NASA Planetary Scientist Christopher McKay on that statement below.


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Who makes the better argument?

Gilbert V. Levin Honorary Professor, Buckingham Centre for Astrobiology of Buckingham University


Or rather, "Hell, yes!" We not only have evidence for life on Mars, we have had proof that there is life in Martian soil for nearly 40 years. Put another way, if the ice machine in a McDonald's on Mars had been tested by the Viking landers for the Greater Olympus Mons City Health Department, they would have shut the place down in 1976. That's the kind of proof NASA has, and it is not that complicated, though some would have us believe that it is.

The Viking LR test conducted on Mars was the standard analysis used to test for microorganisms described in the Standard Method for the Examination of Water and Wastewater, published by the American Public Health Association, the Water Environment Federation and the American Waterworks Association and used by public health departments world-wide for decades.

Here's how it works. An aqueous solution of the sugar lactose is mixed with the sample to be tested for microbial contamination. If they are present, microorganisms metabolizing the lactose produce gas bubbles, the visual observation of which is the evidence for microbial contamination. The Viking LR added several additional organic nutrients, all of which were thought to have formed on early Earth (and Mars) and to have participated in the genesis and evolution of life. In other words, it was a yummy stew for microorganisms, even primeval sorts. But the LR went one step further. It applied the key part of any scientific experiment, a control. This was added to preclude the possibility that some exotic chemical on Mars might cause a false positive. The control was done by heating a duplicate sample of any soil yielding a positive result to a temperature to kill any microbes, but not high enough to destroy chemicals likely to interfere. After cooling, the control was tested in the LR: a negative response testified that the original positive response was biological; a positive response indicated the response had been chemical.

To increase the sensitivity and speed of the method of the Viking tests, so that the experiments might be completed before possible spacecraft or communication failure, the nutrients were tagged with radioactive carbon. Any gas produced from the nutrient solution would be radioactive, permitting its detection long before visible bubbles formed. Thus, the LR was the Standard Method, merely augmented with additional nutrients and radioactive food sources.

Viking 1 landed on July 20, 1976, and Viking 2 landed, some 4,000 miles away on the Martian surface September 3rd of that same year. A total of four tests for life were positive. All five controls that were conducted supported the biological nature of these positive responses. Their amplitudes and kinetics were virtually the same as produced by many of the thousands of tests of microbial soils performed on Earth -- which tests never produced a false result, attesting to the reliability of the method. As noted, this would be proof enough if it were run by the health department in any city on Earth to shut down the local McDonald's.

The Viking LR Mars results were largely discounted in 1976. Why?

The principal reasons initially given by NASA were: the failure of the Viking organic analysis instrument (GCMS -- gas chromatograph mass-spectrometer) to find any organic matter, and/or the presence of an unidentified-highly oxidizing-lethal substance in the Martian soil, and/or the lack of liquid water in the Martian soil. How have these reasons held up over the years since Viking landers completed their missions?

Several post-Viking publications have found a variety of faults with the Viking GCMS, effectively impugning its results. Insensitivity or equipment failure is the most likely cause the GCMS failed in its mission. Furthermore, the Viking Pyrolytic Release (PR) experiment showed that organic compounds were being formed and accumulating in the Martian soil.

Curiosity, NASA's most recent rover sent to Mars, has reported finding several types of low molecular weight organics. But, like the results of the Viking PR, these are not complex enough to support the definitive claim to life -- yet. Curiosity has not, as of this writing, reported on results from its liquid extraction experiment for detecting organics as complex as amino acids. Since the finding of complex organic compounds by the Viking GCMS in 1976 would have led to instant acceptance of the Viking LR results, it seems only logical that, should Curiosity find them now, the same conclusion should result. Thus, data from the liquid extraction method applied to Martian soil are eagerly anticipated.

The Mariner 9 IRIS experiment, several Earth-based observations, and results from Pathfinder, Phoenix, and now Curiosity have demonstrated very clearly that there is no highly oxidizing substance like peroxide, or any other strong oxidant dominating the Martian soil. "The chemical analysis of the sample also revealed compounds in varying states of oxidation -- for example both sulfate and sulfide. This is significant because it demonstrates that the environment was not violently oxidizing (emphasis added)," Joe Michalski, key Curiosity scientist, as reported in Chemistryworld, by Simon Hadlington, Mar. 18, 2013.

Curiosity has also supported Viking's finding of liquid water in the Martian soil. Figure 1 (view here), from Curiosity's analysis of the 4th John Klein rock sample, shows water vapor streaming off the sample when heated from about 0 deg. to about 13 deg. C. Curiosity data also show that the atmospheric pressure never falls below 6.1 mb, the triple point for water, and that the ground temperature frequently goes above 0 deg. C on the soil in Gale Crater. These conditions ensure that water in the sample is routinely in liquid state.

To date, of the countless experiments performed and abiotic theories proposed, none has been proven scientifically tenable in explaining away the Viking test and control data as evidence for life. While some experiments have shown that a variety of oxidants can react with one or more of the LR nutrients to evolve gas, none has met the thermal control data. This includes the recently proposed perchlorates. Either by oversight or by careful selection of data, none of the abiotic explanations addresses all of the control data, especially the one most difficult to explain -- the loss of the activity of the sample after storage for two months in a dark box at the modest temperature of 10 deg. C (a temperature frequently reached diurnally on the Martian surface). These "NASAyers" never discuss the far more likely explanation -- death of the organisms isolated from their native Martian environment.

The proof of life on Mars is the hard data of the Viking LR, adequately replicated at two Martian sites, and supported by virtually every new finding about the habitability of Mars that has arisen in the 40 intervening years. Namely, 1. Positive results from a universally accepted test; 2. Negative responses from well-designed controls; and 3. The absence of any sustainable experiment or theory to provide a non-biological explanation of the data in points 1 and 2, above. Supporting this strong positive proof, are the continual findings of extremeophile organisms living happily in the most Mars-like environments on Earth, and the increasing knowledge of Mars' habitability, as recently confirmed by Curiosity

What more is needed to constitute proof of extant, native microbes on Mars? Maybe further confirmation of that proof by Curiosity's finding of complex organic matter in the Martian soil might add the somehow lacking reassurance the "Flat Mars Society" seems to require. Ultimately, scientific reason will prevail. And, the paradigm will change, alas, once more deflating man's ego as the center of the universe and its home planet being the only one hosting life forms. About time after nearly 40 years NASA, wouldn't you say?

Christopher P. McKay responds to Gilbert V. Levin

Christopher P. McKay Planetary Scientist, the Space Science Division of NASA Ames


In 1976, the Viking Landers reached Mars with the first, and to date, only search for life on another world. Instead of a clear result Viking gave us three puzzles:

  1. The GCMS appeared to show that there were no organics in the soil at the ppb level despite the expected influx of organic rich meteorites on Mars.
  2. The GEx instrument showed the release of O2 when the soil was humidified, with release partially continuing after heating.
  3. The LR instrument showed the consumption of organics and release of CO2, with the activity greatly diminished after heating.

For three decades after Viking the explanations offered to resolve these puzzles were unsatisfactory. The dominant explanation could be called the H2O2 hypothesis. The key element of this hypothesis was the purported production of H2O2 in the Martian atmosphere by UV radiation. This H2O2 was then thought to deposit on the soil and create other, more thermally stable, oxidants such as KO2. These soil oxidants destroyed any Martian organics, explaining puzzle #1. The thermally stable oxidants reacted with water in the GEx instrument releasing O2, explaining puzzle #2, and finally the H2O2 reacted with the organics to produce CO2 and the H2O2 was destroyed by heating, providing an explanation for puzzle #3.

A small, but active, minority led by Gil Levin, the PI of the LR instrument proposed what could be the life hypothesis. The key element of this hypothesis was that the LR results conformed to the pre-launch expectations for a biological result. By ignoring puzzles #1 and #2, the life hypothesis could be maintained. Further to support the life hypothesis it was pointed out that the H2O2 kinetics did not explain the thermal behavior of the LR results, nor was there evidence of H2O2 in the soil and in fact H2O2 is should be unstable in the Martian soil. Indeed no chemical explanation proposed for the two decades following the Viking landing could faithfully reproduce the LR results, prompting Gil Levin to state that from this it must follow that the LR results were biological.

The life hypothesis correctly pointed out the flaws in the H2O2 hypothesis. The flaws in the life hypothesis are obvious; ignoring puzzles #1 and #2, and assuming that the lack of a chemical explanation proves a biological one. Of course the lack of a chemical explanation could have been an indication of our poor knowledge of the Martian soil. In this later case it would be expected that further measurements on the nature of the Martian soil would provide a solution to the three puzzles. And indeed it did.

The solution began to emerge when the Phoenix mission landed on Mars in May 2008, almost 32 years after Viking. The biggest surprise, and the most important result from the Phoenix mission was the discovery that the chlorine in the soil at the Phoenix site was predominantly in the form of perchlorate salt. Chlorine in the soil on Mars had been measured by Viking, Pathfinder, and the two MER rovers as well as mapped from orbit by the Mars Odyssey. It was known that chlorine was widespread on Mars but it had been assumed that it was all in the form of the chloride salt -- for example as sodium chloride. To discover that it was in the form of perchlorate salt was quite unexpected.

Perchlorate salts are curious because; they can keep water liquid to very low temperatures (-70ºC), they can be consumed by certain microorganisms as part of an energy source, they interfere with the thyroid glands of humans and there thus toxic, and finally they decompose at high temperatures (300-500ºC) releasing reactive O and Cl atoms -- which is why perchlorate salts are used in solid rockets. On Earth perchlorate salts are found in the soils and ground water around military test sites and are a major source of concern when they are at the few ppb levels. Finding them on Mars as the major form of chlorine at the percent level was completely unexpected. But perchlorate has proven to be the key to solve all three of the Viking puzzles.

Puzzle #1 Solution: As explained by Navarro-González et al. (in J. of Geophysical Research Volume 115, 2010), because perchlorate salt was in the soil when the Viking landers heated the soil to 500ºC to volatilize the organics the perchlorate salt decomposed releasing reactive O and Cl which would have reacted with the organics forming CO2 and small levels of chlorinated organics. In fact looking back at the Viking GCMS results there were small levels of chlorinated organics detected. The Curiousity Rover results have confirmed the Phoenix discovery of perchlorate salts and have shown that they are present in the shallow subsurface. In addition Curiosity has shown that there are very little organics in the soil and subsurface. This likely is due to the perchlorate being activated by radiation producing bleach-like compounds that destroy organics. This has been shown in the laboratory by Quinn et al. (2013). So the perchlorate effect on organics is two-fold; first it destroys them over time due to the bleach produced by perchlorate radiolysis and second it reacts with any residual soil organics in the instruments when heating to high temperatures. This is why The Viking GCMS did not detect organics. Puzzle #1 solved.

Puzzle #2 Solution: Quinn et al. (in Astrobiology Volume 13, Number 6, 2013) showed that the decomposition of perchlorate salt by cosmic radiation produces oxygen gas which is trapped in the salt compounds. When humidified, the highly soluble perchlorate salt would liquefy and release this gaseous oxygen. Puzzle #2 solved.

Puzzle #3 Solution: The Quinn et al. (2013) results show that perchlorate radiolysis produces bleach-like compounds that can decompose organics and are degraded with temperature. The experiments can faithfully reproduce the LR results. Puzzle #3 solved.

All of the Viking puzzles have been solved, there is no longer any need to suggest that life was detected. Thus, and unfortunately, we have no evidence yet for life on Mars. To find evidence of life, I think we will have to dig deeper, well below the level of perchlorate salt and ionization radiation -- five meters deep or more.

Gilbert V. Levin responds to Christopher P. McKay



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