Scientists have revisited data from NASA's 1976 Viking mission to Mars, and say that the positive detection of microbial life in 1976 coupled with recent evidence of water, organic molecules, and methane in the Martian environment, point to extant life being a "strong possibility" and warrant a reevaluation of the potential for life on Mars.
This goes against the widespread opinion at the time (and ever since) that the positive results of the Viking Labeled Release (LR) experiment in 1976, which involved analysis of Martian soil samples by the Viking landers, can be explained by non-biological processes. The LR experiment consisted of adding a nutrient solution tagged with radioactive Carbon-14 to samples of Martian soil and monitoring it for production of radioactive gases.
Scientists Gilbert Levin and Patricia Ann Straat were experimenters during the original Viking LR mission and have been strong advocates over the years of the biological explanation of the experiment results. Their new paper states “In the years since Viking, many attempts have been made to explain the LR results nonbiologically, but so far, no nonbiological explanation has met all the criteria. Life may therefore still exist, if only in a cryptobiotic state, subject to resuscitation whenever water becomes available.”
Authors Levin and Straat state —
- In the absence of a nonbiological agent that satisfies all Viking findings, and in view of environmental evidence that Mars may well be able to support extant life, it seems prudent that the scientific community maintain biology as a viable explanation of the LR experimental results.
- It seems inevitable that astronauts will eventually explore Mars. In the interest of their health and safety, biology should be held in the forefront of possible explanations for the LR results.
- Plans for any Mars sample return mission should also take into account that such a sample may contain viable, even if dormant, alien life.
The authors recommend that life-seeking experiments be considered for future missions. These should include -
- The continued search for organic molecules of biological importance (e.g., amino acids, simple carbohydrates, lipids, DNA, protein);
- The conduct of further metabolic experiments, including a search for chiral preference in metabolism;
- The close examination of any tantalizing surface features; and
- Perhaps even microscopic examination of martian soil with and without the addition of water or water vapor.
They caution — "that Mars may already have been infected by the many spacecraft that have landed there; although Viking was heat-treated to reduce microbial counts, no other spacecraft have been similarly treated."
Viking and the LR Experiments
Vikings 1 and 2 were launched August 20 and September 9, 1975, and landed safely on Mars, 6400 km apart, on July 20 and September 3, 1976, respectively. The landers contained instruments to dredge and collect soil from a depth of about 4 cm, which were then analyzed with the LR equipment.
In the LR extraterrestrial life detection experiment, a nutrient solution tagged with radioactive Carbon-14 was added to a sample of martian soil, and the mixture was continuously monitored for evolution of radioactive gas. Any microorganisms in the soil would metabolize the nutrients and produce gases such as carbon dioxide and/or methane.
The LR nutrient substrates were sodium formate, sodium lactate, glycine, alanine, and calcium glycolate, all Miller-Urey compounds. Both left-handed and right-handed isomers of alanine and of lactate were included to provide for a possible different chirality with martian life.
The experimental procedure consisted of adding 0.115 mL of nutrient solution to 0.5 cc soil sample in a 3.5 cc cylindrical test chamber that had a 2 cm diameter. This provided a moisture gradient starting with the injected liquid at the center of the sample and progressing to minimal moisture at the periphery of the sample. Other environmental conditions chosen for the experiment were a temperature of 10C – 2C to guarantee liquidity of the nutrient, and martian atmosphere with a helium overpressure of 85 mbar to assure liquidity in the event the martian atmosphere was below the triple point.
The testing was done on soil samples under multiple conditions —
- At nominal 10°C to 2°C temperature. A second dose of nutrients were added after a few days.
- A sample sterilized at 160°C to kill all microbes.
- A sample treated to 50°C to degrade microbes.
- A sample stored in the dark for two months at 10°C.
Viking LR Experiment Testing
Before launching the Viking spacecraft, the researchers tested the LR 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. Then as a control, the researchers heated the samples to 160°C to kill all lifeforms, and then retested. 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.
Viking LR Experiment Results
Both Viking landers, located 6400 km apart, collected soil that tested positive for metabolism. The figure below shows the detection of radioactive gases for Viking Lander 1 (VL1).
Gas evolution in VL1-1 began to level off after around 2 sols, and had nearly plateaued 7 sols after the first nutrient injection. This plateau could indicate either running out of substrate or “death” of the active agent, or both. An additional injection of nutrient on Sol 7 failed to produce additional gas evolution, indicating that the plateau resulted from lack of active agent (microbes) rather than lack of substrate. Instead, there was an immediate depletion in the headspace gas, attributed to reabsorption of CO2, by the wetted alkaline soil.
By contrast, most terrestrial soils responded positively to a second injection of nutrient.
Testing of soil stored for two months at 10–26°C gave negative results. This was considered to be consistent with biology. Such losses are not expected in terrestrial analog soils because these temperatures are within normal range for most terrestrial soils, even over long periods of time. On Mars, however, although soil may temporarily see 10°C or even higher temperatures during a normal diurnal cycle, at least during the warmer times of the martian year, the daily exposure to such elevated temperature would be brief; martian organisms would therefore be expected to be much more likely to succumb to long-term storage at 10–26°C than would terrestrial microorganisms. Thus, possibly stressed by long-term isolation from their natural environment at elevated temperatures, the putative martian organisms may have died.
The list below summarizes the characteristics determined for the active agent. Each of these characteristics is reminiscent of responses by a compendium of terrestrial microorganism species. Further, the Viking LR data show similarity to the SN103 data obtained with California soil held under martian conditions for 3 days prior to testing. All in all, the results of the Viking LR experiment are consistent with a biological explanation.
- Produced positive response when injected with nutrient solution, similar in kinetics and amplitude to responses produced by LR test of a number of terrestrial viable soils.
- Inactivated by preheating to 160°C for 3 h, similar to terrestrial tests of active soils.
- Significantly reduced when preheated to approximately 50°C for 3 h.
- Inactivated when stored approximately 2 months in the dark in the soil distribution box at approximately 10°C.
- Activation of soil not caused by UV exposure.
- A second injection of nutrient solution to positively responding soil caused approximately 25% of gas already evolved to disappear from detector cell (probably reabsorbed into soil), gradually to re-evolve. Active agent no longer available at time of second injection.
- Widespread distribution of active agent (Viking sites 1 and 2 are 6400 km apart).
Please read the full paper at online.liebertpub.com/… for a fascinating discussion of all the results.
Ever since the LR experiments, researchers have been searching for other kinds of nonbiological chemicals and processes that might produce identical results.
One possible candidate is formate, which is a component of formic acid found naturally on Earth. A 2003 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 2009, the Phoenix mission to Mars detected perchlorates in the Martian soil. Although perchlorates could yield a positive result because they produce gas when interacting with some amino acids, they do not break down at 160 °C, and so would continue to give positive results after the sterilization control.
A 2013 study proposed that cosmic rays and solar radiation can cause perchlorate to break down into hypochlorite, which would produce positive results and, unlike perchlorate, is destroyed by heating at 160 °C. For these reasons, hypochlorite is arguably the best candidate yet to explain the LR results.
Nevertheless, Levin and Straat note that hypochlorite has not yet been tested at 50 °C (the temperature at which the activity of the Martian soil was significantly reduced) or after long-term storage in the dark (which produced a negative result for the Martian samples). So at this point, no nonbiological agent has satisfied all of the LR results.
Other Life Experiments in the Viking Mission
Viking contained 3 other experiments to test for life — GCMS (gas chromatograph—mass spectrometer), the gas exchange (GEX) experiment and the pyrolytic release (PR) experiment.
The Viking GCMS (gas chromatograph - mass spectrometer) failed to detect organic matter back in 1976.
The gas exchange (GEX) experiment looked for gases given off by an incubated soil sample. Results were negative.
The PR experiment tested for presence of photosynthetic organisms that would take up radioactive-labeled carbon and was positive.
A 2011 astrobiology textbook notes that the GCMS result 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.”
In a scientific paper published in 1981, Levin and Straat demonstrated that in pre-flight-to-Mars testing of an Antarctic soil sample that contained life (#726), their Viking Labeled Release experiment found microbial activity, while the pre-flight Viking GCMS test model could not detect organic molecules. Yet this would be the instrument used to render the final verdict against any positive evidence of life on Mars that might have been found by the Viking biology instruments.
Levin explains, there are multiple reasons that might explain why the Viking results were negative. "We long ago pointed out the problems with the Viking GCMS," Levin said. "Even its experimenter, Dr. Klaus Biemann, often stressed that the GCMS was not a life-detection experiment. 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." phys.org/…
In a 2002 paper published by Joseph Miller, he speculates that recorded delays in the system's chemical reactions point to biological activity similar to the circadian rhythm previously observed in terrestrial cyanobacteria.
Meteorite ALH84001 as Evidence of Past Biological Activity on Mars
In 1996, a Science magazine article documented possible evidence of nanofossils found on Martian meteorite ALH84001. The research team, led by NASA's David McKay, found carbonate globules that are similar to those found in some terrestrial bacterially induced carbonate precipitates on Earth. Polycyclic aromatic hydrocarbons (PAHs) and secondary minerals and textures in the meteorite provided further evidence of past Martian biota.
According to NASA, 16 million years ago a large asteroid hit Mars and sent ALH84001 into space; from there, it made its way to Earth—where it had laid undisturbed for 13,000 years. At a press conference on August 7, 1996, President Bill Clinton announced that U.S. scientists believed ALH84001 contained evidence of previous biological activity on Mars.
Water on Mars
Various Mars missions, including Curiosity, have showed that there was once abundant flowing water, and ice still exists in pockets at the poles – and possibly elsewhere.
Curiosity, which landed near the Gale crater, has shown that Gale crater was once a water-filled lake that could have lasted for millions of years. Just why it evaporated along with all Mars’s surface water is a mystery that instruments on board NASA’s orbiter, Maven, are trying to solve. In contrast to the fast water that carved the Martian valleys, the water in Gale Crater could have existed for long enough to support life.
New findings from NASA's Mars Reconnaissance Orbiter (MRO) in Sep 2015 provided strong evidence that liquid water flows intermittently on present-day Mars.
Almost all water on Mars today exists as ice, though it also exists in small quantities as vapor in the atmosphere and occasionally as low-volume liquid brines in shallow Martian soil. The only place where water ice is visible at the surface is at the north polar ice cap. Abundant water ice is also present beneath the permanent carbon dioxide ice cap at the Martian south pole and in the shallow subsurface at more temperate latitudes. More than five million cubic kilometers of ice have been identified at or near the surface of modern Mars, enough to cover the whole planet to a depth of 35 meters. Even more ice is likely to be locked away in the deep subsurface. en.wikipedia.org/...
The Martian atmosphere consists of approximately 96% carbon dioxide, 1.9% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane. There has been renewed interest in its composition since the detection of traces of methane in 2003 that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.
On 16 December 2014, NASA reported detecting an unusual increase, then decrease, in the amounts of methane in the atmosphere of the planet Mars
The source of the methane plume remains a mystery. It does not appear to correlate with Martian seasons, or pressure changes or asteroid impacts that might have loosened buried stores of methane. But, in the words of Sushil Atreya, director of the University of Michigan’s Planetary Science Laboratory, whether the source is geological or biological in origin, “it tells us Mars is currently active”.
Methane breaks up in the presence of ultraviolet solar radiation. Based on photochemical models and on the current understanding of the composition of the Martian atmosphere, methane has a chemical lifetime of about 300-600 years, which is very short on geological time scales. This implies that the methane that is observed today cannot have been produced 4.5 billion years ago, when the planets formed.
Curiosity also uncovered evidence of complex organic molecules in drilled samples of sandstone rocks in the Sheepbed mudstone in Gale crater (the Cumberland rock sample), using the Sample Analysis at Mars (SAM) instrument suite.
The organic molecules had chlorine atoms and included high levels of chlorobenzene and several dichloroalkanes, such as dichloroethane, dichloropropane and dichlorobutane.
It's possible that these chlorine-containing organic molecules were present as such in the mudstone. However, according to the team, it's more likely that a different suite of precursor organic molecules was in the mudstone, and that the chlorinated organics formed from reactions inside the SAM instrument as the sample was heated for analysis. Perchlorates (a chlorine atom bound to four oxygen atoms) are abundant on the surface of Mars. It's possible that as the sample was heated, chlorine from perchlorate combined with fragments from precursor organic molecules in the mudstone to produce the chlorinated organic molecules detected by SAM.
Other Missions with LR Experiments
In 1996, the Russian Federal Space Agency launched a Mars-bound probe carrying not only organic chemistry equipment but an upgraded version of Levin’s experiment. Rather than treating regolith samples with a mixture of “right-handed” and “left-handed” forms of organic substrates (known in chemistry as racemic mixtures), the new LR would have treated some samples with a left-handed substrate (L-cysteine) and others with the substrate’s mirror image (D-cysteine).
Had results been the same for L- and D-cysteine, a non-biological mechanism would have seemed all the more likely. However, if the active agent in the Martian regolith favored one compound at the expense of the other, this would indicate life. Even more intriguing: if the active agent favored D-cysteine, it would have suggested an origin of life on Mars separate from the origin of life on Earth, since terrestrial life forms use mostly left-handed amino acids. Such a result would suggest that life originates fairly easily, implying a cosmos teaming with living forms.
But Russia’s Mars ’96 probe crashed in the Pacific Ocean shortly after liftoff.
A few years later, the European Space Agency sent Beagle 2 to Mars, carrying an advanced organic detection package, but this probe too was lost.
Curiosity does not include an LR experiment of any sort, but contains instruments to detect organic molecules.
Currently Planned Missions
The ESA ExoMars Trace Gas Orbiter (TGO), which arrived at Mars this week, will measure and map methane and other important trace gases with high sensitivity to provide insights into the nature of the source through the study of gas ratios and isotopes. The TGO will monitor seasonal changes in the atmosphere’s composition and temperature in order to create and refine detailed atmospheric models. Its instruments will also map the subsurface hydrogen to a depth of a metre, with improved spatial resolution compared with previous measurements.
The ExoMars program will land a rover in the year 2020. It will collect samples with a drill down to a depth of 2 m and analyse them with next-generation instruments in an onboard laboratory. The primary objective is to land the rover at a site with high potential for finding well-preserved organic material, particularly from the very early history of the planet.
In 2020, the next-generation NASA Mars 2020 Rover will be equipped to look for the possibility of life and brings soil samples back to Earth. Newly miniaturised, Mars-proof instruments such as the SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), will point a beam at slabs of rock to locate promising deposits of organic compounds. Those deposits will be extracted and bought back to Earth to be examined more closely.
Conclusions by Levin and Straat
In light of the recent findings, such as the presence of water, organic molecules and periodic appearance of methane, 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.
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.
"It seems prudent that the scientific community maintain biology as a viable explanation of the LR experimental results," they write in their paper. "It seems inevitable that astronauts will eventually explore Mars. In the interest of their health and safety, biology should be held in the forefront of possible explanations for the LR results."
It is understandable that space scientists will not announce the discovery of life, until there is overwhelming evidence for it and there are no alternative non-biological explanations for observations and experiment results. It is all the more reason to raise the level of effort to search for and understand life on Mars, perhaps before we contaminate the Martian environment with humans and their accompanying microbial entourage.
The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment — online.liebertpub.com/...
- Viking 1 — en.wikipedia.org/…
Labeled Release Experiment Description — gillevin.com/...
- NASA Goddard Instrument Makes First Detection of Organic Matter on Mars www.nasa.gov/…
- NASA Rover Finds Active and Ancient Organic Chemistry on Mars — www.jpl.nasa.gov/…
- The Enigma of Methane on Mars — exploration.esa.int/…
- Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001 David S. McKay et al — www.earth.northwestern.edu/...
- Dinosaur asteroid 'sent life to Mars' — www.bbc.com/...
- Mars Landings: Successes and Failures — www.dailykos.com/…
- NASA to Hold Media Call on Evidence of Surprising Activity on Europa — www.dailykos.com/...
Main Image: Artist’s impression shows how Mars may have looked about four billion years ago. The young planet Mars would have had enough water to cover its entire surface in a liquid layer about 140 meters deep, but it is more likely that the liquid would have pooled to form an ocean occupying almost half of Mars’s northern hemisphere, and in some regions reaching depths greater than 1.6 km. Credit: ESO/M. Kornmesser/N. Risinger, Creative Commons.
P.S. There should be one more option in the poll below (too late to add now) —
If life exists on Mars, then we should leave it alone to evolve on its own.
What is you opinion of Life on Mars and its Future?
What is you opinion of Life on Mars and its Future?
Life exists on Mars and we should leave it alone to evolve on its own.
Life exists on Mars and but it needs a helping hand from us.
Life may exist on Mars, or not. In any case, we should leave it alone.
Life may exist on Mars, but it does not matter. Let's export our lifeforms to it.
There is no life on barren Mars. It is ours do as we please.
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