About Chemical Innovation - Subscription Information
May 2001
Vol. 31, No. 5, pp 12—16.
Enabling Science

Table of Contents

Charles W. Schmidt

The chemistry of life on Mars

A 1976 Viking Lander experiment found possible signs of life on the Red Planet. Or did it? After two decades, Gilbert Levin’s assertions are getting a second look.

opening art
Dave Jonason
If your idea of a Martian is a little green man who hops out of a UFO and goes looking for George W. Bush, you’re probably in for a disappointment. A real Martian, should it exist, might resemble a tiny microbe sustained by minimal, intermittent water supplies and the energy flux generated by the oxidation of hydrogen and carbon monoxide (1). If you saw a Martian colony at home, you’d probably try to wipe it off the counter with Formula 409 spray.

Regardless of what it looks like, the discovery of life on Mars would be a stunning revelation that would forever change humanity’s view of its place in the cosmos. And for the past quarter century, much of the controversy over its existence has not revolved around a National Aeronautics and Space Administration scientist, but the inventor of a sugar substitute for diabetics who is now the chief executive officer of a company in Beltsville, MD, which last year grossed $13 million. More than just a businessman, Gilbert Levin holds a Ph.D. in sanitary engineering from the Johns Hopkins University and has published more than 30 papers about life on Mars, a subject about which he is passionate (some would say obsessive), and in which he has a historical stake: Levin designed the only biological test to ever detect what may have been life on the surface of the Red Planet. The results of his experiment, performed during the Viking Lander mission of 1976, have embroiled scientists in a long-standing debate that has recently returned to center stage.

Because the biological test results were in conflict with those from other Viking experiments, the fleeting evidence for life was passed over by scientists who at the time were convinced that conditions on Mars were too extreme to sustain living creatures. But in the past several years, assumptions arguing against life on Mars have been shaken by new discoveries, including telescopic evidence of water and the presence of organics in Martian meteorites (2).

In the face of this new evidence, scientists are reconsidering the biological data, which Levin sees as overdue vindication. “I’ve been shot down for my thoughts concerning life on Mars by people who kept saying the truth is in the simple explanation,” he says. “For a long time, it was easier to explain my results with chemistry, which argued against life. But now, it appears the chemical explanation may be too complicated, and that life may be the simplest answer.”

Viking Lander data
Only two NASA missions from Earth have landed spacecraft on the Martian surface so far: the Viking Mission, which put two landers on Mars roughly a month and a half apart in 1976, and the Mars Pathfinder, which landed there in 1997. Of these, only the Viking Mission investigated biological activity in the Martian environment. NASA called the Viking biology package GEX/LR/PR, an acronym for its three experiments: gas exchange, labeled release, and pyrolytic release. The GEX and PR experiments yielded results that David McKay, the chief scientist for Planetary Science and Exploration at the Johnson Space Center, recently called “unambiguously negative”. However, the LR experiment, the one designed by Levin, scored a positive hit. The goal of this experiment was to identify metabolic wastes from extraterrestrial organisms consuming radiolabeled nutrients squirted on Martian soil. What the LR instrument detected was radioactive carbon dioxide in the air above the samples, suggesting that the nutrients may have been consumed by microbes, which in turn respired byproducts. Also significantly, the radiation counters were quiet after the samples were heated to levels that would kill any life forms in the soil (3, 4).

These profound results would have been taken more seriously were it not for concurrent GC-MS analyses of the same samples showing a complete absence of any organic matter. The conclusion reached by NASA scientists was that without organic material, the LR results could not have been due to biological processes, but to chemical reactions that might have oxidized the nutrients and given off radiolabeled gas. In short: no organics, no life.

Massachusetts Institute of Technology professor and associate director of the Center for Environmental Health Sciences, Arthur Lafleur, who helped design the GC-MS used on the Viking Mission, says these results suited NASA just fine. “There were a lot of people at the time who couldn’t handle the discovery of life on Mars,” he says. “Maybe it was a philosophical thing; they just didn’t want that result. And those prejudices probably influenced the analysis of the data.”

Not so, says Levin, who asserts that in spite of repeated attempts, he was never able to replicate the Martian LR results with chemical reagents. In the face of consistent failure to produce experimental evidence for an alternative chemical explanation, Levin spent years arguing tirelessly that only biology could have explained what the Viking Landers detected.

Calls for reevaluation
Recently, Levin’s call for reevaluating the Viking LR data has garnered some support from other scientists in the field (5). A key issue is the possibility that organic compounds may be present on Mars, despite the negative GC-MS findings on the Viking missions. McKay—who in 1996 made headlines by announcing his discovery of what looked like fossilized bacteria in a 4-billion-year-old Martian meteorite found in Antarctica—examined samples of several other meteorites also believed to have been blasted from Mars’s surface and determined that they contained polynuclear aromatic hydrocarbons (PAHs) and possibly other forms of carbonaceous material. It’s important to note that PAHs, perhaps the most abundant class of organic materials in the universe, can originate from both biotic and abiotic processes. This means that their presence in the meteors does not guarantee that they are the remnants from previous life forms. Furthermore, whether the organics found in the meteors are actually indigenous to Mars is still debated.

But McKay says that a comparison of the carbon isotope ratios found in the meteors with those encountered on Earth convinced him that the meteors are uncontaminated by terrestrial organics. “We found that 80% of the organics in the Nakhla meteor, which fell in Egypt in 1911, have no carbon 14,” he explains. “This means that the organics were there before the meteors landed on Earth.” One of the meteorites is only about 165 million years old, leaving open the possibility that life may have persisted on Mars for billions of years. If the rocks hold remnants of bacteria, McKay suspects that similar microbes may still thrive on Mars, perhaps in ice pockets.

Also at issue is the sensitivity of the Viking GC-MS, which was designed to detect volatilized carbon-containing elements arising from samples heated to several hundred degrees Celsius. Although the GC-MS was at the time considered a highly advanced instrument, some experts now contend that its detection limit of one part per billion may have been inadequate to detect low levels of organic matter consistent with the presence of microbial life. Some of the evidence supporting this view comes from the research of Jeffrey Bada, director of the NASA Specialized Center for Research and Training in Exobiology (La Jolla, CA), who recently published findings showing that at the parts-per-billion level, the degradation products generated by the equivalent of several million bacterial cells per gram of Martian soil would not have been detected by the Viking instrument (1).

“I’m not saying that I accept Levin’s interpretation of the data,” says McKay. “But I would suggest that there is strong evidence for the presence of organics on the surface of Mars. And if the absence of organics was used as proof of false readings in the LR experiment, then we need to go back and look at the results of that experiment more carefully.” McKay recently presented a paper calling for reanalysis of the LR findings at the 32nd Lunar and Planetary Science Conference in Houston (2).

The peroxide conundrum
Any discussion of organics in the Martian environment has to contend with the unanswered question of oxidants, which may be present at levels high enough to destroy all organic matter on the surface, including organics arriving at the planet from outer space. Ever since Viking tests (unrelated to the biology experiments) found that Martian soils exposed to water vapor gave off oxygen, scientists have known that some kind of oxidizing agent was present on Mars, but they didn’t know what it was. The reddish hue on Mars has been attributed to oxidation products of water or rusting iron minerals left over from a few billion years ago when the planet was warm and wet (6). But if there are now oxidants on Mars, their identities are a mystery. Through the years, the top contender has been hydrogen peroxide, which was thought to descend to the surface from its origins in the upper atmosphere. However, in 1997, scientists at NASA’s Goddard Space Flight Center (Beltsville, MD) reported that Earth telescope experiments could find no evidence of H2O2 in the Martian atmosphere (6).

With enthusiasm for H2O2 waning, scientists are shifting their attention to a chemical species called superoxide (O2), which is formed when minerals in the presence of atmospheric oxygen are exposed to ultraviolet rays from the Sun. Albert Yen, a geochemist with NASA’s Jet Propulsion Laboratory (Pasadena, CA), who is leading this research, suggests that negatively charged superoxide ions might persist in Martian soil and decompose organic material that either forms on the planet or is deposited there by meteorites. Unlike H2O2, which as a gas can be detected telescopically in the atmosphere, superoxide would be adsorbed on mineral grains and be undetectable from Earth.

In experiments designed to replicate the Martian atmosphere in a test tube, Yen was able to produce superoxide radical anions from Martian analogue minerals exposed to UV light (7). He contends that superoxide reacted with the nutrients used during the LR experiment and produced the radiolabeled CO2 that Levin sees as evidence for microbial respiration. “I believe the superoxide radicals we produced in the laboratory are the cause of the unusual results seen during the Viking LR experiment,” he said without reservation.

Not surprisingly, Levin vigorously rejects these claims, arguing that the survival of Yen’s oxidant when heated to ~200 °C proves that it cannot be the same as the agent detected by his experiment on Mars 25 years ago (3). “That agent was completely destroyed by room temperature storage in the spacecraft,” he says.

Further complicating Yen’s assumption that peroxides consume Martian organics are the results of a recent study by Stephen Benner, professor of chemistry at the University of Florida, Gainesville (8). Benner’s goal was to oxidize organic materials that might be found on Martian meteorites to see if he could identify stable byproducts that could survive on the Martian surface. What he discovered was that under certain oxidizing conditions, PAHs, alkanes, alkylbenzenes, and naphthalene, among others, gave rise to salts of benzenecarboxylic acids, and perhaps oxalic and acetic acids, that were oxidation-resistant (8). These stable intermediates are also nonvolatile, which is important because the GC-MS used on Viking was designed to analyze volatilized organics given off by heated soil samples. These findings indicate that a whole class of organic compounds may have been missed during the Viking analyses.

Benner emphasizes that the organic compounds analyzed in his laboratory are nonbiogenic. However, he adds, “Organic molecules generated on Mars by entirely hypothetical biological synthesis should suffer similar fates at or near the surface.”

Supporters of reanalysis of the LR data are quick to point to these results as further justification for their view. Says McKay, “These findings are important because they call into question the whole premise that Martian organics are always destroyed by oxidation.”

Is there water on Mars?
In addition to questions about the fate of organics and the sensitivity of the Viking GC-MS, those who believe the LR results are biological have been buoyed by some other recent developments. Among these is the suggestion that water, long thought to be completely nonexistent on Mars, may be present there after all. For decades, most scientists maintained that the Martian atmosphere was too sparse, and temperature and pressure too low, to permit the presence of water. And so it came as a shock when NASA announced in June 2000 that high-resolution images from the Mars Orbiter camera were showing gullies on Martian cliffs and crater walls that could have been formed by liquid water seeping to the surface in the geologically recent past (9, 10).

The freshness of some features suggested that they might still be active today. NASA scientists caution that any number of things could have created the gullies, including liquid CO2 or fluidlike gas flows erupting from volcanoes. But adding weight to the view that water does exist on Mars, Thomas Donahue, a professor in the Department of Atmospheric, Ocean, and Space Sciences at the University of Michigan, Ann Arbor, recently published what he calls a “time-dependent” mathematical theory showing that “a large reservoir of water exists in the Martian crust at depths shallow enough to interact strongly with the atmosphere.” (11). To those who believe life-sustaining conditions may exist on the Red Planet, the observed and calculated evidence for water is a welcome finding.

McKay adds that evidence from Earth indicates that only minimal water supplies might be needed to sustain primitive life forms. “Even in the driest places on Earth, for example, the Atacama desert in Chile, you can still find plenty of bacteria even though it almost never rains. You find the same thing in permafrost conditions; the bacteria that live there have this very simple metabolism that allows them to live on films of water.”

Levin agrees with this observation, adding, “We now have many reports of finding life where it wasn’t expected, including in clouds and miles below the surface of the Earth. If we didn’t know better, we’d think that microbes on the Earth’s surface would have been killed by oxygen poisoning. Obviously, they survived and adapted. Why wouldn’t Martian microbes also adapt to their own environment?”

The future
Ultimately, only another landing on Mars will solve the question of life on the planet once and for all. The next such visit is the British Beagle 2 mission scheduled for June 2003 (12). NASA’s next planned lander voyage with two Rovers also will take off in 2003 (13). Scientists gearing up for the British and U.S. missions are designing experiments that they hope will provide new evidence to supplement or replace the contentious Viking data. For example, a small, lightweight rust detector, the Mars atmospheric oxidant sensor, designed at the NASA Ames Research Center (Mountain View, CA), may catch a ride on the Beagle 2 mission, where its job will be to measure the environment’s oxidation potential. These data should help resolve the questions regarding the fate of organics on the surface.

Bada is working with an instrument, the Mars organic detector, which will search for traces of key organic compounds, including amino acids, amines, and PAHs. An important feature of the amino acid detection phase will be to confirm that the compounds all have the same chirality: either a D- or L-configuration. Such evidence, says Bada, would be “clear evidence for biology”. On Earth, all proteins are based on L-amino acids, which Bada says is “probably a matter of chance”. Finding amino acids with a D-configuration would not only be suggestive of life, but also of a life form unique to Mars rather than one that might have been transported to Mars from Earth on a meteor. This possibility forms the basis of the “Panspermia theory”, which holds that living organisms travel through space and “seed” other planets in the universe (14).

Through it all, Levin believes his Viking data will eventually be proven correct. When the Beagle 2 mission departs for Mars and begins the next phase of data collection, Levin will be 81 years old. “None of the 30 theories published over the past quarter of a century to explain away my results has withstood scrutiny,” he says. “I think it’s time for the critics to accept that the simplest explanation is that living microorganisms were detected on Mars.”

References

  1. Glavin, D. P.; Schubert, M.; Botta, O.; Kminek, G.; Bada, L. Earth Planet. Sci. Lett. 2001, 85, 1–5.
  2. Warmflash, D. M.; Clement, S. J.; McKay, D. S. Organic Matter in SNC Meteorites: Is It Time to Reevaluate the Viking Experimental Data? 32nd Lunar and Planetary Science Conference, Houston, March 12–16, 2001; Abstract 2169.
  3. Levin, G. V. Proc. SPIE-Int. Soc. Opt. Eng. 1997, 3111, 146–161.
  4. DiGregorio, B.; Levin, G. L.; Straat, P. A. Mars: The Living Planet; Frog Ltd.: Berkeley, CA, 1997.
  5. Hedgpeth, D. The Washington Post, Dec 1, 2000; p A1.
  6. Krasnopolsky, V. A.; Bjoraker, G. L.; Mumma, M. J.; Jennings, D. E. J. Geophys. Res. 1997, 102, 6525–6534.
  7. Yen, A. S.; Kim, S. S.; Hecht, M. H.; Frant, M. S.; Murray, B. Science 2000, 289, 1909–1912.
  8. Benner, S. A.; Devine, K. G.; Matveeva, L. N.; Powel, D. H. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 2423–2430.
  9. Kerr, R. A. Science 2000, 288, 2295–2297.
  10. Malin, M. C.; Edgett, K. S. Science 2000, 288, 2330–2335.
  11. Donahue, T. M. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 827–830.
  12. www.beagle2.com (accessed May 2001).
  13. http://mars.jpl.nasa.gov/missions/.
  14. Bada, J. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 797–800.

Charles W. Schmidt is a freelance writer of science-related topics (171 Danforth St., Portland, ME 04102; 207-774-3284; cschmidt@gwi.net).

Return to Top || Table of Contents